Seismic inversion for three-dimensional variations of velocity and attenuation are often used to delineate magma bodies in the crust and upper mantle. Problems related to spatial resolution and data noise can obscure details relevant to investigating magma chambers, and the introduction of smoothing constraints, or damping, causes blurring. Tomographic inversions for P- and S-wave velocity/attenuation are summarized including large calderas, rift zones and smaller scale subduction zone volcanoes. While results vary considerably from place to place, most anomalies are found to be in the range of ?10% perturbation, a range often controlled by the method of smoothing or regularization imposed during analysis. At many volcanoes high velocity anomalies are observed in the shallow regions below active areas where conduits, dykes or sills are expected to be present. At other locations low velocity perturbations are seen and interpreted as magma accumulation. Resolution limitations and regularization play a significant role in determining the level of perturbation observed in tomographic studies, although there may be regions where diffuse accumulations of magma do not exhibit strong anomalies and their identification will be elusive. Back to Lees Publications
I present several new packages for analyzing seismic data for time series analysis and earthquake focal mechanisms. The packages consists of modules that 1) read in seismic waveform data in various common exchange formats, 2) display data as either event or continuous recordings and 3) performs numerous standard analyses applied to earthquake and volcano monitoring. SeisR is designed as a research tool aimed at investigators who need to quickly assess large amounts of time-series as they are related to the spatial distribution of geologic structure and wave propagation. In addition to time series analysis, a spatial mapping program is included that ties waveforms and radiation patterns to geographical data-base and mapping programs. Lees, J. M., N. Symons, O. Chubarova, V. Gorelchik, and A. Ozerov (2007), Tomographic Images of Kliuchevskoi Volcano P-wave Velocity, in Volcanism and Subduction: The Kamchatka Region, edited by J. Eichelberger, E. Gordeev, M. Kasahara, P. Izbekov and J. M. Lees, pp. 293-302, American Geophysical Union, Washington, D.C.
Three-dimensional structural images of the P-wave velocity below the edifice of the great Klyuchevskoy group of volcanoes in central Kamchatka are derived via tomographic inversion. The structures show a distinct low velocity feature extending from around 20 km depth to 35 km depth, indicating evidence of magma ponding near the Moho discontinuity. The extensive low velocity feature represents, at least to some degree, the source of the large volume of magma currently erupting at the surface near the Klyuchevskoy group. Back to Lees Publications
We present a review of great earthquakes and seismicity patterns along the Alaska-Aleutian and Kamchatka-Kurile arcs as an overview of one of the longest subduction zone complexes on the planet. Seismicity patterns, double seismic zones and focal mechanism solutions are described and used to illustrate the distribution of stress in the Pacific plate as it collides with North America and Eurasia. Seismicity along the Alaska-Aleutian arc is relatively shallow as compared to the Kamchatka-Kurile arc where the plate is considerably older and thicker prior to entering the subduction zone. Tomographic inversions of the slab generally show high velocity anomalies where seismicity is high, presumably tracking the cold subducting lithosphere. Back to Lees Publications
First arrivals of seismic waves were recorded along the Kamchatka arc using broadband seismic stations deployed for one year in 1998-1999. Cross correlation methods were used from a high resolution data set for tomographic inversion of body waves. The P-wave teleseismic tomography shows evidence of slab shoaling along the northern terminus of the Kamchatka subduction zone. Tomographic anomalies corroborate trends in seismicity, geochemistry, heat flow, shear wave splitting, and surface wave inversions. Thermal ablation via contact with asthenosphere, under the proper conditions, is offered as a possible explanation of the observed shoaling of the Kamchatka slab edge. Back to Lees Publications
While wireless sensor networks can generically be used for a wide variety of applications, breakthrough innovations are most often achieved when driven by a genuine need or application, with its specific system-level and science-related requirements and objectives. Hence, our work focuses on the development of wireless sensor network system-on-chip devices and supporting software for volcano monitoring, which we call Sensor Network for Active Volcanoes (SNAV). In this paper we present preliminary results of our research and development work on intelligent sensor networks for monitoring hazardous environments especially the SNAV system-on-chip design for active volcanoes monitoring. Back to Lees Publications
We present a science-centric evaluation of a 19-day sensor network deployment at Reventador, an active volcano in Ecuador. Each of the 16 sensors continuously sampled seismic and acoustic data at 100 Hz. Nodes used an event-detection algorithm to trigger on interesting volcanic activity and initiate reliable data transfer to the base station. During the deployment, the network recorded 229 earthquakes, eruptions, and other seismoacoustic events. The science requirements of reliable data collection, accurate event detection, and high timing precision drive sensor networks in new directions for geophysical monitoring. The main contribution of this paper is an evaluation of the sensor network as a scientific instrument, holding it to the standards of existing instrumentation in terms of data fidelity (the quality and accuracy of the recorded signals) and yield (the quantity of the captured data). We describe an approach to time rectification of the acquired signals that can recover accurate timing despite failures of the underlying time synchronization protocol. In addition, we perform a detailed study of the sensor network?s data through a direct comparison to a standalone data logger, as well as an investigation of seismic and acoustic wave arrival times across the network. Back to Lees Publications
Augmenting heavy and power-hungry data collection equipment with lighter, smaller wireless sensor network nodes leads to faster, larger deployments. Arrays comprising dozens of wireless sensor nodes are now possible, allowing scientific studies that aren?t feasible with traditional instrumentation. Designing sensor networks to support volcanic studies requires addressing the high data rates and high data fidelity these studies demand. The authors? sensor-network application for volcanic data collection relies on triggered event detection and reliable data retrieval to meet bandwidth and data-quality demands. Back to Lees Publications
We studied regional QLg-1 at 1 Hz (Qo-1) around the Korean Peninsula based on broadband, vertical component seismic records of 6 IRIS Global Seismographic Network stations, and 19 Korean stations of the Korea Institute of Geoscience and Mineral Resources. Using 177 seismic events with M between 5.3 and 5.7, and depths less than 50 km, the reversed two station method was applied and 94 high quality interstation paths were selected from 869 possible pairs. These results show high Qo-1 paths around the Sea of Japan (East Sea) reflecting the typical oceanic structure, and low Qo-1 paths around the northeastern China related to inactive seismicity. Assigning these path values into 193 cells around S. Korea with a size of 1 by 1?, we observed that the regional Qo-1 decreased gradually from east to west between 2 and 1 ? 10-3. Back to Lees Publications
A detailed analysis of discrete degassing pulses, chugs, at Sangay volcano, was performed on seismic and infrasonic records to determine the physics of the conduit. Infrasonic chugging signals appear as repetitive pulses with small variations in amplitude and time lag. An automated time-domain analysis was developed to measure with high precision time intervals and amplitudes at different wave arrivals, reducing the possibility error associated with hand picking. Using this automated method, a strong positive correlation of acoustic amplitude with repose time between individual pulses on chugging signals of Sangay was found on numerous oscillating sequences. Frequency gliding of apparent harmonic frequencies generally trends from high to low frequency at Sangay, in contrast to trends at Karymsky Volcano, Russia. A new description of chugging events using wavelet transform methods, appropriate for non-stationary signals, shows subtle changes in the waveforms relate to physical processes in the volcano. A system of nonlinear feed back, based on choked flow at the vent, is postulated as the most likely source of this volcanic tremor. Back to Lees Publications
Reventador Volcano entered an eruptive phase in 2005 which included a wide variety of seismic and infrasonic activity. These are described and illustrated: volcano-tectonic, harmonic tremor, drumbeats, chugging and spasmodic tremor, long period and very long period events. The recording of this simultaneous activity on an array of three broadband, seismo-acoustic instruments provides detailed information of the state of the conduit and vent during this phase of volcanic eruption. Quasi-periodic tremor at Reventador is similar to that observed at other volcanoes and may be used as an indicator of vent aperture. Variations in the vibration modes of the volcano, frequency fluctuations and rapid temporal fluctuations suggest the influx of new material, choking of the vent and possible modification of the conduit geometry during explosions and effusion over a period of six weeks. Back to Lees Publications
In parts of Ecuador's northeastern provinces airwaves are saturated
with infrasound (sub-audible acoustic energy < 20 Hz). Here we
identify the locations and characterize three distinct sources of
local infrasound, including two types of infrasonic sources, which are
not commonplace discussed in the literature. The first of these novel
sources is an intense and continuous infrasound radiator with a fixed
location corresponding to the San Rafael Waterfall. The signal from
the river exhibits a tremor-like envelope that is very well correlated
across the 3-element infrasound network. Beyond the river, we also
observe and map spatially variable sources corresponding to lightning.
These transient signals have impulsive onsets, but are not well
correlated across the network and are attributable to
spatially-distributed source regions. Finally, we identify plentiful
infrasound corresponding to Reventador's vent that is associated with
unrest at the volcano. This study demonstrates the great utility of
dispersed infrasound networks for distinguishing sources and improving
interpretation of local types of infrasound radiators.
Back to Lees Publications
Wireless sensor networks have the potential to
greatly advance volcano monitoring. Augmenting
heavy and power-hungry monitoring equipment with lighter, small
wireless sensor network nodes leads
to faster, larger deployments. Arrays consisting of dozens of wireless
sensor nodes are now feasible,
with the additional scale and resolution permitting studies not
possible with existing instrumentation.
Designing sensor networks for volcanic monitoring requires addressing
the high data rates and high
data fidelity demanded by this scientific application. We have designed
a sensor network application for
volcanic monitoring that relies on triggered event detection and
reliable data retrieval to meet bandwidth
and data quality demands. Here we describe our design and relate our
experience deploying our network
on Volc´an Reventador, an active volcano in northern
Ecuador.
Back to Lees Publications
An experiment on the uses of shear-wave
splitting as an
imaging tool in fracture-controlled geothermal reservoirs
was conducted at Krafla, Iceland. Fifteen days after the
beginning of the seismic recording the injection was stopped
for eleven days and then restarted, a sequence designed to
determine whether shear-wave splitting measurements can
detect the transient response of the subsurface crack system
to changes in fluid pressure. It was observed that time
delays between the fast and slow split shear waves
changed significantly and promptly with the stoppage and
resumption of injection. Large time delays occurred only
during injection, decreased substantially during the
stoppage phase, and increased again as injection
restarted. Comparisons of these results with similar
observations at the Coso geothermal field in California
strongly suggest that the time delay of split shear waves
can be a useful proxy to monitor fluid pressure in the
cracks and changes in crack density.
Back to Lees Publications
An important objective in volcanology is the
quantification of eruption intensity through the study of the elastic
energy propagated through atmosphere and ground. To this end we
deployed a time-synchronized, seismo-acoustic-video installation at
Tungurahua Volcano (Ecuador) in November-December 2004 in an attempt to
understand the relation between elastic wave radiation and eruptive
manifestation. Our results indicate that plume expansion scales very
poorly with both the recorded seismic and acoustic trace energy and
with the seismic and acoustic signal amplitude. Heightened material
flux during Tungurahua eruptions evidently appears not to be
representative of elevated source accelerations, which are the primary
influences on elastic energy radiation.
Back to Lees Publications
Tungurahua volcano has exhibited 5 eruptive
periods since 1999. The last one began in May, 2004, reached its peak
in July, and remained with minor bursts until December, 2004. Between
June 30 and August 12, 2004, three temporary seismic and infrasonic
stations were installed on the southwest, northwest and northeast
flanks of the volcano. About 2,000 degassing signals were recorded
jointly on high fidelity infrasonic and seismic instrumentation.
Recorded signals, classified by waveform character, include: - 1)
Explosion events: impulsive, short duration blasts on infrasound and
spindle-shaped long-duration seismic signals. Amplitudes of blast
signals span three orders of magnitude from 0.1 to 180 Pa (July 21,
03h32 GMT, the largest signal recorded on the closest station). - 2)
Roaring events, composed of seismic and infrasound complex signals with
broad frequency bands at stations close to the vent. - 3) Chugging
events, composed of saw-tooth shaped infrasound signals. Collocated
infrasonic and seismic instruments provided a basis for cluster
analysis classification of the most conspicuous type of infrasonic
signals (blast explosion events). Travel time analysis of seismic first
arrivals and infrasonic waves indicates that blast explosions start
with a seismic event at a shallow depth (55-218 m), followed ~1 s later
by an out-flux of gas, ash and solid material through the vent,
creating pressure disturbances with different patterns that do no
correlate with size, location or temporal patterns.
Back to Lees Publications
This paper describes our experiences using a
wireless sensor network to monitor volcanic eruptions with
low-frequency acoustic sensors. We developed a wireless
sensor array and deployed it in July 2004 at Volc´an
Tungurahua, an active volcano in central Ecuador. The network
collected infrasonic (low-frequency acoustic) signals
at 102 Hz, transmitting data over a 9 km wireless link to a
remote base station. During the deployment, we collected
over 54 hours of continuous data which included at least
9 large explosions. Nodes were time-synchronized using a
separate GPS receiver, and our data was later correlated
with that acquired at a nearby wired sensor array. In
addition to continuous sampling, we have developed a
distributed event detector that automatically triggers data
transmission when a well-correlated signal is received by
multiple nodes. We evaluate this approach in terms of
reduced energy and bandwidth usage, as well as accuracy
of infrasonic signal detection.
Back to Lees Publications
During the summer of 2004 we recorded the
seismicity at the Krafla geothermal field for forty
days with an array of twenty PASSCAL L-28 4.5-Hz
sensors. The Krafla field is located approximately 60
km East of Akureyri in northern Iceland. The array
covered an area approximately 5 km N-S by 4 km EW.
The field area is located on Holocene lava flows
on the Mid-Atlantic Ridge. The array recorded
approximately 5 micro-earthquakes per day at a
sampling rate of 500 Hz. This high sampling rate is
required to exploit newly developed theories on the
frequency-dependence of shear-wave splitting
(SWS). During the experiment, the injection well was
stopped for ten days to study the response of the
subsurface crack system to changes in water pressure.
SWS is an exploration method based on the analyses
of polarizations and time delays of shear waves that
have been distorted by the anisotropy of the medium
through which the seismic waves have propagated.
Epicenters roughly align along the E-W direction,
while hypocenters are shallow around the injection
well and appear to be related to the on-going
injection. Observations of SWS at Krafla have
provided evidence for at least two major crack
systems oriented approximately N-S and E-W. This
last, rather unexpected direction is consistent with
results from a simultaneous MT (magneto-telluric)
survey. Further SWS study will lead to a more
detailed understanding of the fracture locations, sizes,
and orientations in the geothermal field.
Back to Lees Publications
A high resolution tomographic study, using
cubic B splines parameterization and employing a systematic approach to
the choosing of appropriate damping and smoothing parameters, provided
a three dimensional P wave velocity map of the Loma Prieta area. 11,977
high quality raypaths from 844 aftershocks of the 1989 Loma Prieta
earthquake were used in the inversion. The velocity model exhibits a
low velocity feature between the San Andreas and Zayante Vergeles
faults in the top 10 km of the crust. This low velocity feature is
interpreted as a sedimentary unit exposed to the northwest and
separated from the Salinian block by the Zayante Vergeles fault. Below
10 km no consistent change is observed between the Salinian and the
Franciscan blocks. There appears to be a high correlation of aftershock
activity and localized high velocity anomalies southeast of the Loma
Prieta main shock. Whereas this anomaly may represent brittle rocks
associated with a fault zone asperity that failed after the main shock,
there is evidence to suggest it may be a body of serpentinite. The
serpentinite exhibits high velocities and is potentially less competent
than surrounding country rock, thus providing a sector along the fault
more likely to be associated with many smaller earthquakes or creep
behavior.
Back to Lees Publications
Pacific plate subduction at the
Aleutian–Kamchatka juncture, or corner, could be accommodated by
either a large bend or a
tear in the oceanic lithosphere. In this paper, we describe a number of
observations which suggest that the Pacific plate
terminates abruptly at the Bering transform zone (TZ). Seismicity
shoals along the subduction zone from Southern Kamchatka
(600 km) to relatively shallow depths near the Kamchatka–Bering
Fault intersection (100–200 km). This seismicity shoaling is
accompanied by an increase in the heat flow values measured on the
Pacific plate. Moreover, unusual volcanic products related
to adakites are erupted on Kamchatka peninsula at the juncture. Simple
thermal modeling shows that a slab torn and thinner
along the northern edge of the Pacific plate would be compatible with
the observations. Delayed thickening of the lithosphere
due to the Meiji–Hawaiian hotspot may be responsible for the
required thinning.
Back to Lees Publications
Eruptive activity at Stromboli Volcano was
significantly elevated over background levels in May 2001. During 63
hours of observation, eight vents produced, on average, 17
explosions/hr with an average repose interval of three minutes. During
this period the Stromboli vents exhibited consistent seismic and
acoustic signatures, based on cross correlation cluster analysis.
Dendrogram clustering, based on waveform cross-correlation, was used to
illustrate the complexity of the near surface plumbing system of
Stromboli's multi-vent edifice. Cross correlations of displacement
seismograms produced by explosions at specific craters, such as the
Northeast crater (NEC), form dense waveform clusters with correlation
coefficients between 0.96-0.99, while displacement waveforms from other
craters, such as the Southwest crater (SWC), exhibit loose clusters
with correlations between 0.88-0.96. The inconsistency of SWC events,
as compared to the NEC, suggests that the vent system there is more
heterogeneous. Cluster linkage distance between the NEC cluster and the
Central crater (CC) cluster is shorter than the linkage distance
between the NEC and SWC clusters, indicating that NEC and CC are more
closely related. Infrasonic observations were used to locate vent
explosions confirming that the clusters of events are associated with
specific vents or craters. Qualitative analyses of acoustic waveforms
from approximately 500 explosions reveal that impulsive acoustic
signals were associated with short, mechanically simple ground
displacement responses. These events may correspond to the bursting of
an individual gas slug. Similar degassing mechanisms from vents in the
NEC and the CC show common characteristics in their displacement
waveforms.
Back to Lees Publications
Explosions of Karymsky volcano often
produce signals containing a sequence of repeating pulses recorded on
acoustic and seismic sensors, known as chugging. The amplitudes of
these pulses correlate with the time interval between pulses. For a
given measured acoustic pressure, seismic amplitudes take on arbitrary
values up to a specific, empirically determined threshold. Conversely,
small seismic amplitude events yielded acoustic waves with large
variations and large amplitude seismic events corresponded to large
acoustics waves. These observations are not consistent with a source
modeled by a resonating conduit. Rather, a model consisting of a
sequence of discrete pulses explains the data and provides a framework
for understanding the dynamics of degassing at the vent. The physical
model for chugging involves a time varying narrowing vent where gasses
are released in a series of oscillations which appear to be harmonic
but instead are modeled as short term transients, or discrete pulses,
suggestive of choked flow.
Back to Lees Publications
Large amplitude, secondary arrivals are
modeled as scattering anomalies near the Coso, California, Geothermal
field. Polarization and raytracing methods determine the orientation
and location of the scattering body. Two models are proposed for the
scatterer: 1) a point scatterer located anywhere in a one-dimensional,
layered velocity model, 2) a dipping interface between two homogeneous
half spaces. Each model is derived by non-linear, grid search inversion
for the optimal solution which best predicts observed travel times. In
each case the models predict a nearly vertical scatterer southwest of
station S4 and Y4, each southeast of Sugarloaf Mountain, a large
rhyolite dome. The interface model includes five unknown parameters
describing the location and orientation of the interface in addition to
the S-wave velocity of the half-space. The S-wave velocity, 3.25 km/s,
agrees with independently derived one-dimensional models in this area.
The large amplitude, vertical impedance contrast interface coincides
with steep gradients of heat flow measured near the surface and with
structural boundaries observed in surface geology. The reflector is
most probably the sharp boundary between the northern part of the field
where there is significant fluid flow versus the southern part where
hydrothermal fluids are absent. The interface coincides with geological
boundaries and faults recently observed in this region, most likely
representing the hydrothermal barrier which channels hot fluids
northward.
Back to Lees Publications
A model is proposed to explain temporal patterns of
activity in a class of periodically exploding Strombolian-type
volcanos. These patterns include major events (explosions) which follow
each other every
10-30 minutes and subsequent tremor with a typical period of 1 second.
This
two-periodic activity is thought to be caused by two distinct
mechanisms of accumulation of the elastic energy in the moving magma
column:
compressibility of the magma in the lower conduit and viscoelastic
response of the almost solid magma plug on the top. A release of the
elastic energy happens when a stick-slip dynamic phase transition in a
boundary layer along the walls of the conduit occurs; this phase
transition is driven by the shear stress accumulated in the boundary
layer. The first-order character and intrinsic hysteresis of this phase
transition explains the long periods of inactivity in the explosion
cycle.
Temporal characteristics of the model are found to be qualitatively
similar to
the acoustic and seismic signals recorded at Karymsky volcano in
Kamchatka.
Back to Lees Publications
Teleseismic receiver functions (RFs) from a
yearlong broadband seismological experiment in Kamchatka reveal
regional
variations in the Moho, anisotropy in the supra-slab mantle wedge, and,
along the eastern coast, Ps converted phases from the
steeply dipping slab. We analyze both radial- and transverse-component
RFs in bin-averaged epicentral and backazimuthal
sweeps, in order to detect Ps moveout and polarity variations
diagnostic of interface depth, interface dip, and anisotropic fabric
within the shallow mantle and crust. At some stations, the radial RF is
overprinted by near-surface resonances, but anisotropic
structure can be inferred from the transverse RF. Using forward
modeling to match the observed RFs, we find Moho depth to
range between 30 and 40 km across the peninsula, with a gradational
crust–mantle transition beneath some stations along the
eastern coast. Anisotropy beneath the Moho is required to fit the
transverse RFs at most stations. Anisotropy in the lower crust
is required at a minority of stations. Modeling the amplitude and
backazimuthal variation of the Ps waveform suggests that an
inclined axis of symmetry and 5–10% anisotropy are typical for
the crust and the shallow mantle. The apparent symmetry axes
of the anisotropic layers are typically trench-normal, but
trench-parallel symmetry axes are found for stations APA and ESS,
both at the fringes of the central Kamchatka depression. Transverse RFs
from east-coast stations KRO, TUM, ZUP and PET are
fit well by two anisotropic mantle layers with trench-normal symmetry
axes and opposing tilts. Strong anisotropy in the supraslab
mantle wedge suggests that the mantle
‘‘lithosphere’’ beneath the Kamchatka
volcanic arc is actively deforming, strained
either by wedge corner flow at depth or by trenchward suction of crust
as the Pacific slab retreats.
Back to Lees Publications
In the most basic seismo-acoustic studies at
volcanoes, infrasound monitoring enables differentiation between
sub-surface seismicity and the seismicity associated with gas release.
Under optimal conditions, complicated degassing
signals can be understood, relative explosion size can be assessed, and
variable seismo-acoustic energy partitioning
can be interpreted. The extent to which these points may be
investigated depends upon the quality of the infrasonic
records (a function of background wind noise, microphone sensitivity,
and microphone array geometry) and the type
of activity generated by the volcano (frequency of explosions,
bandwidth of the signals, and coupling efficiency of the
explosion to elastic energy). To illustrate the features, benefits, and
limitations of infrasonic recordings at volcanoes,
we showcase acoustic and seismic records from five volcanoes
characterized by explosive degassing. These five
volcanoes (Erebus in Antarctica, Karymsky in Russia, and Sangay,
Tungurahua, and Pichincha in Ecuador) were the
focus of seismo-acoustic experiments between 1997 and 2000. Each case
study provides background information
about the volcanic activity, an overview of visual observations during
the period of monitoring, and examples of
seismo^acoustic data. We discuss the benefits and utility of the
infrasound study at each respective volcano. Finally,
we compare the infrasound records and eruptive activity from these
volcanoes with other volcanoes that have been the
focus of previous seismo-acoustic experiments.
Back to Lees Publications
This paper reviews geophysical and
seismological imaging in the Coso geothermal field, located in east
central California. The Coso geothermal production area covers an area
approximately 6 10 km2. Although regional seismicity is addressed, as
it sheds light on the magma, or heat, sources in the field, the main
focus of this paper is on the main production area. Three-dimensional
inversions for P- and S-wave velocity variations, distribution of
attenuation, and anisotropy are presented side-by-side so that
anomalies can be compared spatially in a direct manner. Velocity
inversions for P- and S-waves are combined for direct determination of
Poisson's ratio and indirect estimation of variations of porosity in
the field. Anomalies southeast of Sugarloaf Mountain are prominent on
nearly all analyses. The anomalies coincide with high levels of
seismicity and with stress anomalies as determined from earthquake
focal mechanism analysis and seismic anisotropy distribution. They also
correlate with high heat flow in the field and the termination of
geothermal production to the south. I speculate that an intrusion is
present in this region that causes significant perturbation of stress
in the field.
Back to Lees Publications
The temporal and spatial distribution of
seismicity in the Coso and Indian Wells valley region of eastern
California are discussed in this study. An analysis of fault-related
seismicity in the region let us conclude that the Little Lake fault and
the Airport Lake faults are the most significant seismogenic zones. The
faulting pattern clearly demarcates the region as a transition between
the San Andreas type strike-slip regime to the west and Basin and Range
extension to the east. We present the spatial and temporal variations
in seismicity immediately following significant earthquakes in nearby
regions over the last fifteen years with special emphasis on larger
earthquakes (M 5) over the last five years. The Ridgecrest earthquakes
of 1995 show a complicated faulting pattern as the rupture changes from
normal-slip to right-slip at depth. The inter-relationships between the
Coso earthquakes of 1996 and 1998 are presented as a set of conjugate
events. Analysis of earthquake source mechanisms shows evidence for
lateral variations of faulting pattern in eastern California.
Earthquake focal mechanisms are used to estimate local stress
orientation within the Coso geothermal field. We have identified a
boundary between a transpressional regime and a transtensional regime
inside the field which correlates with observed spatial variations of
heat flow and seismic attenuation, velocity and anisotropy.
Back to Lees Publications
New PASSCAL data have been acquired along the
extent of the Kamchatka Peninsula to examine the interaction of the
Pacific Plate and the mantle in the corner junction of the Aleutian and
Kamchatka trenches. The project, called the Side Edge of Kamchatka
Slab, is a collaborative effort between researchers at Yale University
and the Russian Academy of Sciences Institutes in
Petropavlovsk-Kamchatski, in particular the Institute of Volcanology
(Alexei Ozerov) and KOMSP (Evgenii Gordeev). In Russia, Kamchatka is
known as the caboose, the last car on the train. It is practically the
farthest one can get from Moscow, the center of cultural life in
Russia. Being nine time zones away from the financial centers creates a
sense of isolation and liberation. For Russians, Kamchatka is a land of
dreams and possibilities, much as Alaska is the last frontier for
Americans. This gives one the sense that being in Kamchatka is like
being on the edge of the world. But the feeling has more significance
than the mere waywardness of the place. The majestic volcanoes, the
continuous seismicity and the incredible geology are omnipresent
reminders that this is a special place. The relentless subduction of
the Pacific plate plunges beneath Kamchatka, 70mm per year. Kamchatka
is home to the most magnificent volcanoes in the Pacific Rim (28 active
volcanoes) and neighbor to the Bering strike-slip fault, marking the
western end of the Aleutian-Komandorsky Islands. This land mass
provides an exceptional platform for investigating the interactions of
volcanism, tectonics, and mantle dynamics.
Back to Lees Publications
The junction of the Aleutian Island and the
Kamchatka peninsula defines
a sharp turn in the boundary of the Pacific and North American plates,
terminating the subduction zones of the northwest Pacific.
The regional pattern of shear-wave birefringence near the junction
indicates that trench-parallel strain follows the seismogenic Benioff
zone, but rotates to trench-normal elsewhere.
Asthenospheric mantle is inferred to flow around and beneath the
disrupted slab edge, and may influence the shallowing dip of the
Benioff zone at the Aleutian junction.
Back to Lees Publications
Data are presented to show that slab
melt-related volcanism (adakitic volcanism) is occurring in the
Aleutian-Kamchatka junction, and in the western Aleutians where the
plate boundary switches from convergent to strike-slip motion. The
subducting lithosphere in these areas is relatively old (>40Ma), and
is therefore not expected to melt during subduction (i.e., Defant and
Drummond, 1990). It is argued that the slab is melting in these areas
because the side edge of the down-going plate is warmed by
asthenospheric flow on three sides. The absence of deep or
intermediate-depth seismicity in these areas supports a warm slab
interpretation. An anomalously large slab melt contribution to arc
volcanic rocks in the Aleutian-Kamchatka junction and in the western
Aleutians is interpreted to be a geochemical edge effect. The presence
of this effect in these areas is consistent with the idea that the
Pacific plate is being torn in an unzipping motion as it moves westward
from the convergent to the strike-slip portions of the Aleutian arc.
Back to Lees Publications
A new program for exploratory data analysis in
three and four dimensions is presented and described. Interactive
communication between diverse datasets is stressed as the main gestalt
of the Geotouch program. The primary kinds of data include point,
vector, raster and wireframe data sets, in addition to specialized
forms such as focal mechanisms and ellipsoidal information. The
software includes methods for cutting cross sections at arbitrary
angles, spinning objects in three space and animating time series of
punctual data, such as hypocenter series and volcano eruptions. The
program is written in POSIX compliant C using X-windows for Unix
systems and has been ported to Linux. Free access, via the Internet, to
executable binary, source code and documentation make this package an
attractive alternative to expensive or unwieldy commercial options.
Back to Lees Publications
Frequent
degassing explosions, occuring at intervals of minutes to tens
of minutes, are common at many active basaltic and andesitic volcanoes
worldwide. In August 1997, April 1998, and September 1998 we recorded
seismic and acoustic signals generated at two andesitic volcanoes with
'Strombolian-type' activity. Despite variations in explosion
frequency (5 to 15 per hour at Karymsky as opposed to 1 to 3 per hour
at Sangay), the signatures of the explosions are remarkably similar at
these two, diverse field sites. In all explosions, gas emission
begins rapidly and is correlated with an impulsive acoustic pressure
pulse. Seismic waveforms are extremely emergent and begin 1 to 2
seconds before the explosion itself. We classify explosion events at
the two volcanoes as either short-duration (less than 1 minute) simple
impulses or long-duration (up to 5 minutes) tremor events. Many
tremor events have harmonic frequency spectra and correspond to regular
1 second
acoustic pulses, often audible, that sound like chugging from a
locomotive. Chugging events are intermittent, suggesting that the
geometry or geochemistry of the process is variable over short time
scales. We attribute the 1 Hz periodic chugs to a resonant phenomena
in the upper section of conduit.
Back to Lees Publications
High resolution three-dimensional compressional
and shear wave velocity models, derived from microearthquake
traveltimes, are used to map the distribution of Poisson's ratio and
porosity at Coso Geothermal Area, Inyo County, California.
Spatial resolution of the three-dimensional Poisson's ratio and
porosity distributions is estimated to be 0.5 km horizontally and 0.8
km vertically.
Model uncertainties, \pm 1% in the interior and
\pm 2.3\% around the edge of the model, are estimated by a jackknife
method.
We use perturbations of r = V_p / V_s ratio and \Psi = V_p * V_s
product to derive distributions of Poisson's ratio, \sigma, and
porosity, which are
then used to constrain and delineate possible zones of intense heat,
fracture accumulation
and fluid saturation.
Poisson's ratio at Coso ranges from 0.15 to 0.35 with
an average of 0.224, lower than the crustal average of 0.25.
High Poisson's ratios are more extensive in shallower depths (<.5
km)
while lower Poisson's ratios are found in the deeper section (1.5--3.0
km) of the target area.
Two major features with low Poisson's ratio are identified at
geothermal
production depth (1--3 km) around stations S2-S6 and S1-S3-S4.
The two low \sigma features are separated by a northwest-southeast
trending high \sigma belt with variable width of 1 ~ 3 km.
A high \Psi body is found around S2 and S6, and extends down in depth.
A circular, low \Psi belt corresponding to the high \sigma
belt, is located around S2-S6 and
is linked to a previously reported structure in V_s tomography.
This low \Psi (highly porous) belt is probably a horizontal
conduit/reservoir of geothermal fluid.
A vertical, low \sigma and high \Psi
channel beneath triangle S1-S3-S4 corresponds to a high attenuation,
dome-like feature.
We propose an upwell-and-spread magma intrusion model for the last
major magmatism in the Coso region.
The magmatic upwelling is centered in the S1-S3-S4 area.
The model predicts potential geothermal resources to the south and
west of triangle S1-S3-S4 based on local faulting patterns.
Back to Lees Publications
A new inversion method for P-wave anisotropy
[{\em Wu and Lees}, 1999a] has been applied to high-precision,
microseismic travel-time data collected at Coso geothermal region,
California.
Direction-dependent P-wave velocity, and thus its perturbation, are
represented by a symmetric positive definite matrix A instead of a
scalar.
The resulting anisotropy distribution is used to estimate
variations in crack density, stress distribution and permeability
within the producing geothermal field.
A circular dome-like structure is observed at the southwestern part of
the geothermal region southwest of Sugarloaf Mountain.
Using a
linear stress-bulk modulus relation and deviatoric stress is estimated
to be 3-6 MPa at geothermal production depths (1-2 km), assuming all
the anisotropy is realted to stress.
The stress field is compressional NNE-SSW and dilational WNW-ESE,
coinciding with a previous, independent study using earthquake focal
mechanisms.
Following a theory on flat, elliptic cracks, residual crack density
estimated from P anisotropy is about 0.0078, assuming crack aspect
ratios >>1:60 and is ~0.041 when crack aspect ratios are close to
1:60.
Residual crack orientation distribution is related to velocity
anisotropy.
Based on the anisotropic part of crack density distribution function,
the anisotropic part of permeability distribution may be calculated
by a statistical approach via simple parallel fluid flow along cracks.
Back to Lees Publications
Two recent earthquake sequences near the
Coso geothermal field show clear
evidence of faulting along
conjugate planes. We present results from analyzing an earthquake
sequence
occurring in 1998, and compare
it with a similar sequence that occurred in 1996. The two sequences
followed mainshocks which occurred
on November 27, 1996 and March 6, 1998. Both mainshocks ruptured
approximately co-located regions of the same fault system.
Following a comparison with the background seismicity of the Coso
region,
we have detected evidence of stress loading within the geothermal field
that appears to be
in response to the
1998 earthquakes. The M_L = 5.2 mainshock in the 1998 sequence occurred
at 5:47 am UTC, and waslocated approximately 45 km north of the town of
Ridgecrest in the Coso
range.
The mainshock of the 1996 sequence had a M_L magnitude of 5.3.
There have been no observable surface ruptures associated with either
of
these sequences.
Though the mainshocks for both sequences were located about 900 m apart
and have nearly the same local magnitudes, the sequences differ in both
their temporal and spatial characteristics.
An analysis of the fault plane solutions of the mainshocks and the
aftershock locations suggests that the two sequences ruptured fault
planes which are
perpendicular to one another.
We observe a much faster temporal decay of the 1998
sequence compared to the one in 1996; moreover, while the 1996 sequence
was not followed by any sizeable (i.e., M_L > 4.0)
aftershocks, the 1998 sequence had four such events.
From an estimate of the tectonic stressing rate on the fault that
produced
the 1998 sequence,
we infer a repeat cycle of 135 years for an earthquake of comparable
magnitude at Coso.
Back to Lees Publications
We use a multiple-empirical Green's function
method to determine
source properties of small (M-0.4 to 1.3) earthquakes and
P and S-wave attenuation at the Coso Geothermal field, California.
Source properties of a previously-identified set of
clustered events from the Coso geothermal region are first analyzed
using an empirical Green's function (EGF) method. Stress drops values
of at least 0.5-1 MPa are inferred for
all of the events; in many cases, the corner frequency is outside the
usable bandwidth and can only be constrained as being higher than ~3
MPa. P- and S-wave stress drop estimates are identical to the
resolution limits of the data. These
results are indistinguishable from numerous EGF studies
of M2-5 earthquakes, suggesting a similarity in rupture
processes that extends to events that are both tiny and
induced and providing further support for Byerlee's Law. Whole-path Q
estimates for P and S waves are determined using the multiple-empirical
Green's function (MEGF) method of {\it Hough}, (1997), whereby spectra
from clusters of colocated events at a given station are inverted for a
single attenuation parameter, \kappa, with source parameters
constrained from EGF analysis. The \kappa estimates, which we infer to
be resolved
to within 0.003 sec or better, exhibit almost as much scatter as a
function
of hypocentral distance as do values from previous single-spectrum
studies for which much higher uncertainties in individual \kappa
estimates
are expected. The variability in \kappa estimates determined here
therefore suggests real lateral variability in Q structure. Although
the raypath
coverage is too sparse to yield a complete three-dimensional
attenuation
tomographic image, we invert the inferred \kappa value for
three-dimensional structure using a damped least-squares method and
the results do reveal significant lateral variability in Q structure.
The inferred attenuation variability corresponds to the heat-flow
variations
within the geothermal region. A central low-Q region corresponds well
with the central high-heat flow region; additional detailed structure
is
also suggested.
Back to Lees Publications
We present results from a local earthquake
tomographic imaging experiment
in the greater Mount Rainier area. We inverted P-wave arrival times
from
local earthquakes recorded at permanent and temporary Pacific Northwest
Seismograph Network seismographs between 1980 and 1996. We used a
method
similar to that described by Lees and Crosson [1989], modified to
incorporate the parameter separation method for decoupling the
hypocenter
and velocity problems. In the upper 7 km of the resulting model there
is
good correlation between velocity anomalies and surface geology. Many
focal mechanisms within the St. Helens seismic zone have nodal planes
parallel to the epicentral trend as well as to a north-south trending
low-velocity trough, leading us speculate that the trough represents a
zone of structural weakness in which a moderate (M 6.5-7.0) earthquake
could occur. In contrast, the western Rainier seismic zone (WRSZ) does
not correlate in any simple way with anomaly patterns or focal
mechanism
fault planes, leading us to infer that the WRSZ is less likely to
experience a moderate earthquake. A ~10 km-wide low-velocity anomaly
occurs 5 to 18 km beneath the summit of Mount Rainier, which we
interpret
to be a signal of a region composed of hot, fractured rock with
possible
small amounts of melt or fluid. No systematic velocity pattern is
observed in association with the southern Washington Cascades
conductor. A
mid-crustal anomaly parallels the Olympic-Wallowa lineament (OWL) as
well
as several other geophysical trends, indicating that the OWL may play
an
important role in regional tectonics.
Back to Lees Publications
High precision P and S wave travel times
for 2104 microearthquakes with focus
<6 km are used in a non-linear inversion to derive high-resolution
three-dimensional
compressional and shear velocity structures
at the Coso Geothermal Area in eastern California.
Block size for the inversion is 0.2 km horizontally and 0.5 km
vertically
and inversions are investigated in the upper 5 km of the geothermal
area.
Spatial resolution, calculated by synthetic modeling of a cross model
at critical locations, is estimated to be 0.35 km for Vp
and 0.5 km for Vs.
Model uncertainties are estimated by a jackknife approach and
simulation of random and associated picking errors.
Low-velocity zones for both P and S waves are identified at geothermal
production depths (1-3 km).
A large, low Vp (-6%) zone is found at depth 2-2.5 km 2 km southwest of
Sugarloaf Mountain
where high attenuation has been previously reported.
However, a general high-Vp zone is seen under Coso Hot Springs
with a slightly low Vs zone, which is characteristic of fluid
saturation.
The overall distributions of Vp and Vs perturbations do not correlate.
An isolated high-Vs (+9%) feature, about 2 km in diameter, is
unambiguously
seen 2 km
due west of Sugarloaf extending from surface to depth.
This feature is surrounded by a circular, low-Vs belt of
~1 km width.
The surrounding belt is probably the cracked, high-porosity
reservoir/conduit of geothermal fluid flow.
In the 2 km
southwest Sugarloaf
region, we found low Vp and high Vs at geothermal production depths
from 1 to 2.5 km.
Combined with attenuation results, this may represent a hot,
fluid-depleted center of magmatic activity.
Back to Lees Publications
The tradeoff between isotropy and anisotropy
in practical tomography,
which differs from the fundamental deficiency of anisotropic traveltime
tomography (Mochizuki, 1997), is shown to be ~1, i.e., their
effects are of the same order. We conclude that anisotropic
considerations may be important in velocity inversions
where ray coverage is less than optimal. On the other hand, when the
ray directional coverage is complete and balanced, effects of
anisotropy sum to zero and the isotropic part gives the result obtained
from inverting for isotropic variations of velocity alone.
Synthetic test datasets are inverted, demonstrating the
effectiveness of the new inversion approach. When ray coverage is
fairly complete, original anisotropy is well recovered, even with
random noise introduced, although anisotropy ambiguities arise where
ray coverage is limited. Random noise was found to be less important
than ray directional coverage
in anisotropic inversions.
Back to Lees Publications
During the summer of 1997, Karymsky Volcano
produced summit explosions
about six times each hour. Typical explosive episodes lasted between
30 seconds and three minutes, produced gas and ash columns several
hundred meters high, and ejected some incandescent material. To better
understand the physical source mechanisms responsible, we recorded
hundreds of explosions with a three component broad-band seismometer
and an infrasonic pressure sensor located 1650 meters from the active
vent.
Nearly every explosion is recorded as an emergent yet identical
seismic wavelet which is followed 4.15 s later by an impulsive
acoustic arrival. We interpret the signals as a near-surface gas
volume burst which fractures the vent `plug,' lowers the lithostatic
pressure within the magma column, and often induces further degassing.
When degassing continues, it is generally manifested as either a
series of regular one second `chugging' explosions, steady higher
frequency `jetting', or a hybrid combination. We believe that the
seismic signature for `chugs,' short duration harmonic tremor with
integer overtones, is the result of repeated gas volume bursts at the
vent. In contrast, seismograms for jetting are non-harmonic and
contain higher frequencies. We believe that the competing degassing
behaviors are influenced by the gas flux as well as the plug/conduit
characteristics. We propose that a plug exists due to a viscosity
gradient caused by volatile depletion in the upper conduit.
Back to Lees Publications
We have searched the Coso geothermal field for
microseismicity in seismic doublets, co-located hypocenters that appear
to have nearly
identical waveforms. Using 1085 high quality events from 1993-1994, we
identified
numerous doublets, some occurring within minutes of each other. We
subdivided hypocentral data into
spatial clusters to reduce the computational burden and evaluated
multiple cross correlation pairs, assigning
scores to each pair. As an example, one spatial cluster includes 183
events
yielding 96 high correlation (>0.6) paired events.
To isolate potential multiplets, equivalence class analysis and
cluster analysis routines were used. Among the 96 high correlation
pairs
24 equivalence classes have been isolated.
While most of these are doublets, 8 classes include
3 or more cluster members and one class include 16 members.
Relative locations were calculated
using phase shifts between corresponding events.
Detailed analysis of hypocenter relocations shows elongate, vertical
structure with apparent random temporal
variations. The multiplets do not
appear to be true repeating events; rather they are
clusters of small, nearly identically oriented
ruptures, perhaps representing swarms of fractures
activated by fluid pressure fluctuations.
Using the small volumes encompassing each multiplet, we estimate
fracture densities measure between 0.02 to 0.4 1/m
and are largest near injection wells.
Back to Lees Publications
Microseismicity in the Coso geothermal field is
spatially but not temporally related to regional seismicity extending
southeast
of the field. The spatial distribution of these events defines a
northwest-trending seismic-fracture zone, consistent with a previously
defined northwest-striking zone. The abrupt decrease of seismicity
below this fracture zone may provide seismic evidence for the existence
of a brittle and ductile
transition zone at shallow depth beneath the Coso geothermal field.
Back to Lees Publications
Coherency between seismic signals recorded in
tight
arrays is investigated by reorganizing data via waveform cluster
analysis. Multitaper spectral estimates and coherency functions
suggest that pair-wise coherency between stations located only a few
meters apart at Pinyon Flat, CA, is low above approximately 15 Hz.
This observation is examined in detail by searching for clusters of
similar waveforms using single link cluster analysis on suites of
stations located in the 1990 high frequency, 60 station Pinyon Flat,
CA, deployment. The analysis requires that noisy line spectra,
introduced by instrument noise and shallow resonances be removed prior
to analysis. Spectrum reshaping is applied
to remove
biases associated with smearing periodic signals in the frequency
domain. Cluster analysis
is performed on coherency and correlation scores derived from
multi-taper analysis of 60 signals in the surface and borehole
arrays. Cluster analysis shows that while simple pair-wise
comparisons of coherency suggests that local scattering dominates the
spectrum above 15 Hz, spatial clustering indicates that there is more
structure in arriving waveforms than is apparent in the simple
analysis. Stations that are clustered together
spatially also cluster in coherency score, at frequencies as high as
50 Hz. By re-arranging seismograms according to associated
clusters, signals along north-south and east-west arms of the
array are separable at frequencies ranging
from 5-50 Hz.
These observations have important implications for the use of high
frequency seismic spectra in determination of source and path effects
for small magnitude, local earthquake analyses.
Back to Lees Publications
A high resolution tomographic study, using
cubic B-splines
parameterization and employing a systematic approach to the choosing of
appropriate damping and smoothing parameters, provided a
three-dimensional P-wave velocity map of the Loma Prieta area. 11,977
high quality raypaths
from 844 aftershocks of the 1989 Loma Prieta earthquake were used in
the
inversion. The velocity model exhibits a low-velocity feature between
the
San Andreas and Zayante-Vergeles faults in the top 10 km of the crust.
This low-velocity feature is interpreted as a sedimentary unit exposed
to the
northwest and separated from the Salinian block by the Zayante-Vergeles
fault.
Below 10 km no consistent change is observed between the Salinian and
the Franciscan blocks. There appears to be a high correlation of
aftershock activity and localized high-velocity anomalies southeast of
the Loma Prieta main shock. Whereas this anomaly may represent brittle
rocks associated with a fault zone asperity that failed after the main
shock, there is evidence to suggest it may be a body of serpentinite.
The serpentinite exhibits high velocities and is potentially less
competent than surrounding country rock, thus providing a sector along
the fault more likely to be associated with many smaller earthquakes or
creep behavior.
Back to Lees Publications
This study investigates the use of a station influence statistic to
identify velocity model shortcomings in the earthquake hypocenter
location problem. Two groups of microearthquake events are examined.
The first is a group of 81 events from the Mount St. Helens region
which occurred between November 1987 and September 1991; the second,
110 well located events from the 1992 Joshua Tree aftershock sequence.
We describe a method for validating a postulated earth model. Let
denote the hypocenter estimates that Geiger's
method obtains.
Systematically remove each station observation from the location
problem
and recompute the location estimate. Call this estimate (i)
when the i-th station is removed. For a single event define
a station's influence (SI) as a weighted difference between lambda
and (i). Distributional summaries of SI statistics
across
events are used to identify model shortcomings: given a specified
velocity model, SI distributions which are not homogeneous across
stations provide evidence of model inadequacies and/or failures in
the weighting scheme. We show that velocity model shortcomings
detected using SI statistics for the Mount St. Helens sequence under
a one-dimensional model appear to correlate with known physical
anomalies;
while SI distributions evaluated under a 3-dimensional model are more
homogeneous and reflect a modest improvement over the 1-dimensional
model.
SI distributions provide evidence of model failure for the Joshua Tree
sequence under a 1-dimensional model, but no evidence of failure under
a 3-dimensional model. Finally, the weighting scheme's validity is
verified for the Joshua Tree sequence under the 3-dimensional model.
Back to Lees Publications
Pulse width data are used to invert for attenuation structure in the
Coso geothermal area, California.
The dataset consists of pulse width measurements of 838 microseismic
events recorded on a seismic array of 16 downhole stations
between August 1993 and March 1994.
The quality factor Q correlates well with surface geology and surface
heat flow observations.
A broad region of low Q(~30-37) is located at 0.5-1.2 km depth below
Devil's
Kitchen, Nicol Prospects and Coso Hot Springs.
A vertical, low Q(~36) in contrast with surrounding rock of 80) region
is interpreted as a channel through which hydrothermal energy
is transported from depth to the surface.
The location of the channel is between stations S1 and S4 and its
dimension is about 1 km.
At the deep end of the channel, a large, broad body of low Q is also
located at 3 km depth 2-4 km
to the southwest of Nicol Prospects and Devil's Kitchen.
Since it lies at the bottom of the target region and beyond the scope
of seismicity, further research
is needed to constrain its extent. Numerical modeling with a
pseudospectral method is also done to
investigate the applicability of the inversion scheme to fractured
regions.
A linear relationship between pulse width broadening and travel time
is upheld, and the proportional constants are estimated.
Back to Lees Publications
A new method for calculating boundary conditions
at the free surface and along absorbing boundaries of a finite grid is
presented.
A finite, twice differentiable reduction function
which achieves a 99% reduction over 3 wavelengths
is proposed and tested.
In the context of pseudospectral wave propagation, this implies
a boundary layer of at least 6 grid nodes.
The method is analyzed in one and two dimensions and the
problems of waves impinging on corners are addressed.
The reduction function recommended is
gamma_R = \alpha (1+cos(pi x))^2
where \alpha is a parameter to be determined by optimization.
Tests of the performance of the new method versus
other common schemes are presented and analyzed.
We provide a strategy for determining the optimal
parameter in the reduction function.
Synthetic Rayleigh waves are observed
at the free surface of the simulation.
Experiments with a vertical fault plane show
the presence of direct, reflected, transmitted and
head waves. The presence of head waves may be used
to analyze velocity contrasts across fault zones.
Back to Lees Publications
Three-dimensional Q-1 variations in the aftershock region of Loma
Prieta are derived by tomographic inversion. The data set consists of
over 4000 aftershock recordings at 22 PASSCAL (Program for Array
Seismic Studies of the Continental Lithosphere) stations deployed
after the Loma Prieta mainshock of 1989. Estimates of attenuation are
determined from nonlinear least squares best fits to the Fourier
amplitude spectrum of P and S wave arrivals. The linear attenuation
inversion is accomplished by using three-dimensional velocity
variations derived previously in nonlinear velocity inversions. Low Q
is observed near the surface and Q generally increases with depth.
The southwest side of the San Andreas fault exhibits lower Q than does
the northeast side and this feature apparently extends to
approximately 7 km depth. The fault zone, as determined by the
dipping plane of aftershock activity, is characterized by slightly
higher Qp and lower Qs, compared to regions immediately adjacent to
the fault. These correlate with high- velocity anomalies associated
with seismicity at depth. The results are in agreement with earlier
observations regarding the association of high-velocity anomalies,
seismicity, and fault zone asperities.
Back to Lees Publications
Tomographic inversions for velocity variations in western Washington
indicate a high correlation with surface geology and geophysical
measurements, including gravity observations. By assuming a simple
linear relationship between density and velocity (Birch's law) it is
possible to calculate the gravity field predicted from the velocity
perturbations obtained by local tomographic inversion. While the
predicted gravity matches observations in parts of the model the
overall correlation is not satisfactory. In this paper we suggest a
method of constraining the tomographic inversion to fit the gravity
observations simultaneously with the seismic travel time data. The
method is shown to work well with synthetic data in 3 dimensions where
the assumption of Birch's Law holds strictly. If the sources of the
gravity anomalies are assumed to be spatially localized, integration
can be carried out over a relatively small volume below the
observation points and sparse matrix techniques can be applied. We
have applied the constrained inversion method to western Washington
using 4,387 shallow earthquakes, to depths of 40.0 km, (36,865
raypaths) covering a 150´250 km region and found that the
gravitational constraints may be satisfied with minor effect on the
degree of misfit to the seismic data.
Back to Lees Publications
Tomographic inversion of P-wave arrival times from aftershocks of
recent 1992 Southern California earthquakes is used to produce
three-dimensional images of subsurface velocity. The preliminary 1992
dataset, augmented by the 1986 M 5.9 North Palm Springs sequence,
consists of 6458 high-quality events recorded by the permanent
regional networkÑproviding 76, 306 raypaths for inversion. The
target area consisted of a 104 ´ 104 ´ 32 km3 volume
divided
into 52 ´ 52 ´ 10 rectilinear blocks. Significant velocity
perturbations appear to correlate with rupture properties of recent
major earthquakes. Preliminary results indicate a low-velocity
anomaly separates the dynamic rupture of the M 6.5 Big Bear event from
the M 7.4 Landers mainshock; a similar low-velocity region separates
the M 6.1 Joshua Tree sequence from the Landers rupture.
High-velocity anomalies occur at or near nucleation sites of all 4
recent mainshocks (North Palm Springs- Joshua Tree-Landers-Big
Bear). A high-velocity anomaly is present along the San Andreas fault
between 5 and 12 km depth through San Gorgonio Pass; this
high-velocity area may define an asperity where stress is concentrated
and where future large earthquakes may begin.
Back to Lees Publications
An automated method for removing line spectrum elements embedded in
colored spectra is presented. Since smooth spectrum estimates are
desired, line spectra tend to smear out over an effective smoothing
window. This introduces a bias in the spectrum estimation that can
seriously degrade determination of signal-to-noise ratios, spectral
deconvolution or any other operation where spectrum shape is important
in analysis. Multi-taper analysis provides a simple algorithmic
approach to this problem and a simple method of determining where
spectral peaks are both significant and contain signal power is
suggested. While the method is completely general, an illustration of
the technique applied to seismic signals is provided. Examples
include estimation of signal-to-noise ratio at the high frequency
array at, Pinyon Flat, CA. A comparison of noise spectra line
segments and signal spectra line spectra reveals similarities
associated with instrument noise and shallow resonances that are
stimulated by incoming seismic signals. Identification and removal of
the resonances provides a better means of estimating background noise
spectrum for the purposes of modeling earthquake source spectra and
path effects associated with attenuation.
Back to Lees Publications
Tomographic inversion is applied to delay times from local
earthquakes
to image 3-D velocity variations surrounding the main rupture of the
1989 Loma Prieta earthquake. The 55x45 square km region is
represented by blocks of 1 km per side laterally and by 8 layers of
varying thickness to 18 km depth. High quality P-wave arrival times
recorded on the USGS CALNET array from 549 crustal earthquakes with
depths of 0 to 25 km were used as sources. Preliminary results
several velocity variations (5-12%) that correlate with specific
characteristics of the 1989 rupture. These include prominent
high-velocity anomalies near the mainshock hypocenter and prominent
low-velocity anomalies where the dip of the San Andreas fault appears
to change significantly. The termination of prominent low velocity
features existing primarily in the hanging wall to depths of 7-9 km,
correlates with the top of the rupture zone. High-velocity variations
along the fault dominate where aftershock activity is high. The high
velocity anomaly located at depth along the fault is interpreted as
imaging the asperity on which the Loma Prieta earthquake occurred.
Back to Lees Publications
We investigate the stability of tomographic analysis by comparing
the
results of two different methods of parameterizing the
three-dimensional P-wave velocity variations in the vicinity of the
1989 Loma Prieta earthquake. A block inversion is implemented using
55x45x10 blocks of 1 km width and varying thickness to a depth of 25
km below the surface. Linear and non-linear analysis are presented.
The non-linear analysis is achieved by iterating over
three-dimensional raytracing and earthquake relocation relative to
current three-dimensional models until solutions show only small
improvements. A second parameterization is achieved by using cubic
B-spline functions to span the space of the model which is rotated by
46.5û. Non-linear results are presented with several different
starting models illustrating the robustness of the technique to the
initial conditions. All the non-linear results produced essentially
the same final model, which was structurally the same as the model
obtained by linear analysis using a reasonable starting model.
Back to Lees Publications
High resolution, three-dimensional images of P-wave velocity
anomalies
below Mt. St. Helens, Washington, were derived using tomographic
inversion. The model is a 27.5´21´20 km target volume
parameterized by blocks .5 km per side. The area included 39 stations
and 5454 local events leading to 35,475 rays used in the inversion.
To diminish the effects of noisy data, the Laplacian was constrained
to be small within horizontal layers, providing smoothing of the
model. Non-linear effects were compensated for by iterating
three-dimensional ray tracing (using pseudo-bending) between
inversions and relocating earthquakes relative to the updated
three-dimensional model. The structural differences between the
linear and non-linear inversions appear to be insignificant, although
the amplitudes of the anomalies are larger in the non-linear models.
Results indicate a low- velocity anomaly (>7%), approximately 1 km
in
lateral extent, from 1.5 to 3 km depths. Between 3 and 6 km depth the
anomaly appears to spread out. Below 6 km depth the low velocity
feature changes to a higher velocity perturbation with lower velocity
perturbations flanking around the perimeter of the volcano. The
higher velocity material, which correlates with the higher seismicity
at that depth, is interpreted as being a plug capping the low velocity
magma chamber which begins below 9 km depth.
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Tomographic inversion is applied to delay times from local
earthquakes
to image three dimensional velocity variations in the Puget Sound
region of Western Washington. The 37,500 square km region is
represented by nearly cubic blocks of 5 km per side. P-wave arrival
time observations from 4,387 crustal earthquakes, with depths of 0 to
40 km, were used as sources producing 36,865 rays covering the target
region. A conjugate gradient method (LSQR) is used to invert the
large, sparse system of equations. To diminish the effects of noisy
data, the Laplacian is constrained to be zero within horizontal
layers, providing smoothing of the model. The resolution is estimated
by calculating impulse responses at blocks of interest and estimates
of standard errors are calculated by the jackknife statistical
procedure. Results of the inversion are correlated with some known
geologic features and independent geophysical measurements. High
P-wave velocities along the eastern flank of the Olympic Peninsula are
interpreted to reflect the subsurface extension of Crescent
terrane. Low velocities beneath the Puget Sound further to the east
are inferred to reflect thick sediment accumulations. The Crescent
terrane appears to extend beneath Puget Sound, consistent with its
interpretation as a major accretionary unit. In the southern Puget
Sound basin, high velocity anomalies at depths of 10-20 km are
interpreted as Crescent terrane and are correlated with a region of
low seismicity. Near Mt. Rainier, high velocity anomalies may reflect
buried plutons.
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Three-dimensional tomographic inversion of P-wave travel-time data
is
used to investigate the seismic velocity structure of the crust in
Thessaloniki and Chalkidiki, N. Greece. Local earthquakes recorded by
two networks operating in the area are used as natural seismic
sources. Two different target volumes, defined on the surface by
39o50' - 41o50'N and 21o25' - 24o20'E, and 40o10' - 41o 00'N and 22o
45' - 23o 50'E, are investigated. The first dataset is recorded by 13
stations and the second by 29. The size of the blocks used to
parameterize the areas is 10 x 10 km and 3 x 3 km in the horizontal,
respectively, with varying depth layering. The major seismic velocity
anomalies within the crust, obtained by the tomographic inversion, are
resolved with a horizontal spatial resolution of about 20 km and 7 km
for the first and second target volume, respectively. Our particular
interest is to illuminate velocity anomalies and more detailed
characteristics of the two main Neogene- Quaternary basins in this
region (Vardar-Axios and Struma-Strymon). These basins are identified
as low velocity features overlying relatively higher P-wave velocity
structures in the lower crust. The complex Mygdonian area reveals a
similar pattern of low-velocity basin overlying higher-velocity
basement. Overall the velocity patterns correlate well with the
location and strike of the main geological and tectonic units in the
area, as well as the basic assumptions on basin development. This
highlights the utility of local tomography to illuminate structural,
tectonic and rheological properties within the crust.
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Tomographic inversion is applied to delay times from aftershocks of
the 1986 ML 5.9 North Palm Springs (NPS) earthquake to image 3-D
velocity variations within the northern Coachella Valley. P-wave
arrival times from 1074 earthquakes, with depths ranging from 3 to 20
km, were used as sources recorded by 12 portable and 4 permanent
stations. Preliminary results show well-defined high- and low-velocity
anomalies (2-7%) that correlate with the rupture distribution of
the 1986 mainshock. At depths less than 8 km, a low-velocity anomaly
predominates between the two NE-dipping Banning and Mission Creek
faults. From 8 to 12 km, where the NPS mainshock and most of the
aftershocks occur, a high-velocity anomaly is observed. This
high-velocity feature is interpreted as imaging the asperity
responsible for the 1986 rupture; and suggests that velocity
information may help to define important elements, such as asperities,
that control fault rupture, and thus, may help to predict the location
and size of future events.
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Unzen Volcano, located in south west Japan,
erupted on November 17,
1990 after 198 years of dormancy and has been ejecting lava since May
20, 1991. In this paper, we present three-dimensional P and S-wave
velocity variations below Unzen Volcano using 22,473 P-wave, and
14,349 S-wave arrival times from 3,457 local earthquakes recorded on a
local network of 12 seismic stations. The model was parameterized by
24,000 approximately 2.0 km cubic blocks, targeting a volume of
120X100X20 km. A prominent low velocity anomaly
(greater than 4\% slowness perturbation) starting from 2.5 km to 5.0
km depth beneath the volcano is observed on both the P and S wave
inversions. Below Unzen volcano slightly lower amplitude low velocity
anomalies are further observed to a depth of 12.5 km for P and 7.5 km
for S-waves. Shallow low velocity anomalies are observed below the
Chijiwa Bay area (northwest of Unzen), and deeper anomalies are seen
below Shimabara Bay (southwest off the Shimabara Peninsula). This
northwest to southeast trending feature represents an elongated system
of dykes which supply the shallow magma reservoirs.
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A technique for improving the efficiency of
shortest path raytracing
(SPR) [Moser, 1991] is presented. We analyze situations where SPR
fails and provide quantitative measures to assess the performance of
SPR raytracing with varying numbers of nodes. Our improvements
include perturbing the ray at interfaces according to Snell's Law, and
a method to find correct rays efficiently in regions of low velocity
contrast. This approach allows the investigator to use fewer nodes in
the calculation, thereby increasing the computational efficiency. In
two dimensional cross-borehole experiments we find that with our
improvements, we need only use 2/3 as many nodes, saving up to 60\% in
time. Savings should be even greater in three dimensions. These
improvements make SPR more attractive for tomographic applications in
three dimensions.
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Tomographic inversion is applied to 17,659
P phase
observations at 21 stations from 2023 earthquakes in
the vicinity of Mount St. Helens to study the
three-dimensional velocity structure.
Block size for the inversion is 2 km horizontally and 2 km or more
vertically. Locations of hypocenters are assumed
known and are based on a reference one-dimensional, layered
velocity structure.
A conjugate gradient technique (LSQR) is used
to invert the large sparse system of equations, augmented
by regularization with a Laplacian roughening matrix.
Resolution is estimated by computing the impulse
response of the inversion for various critical
locations, and uncertainties of the estimates are
determined by a jackknife approach.
The results of the inversion show a remarkable
correlation with known geological and
geophysical features. The Spirit Lake and Spud Mt. plutons
are characterized by high-velocity regions
extending to approximately 9 km depth.
The St. Helens seismic zone, a band of diffuse seismicity
extending NNW from the volcano is evident as a
prominent low-velocity lineation.
The change in character of the velocity anomalies south of St. Helens
corresponds well with the near cessation
of seismic activity there.
A low-velocity anomaly beneath the crater from 6 to 16 km depths
may represent modern magma accumulations.
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Tomographic inversion is applied to delay
times from local earthquakes
to image three dimensional velocity variations near Parkfield,
California. The 25 ´ 20 square km region is represented by nearly
cubic blocks of 0.5 km per side. Arrival times of P waves from 551
local earthquakes, with depths of 0 to 15 km, were used as sources
producing 3135 rays covering the target region. The data were recorded
on low-noise downhole seismographs. A conjugate gradient method is
used to invert the resulting sparse system of simultaneous equations.
To diminish the effects of noisy data, the Laplacian of the model
parameters is constrained to be small within horizontal layers,
providing smoothing of the model. The resolution of the model is
estimated by calculating point spread functions at blocks of interest.
Estimates of standard errors of the model parameters are calculated by
the jackknife statistical procedure. The results of the inversion
show correlation with some of the local geological and geophysical
features. Station corrections removed the long- wavelength anomaly
associated with the contrast of the Salinian block southwest of the
San Andreas fault versus the Franciscan to the northeast. A velocity
low located a few kilometers northwest of Parkfield (depth 2.5-3.5
km), appears to lie along the gradient of the large Bouguer gravity
anomaly associated with the Parkfield syncline. The south-
southeastward extension of the low velocities may relate to
reflections observed on the Parkfield, Consortium for Continental
Reflection Profiling (COCORP) lines. We speculate on the geological
meaning of these features and interpret them either as part of the
local strike slip tectonics or a shallow crustal detachment. The
correlation of higher-velocity features and seismic activity may
indicate that earthquakes are occurring in more competent zones while
aseismic slip takes place in zones of lower-velocity, less competent
rocks.
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There is a high correlation between the P and
S-wave inversions. Between 5-15 km depth both 3D images indicate low
velocity anomalies trending northward in the west along the Fujikawa,
from Suruga Bay to northwest of Mt. Fuji. North of Izu Peninsula there
is a broad high velocity zone. These results correlate with Bouguer
gravity anomalies previously observed. Starting at 5 km depth, a high
velocity, northeast dipping anomaly beneath Sagami Bay is inferred to
be the shallow subducting PHS plate. To the north, a high velocity zone
in the vicinity of the East Yamanashi seismic swarm (15-32 km depth) is
clearly observed on the P-wave inversion and to a lesser extent on the
S-wave inversion. Because of the high seismicity in this region, we
speculate this is the zone where the PHS plate is grazing the EUR plate
to the northwest, and the high velocity anomaly represents a large
scale asperity. To the west, below the Mt. Fuji area, a broad low
velocity is observed between 15-32 km depth. We interpret the deep, low
velocity structure, which exhibits little seismicity, to be either the
subduction of the volcanic arc or a region of elevated temperature
associated with hot rising magma. Near the swarm area on the eastern
edge of Izu Peninsula, low velocity anomalies are observed near the
surface on both P and S wave inversions. Along the eastern part of Izu
Peninsula a prominent low velocity anomaly is observed between 10-20 km
on the S-wave image but is less pronounced on the P-wave inversion.
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We compare two approaches for solving the
large, sparse
linear systems that arise in tomographic velocity
inversion problems.
When noise is present in the data, the system typically
is inconsistent and quasi-overdetermined, and
some form of regularization must be implemented
to avoid the strong, undesired influence
of small singular values dominating the solutions.
First, a Bayesian ART (Algebraic Reconstruction Technique)
algorithm is applied
to the system where we solve for a set of model
parameters and residuals simultaneously.
Careful choice of relaxation parameters and smoothing filters
insures convergence and acceptable results.
This approach avoids the undesirable effects of implicit row weighting
inherent in simple ART or SIRT (Simultaneous Iterative Reconstruction
Technique) applications.
Second, a conjugate gradient approach is implemented via
algorithm LSQR where regularization is achieved
by augmenting the system with additional constraint equations
which minimize the roughness of the model.
Specifically, applied to a 3-dimensional tomographic inversion
we constrain the second derivative (the Laplacian) to be zero
within horizontal layers.
Comparison on synthetic data reveals that these techniques
produce nearly equivalent results.
By applying these methods to
local earthquake data in the vicinity of Mount St. Helens, Washington,
we have produced a 3-dimensional, laterally varying velocity
structure in the top 40 km of the crust which correlates
well with known geological and geophysical features
and delineates possible accumulation of magma
beneath the crater.
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Microseismicity in the Coso geothermal field
has been searched for seismic doublets, hypocenters co-located
that appear to have identical waveforms. Using 1085 high quality events
from 1993-1994,
numerous doublets were identified, some occurring within minutes of
each other. Hypocentral data were subdivided into
spatial clusters to reduce the computational burden and
multiple cross correlation pairs were evaluated, assigning
scores to each pair. In one spatial cluster including 183 events
(16471 pairs) yielded 96 high correlation (>0.6) paired events.
Equivalence class analysis and
cluster analysis routines were used to isolate potential multiplets.
Among the 96 high correlation pairs
24 equivalence classes were isolated.
While most of these are doublets, 8 classes included
3 or more cluster members and one class included 16 members.
Relative locations are calculated
using phase shifts between corresponding events.
Detailed analysis of hypocenter relocations shows elongate, vertical
structure aligned along
ambient stress fields as measured by conventional focal mechanism
analysis.
Multiplet hypocenters are generally oriented sub-parallel to faults
observed at the surface.
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R is a free software for statical computing and graphics. It compiles and runs
not only on UNIX platforms but MS Windows. The R commands are easy and offer
interactive help. R is used in extensive field by implementing packages. RSEIS,
the package of R, enables us to do easy graphical processing of seismic data. Here we
illustrate by showing an example of seismic data processing using RSEIS.
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Although widely used for preparation of geographic maps in the field of earth
sciences, the Generic Mapping Tools(GMT) software is difficult for users to understand,
and does not work well with Microsoft (MS) Window PC. By utilizing R
package, GEOmap, we can do mapping work at MS window PC with commands easier
than those of GMT.
In addition, the R commands provide interactive help. Here we briefly introduce a few
features of GEOmap, and illustrate a procedure for preparing topographic maps
as an example.
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Prof. Jonathan M. Lees
Department of Geological Sciences
CB #3315, 104 South Road
University of North Carolina
Chapel Hill, NC 27599-3315
(919) 962-0678
FAX (919) 966-4519