Recent SCEC Published Research
The following list of recently published
papers are based on research sponsored by SCEC. These papers
are NOT available from SCEC. Most of the journals containing
these papers are available at university libraries, and authors
may also have reprints of their papers available by request.
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Other SCEC research papers are listed in an online
List of publications
List of publications
Recent Research as of
(SCEC Contribution numbers are in bold)
J.L., S.M. Day, H. Magistrale, R. Castro, L. Astiz, C. Rebollar,
J. Eakins, F.L. Vernon, and J.N. Brune, Crustal thickness of
the Peninsular Ranges and Gulf Extensional Province in the Californias,
Journal of Geophysical Research, 106, pp. 13599-13611,
Previous crustal thickness studies in southern
California, U.S.A., indicate a steeply dipping Moho beneath the
topographically high eastern Peninsular Ranges. We continue these
related studies to the south in northern Baja California, México,
using an 11-station array that crosses the Baja California peninsula
and Gulf of California at ~31ºN. Moho Ps conversions, identified
in teleseismic receiver functions, are used to estimate the crustal
thicknesses beneath these stations. Depth to the Moho is ~33
km near the Pacific coast of Baja California and increases gradually
toward the east, reaching a maximum depth of ~43 km beneath the
western part of the Peninsular Ranges batholith. The crust then
thins rapidly under the topographically high eastern Peninsular
Ranges and across the Main Gulf Escarpment. Crustal thickness
is ~15-18 km within and on the margins of the Gulf of California.
The Moho shallowing beneath the eastern Peninsular Ranges represents
an average apparent westward dip of ~25º. This range of
Moho depths within the Peninsula Ranges, as well as the sharp
~east-west gradient in depth in the eastern part of the range,
are in agreement with the earlier observations from north of
the international border. The Moho depth variations do not correlate
with topography of the eastern batholith. These findings suggest
that a steeply dipping Moho is a regional feature beneath the
eastern Peninsular Ranges, and that a local Airy crustal root
does not support the highest elevations. We suggest that Moho
shallowing under the eastern Peninsular Ranges reflects extensional
deformation of the lower crust in response to adjacent rifting
of the Gulf Extensional Province that commenced in the late Cenozoic.
Support of the eastern Peninsular Ranges topography may be achieved
through a combination of flexural support and lateral density
variations in the crust and/or upper mantle.
530. Olsen, K.B., Three-dimensional
ground motion simulations for large earthquakes on the San Andreas
fault with dynamic and observational constraints, Journal
of Computational Acoustics, 9, no. 3, pp. 1203-1215,
I have simulated 0-0.5 Hz viscoelastic
ground motion in Los Angeles from M 7.5 earthquakes on the San
Andreas fault using a fourth-order staggered-grid finite-difference
method. Two scenarios are considered: (a) a southeast propagating
and (b) a northwest propagating rupture along a 170-km long stretch
the fault near Los Angeles in a 3D velocity model.
The scenarios use variable slip and rise time distributions inferred
from the kinematic inversion results for the 1992 M 7.3 Landers,
The rough static slip distribution used in this study, unlike
that modeled in a recent study, is in agreement with constraints
provided by rupture dynamics. I find peak ground velocities for
(a) and (b) of 49 cm/s and 67 cm/s, respectively, near the fault.
The near-fault peak motions for scenario (a) are smaller compared
to previous estimates from 3D modeling for both rough and smooth
faults. The lower near-fault peak motions are in closer agreements
with constraints from precarious rocks located near the fault.
Peak velocities in Los Angeles are about 30% larger for (b) (45
cm/s) compared to those for (a) (35 cm/s).
541. Hardebeck, J. L. and Hauksson,
E., The crustal stress field in southern California and its implications
for fault mechanics, Journal of Geophysical Research,
106, pp. 21859-21882, 2001.
We present a new, high spatial resolution
image of stress orientation in southern California based on the
inversion of earthquake focal mechanisms. We use this image to
study the mechanics of faulting in the plate boundary region.
The stress field contains significant spatial heterogeneity,
which in some cases appears to be a result of the complexity
of faulting, and in other cases, a cause. Temporal changes in
the stress field are also observed, primarily related to major
earthquakes. The observed 15 degree rotation of the stress axes
due to the 1992 M7.3 Landers mainshock implies that the deviatoric
stress magnitude in the crust is low, on the order of 10 MPa.
This suggests that active faults in southern California are weak.
The maximum principal stress axis near the San Andreas Fault
is often at 50 degrees to the fault strike, indicating that the
shear stress on the fault is comparable to the deviatoric stress.
The San Andreas in southern California may therefore be a weak
fault in a low-strength crust.
554. Peyrat, S., K.B. Olsen, and R. Madariaga, Dynamic
modeling of the 1992 Landers earthquake, Journal of Geophysical
Research, 106, B11, pp. 26467-26482, 2001.
We have modeled dynamic rupture propagation
for the 1992 Landers earthquake using a fourth-order finite-difference
method (Madariaga et al., 1998). In this method, the dynamic
rupture grows spontaneously under the simultaneous control of
the initial load and rupture resistance by friction, modeled
using a simple slip weakening law. We used a simplified Landers
fault model where the three segments were combined into one vertical,
planar fault. By trial-and-error we modified the initial stress
field (Olsen et al., 1997) until the dynamic rupture generated
a rupture history and final slip distribution that approximately
fit those determined by the kinematic inversions by (Wald and
Heaton, 1994; Cohee and Beroza, 1994; and Cotton and Campillo,
1995). We find that rupture propagation is extremely sensitive
to small changes in the distribution of prestress and that energy
release rate controls the average rupture speed. Clearly, dynamic
models have many more degrees of freedom than kinematic inversions
where the rupture velocity is typically heavily constrained.
Our results suggest that, to some extent, the slip distribution
and rupture velocity obtained by kinematic inversions reflect
the initial stress distribution and rupture resistance, respectively.
In order to validate our model we generated synthetic 0.5 Hz
accelerograms from our dynamic simulation with the Green's functions
propagation method (Bouchon, 1981; Kennett and Kerry, 1979).
This method enables us to propagate the radiation generated by
the dynamic rupture to distances greater than those feasible
using the finite-difference method. We find a satisfactory fit
between our synthetics and the strong motion data.
556. Ben-Zion, Y., Dynamic rupture in recent models of
earthquake faults, J. Mech. Phys. Solids, 49, pp.
We discuss several problems of dynamic
rupture relevant to mechanics of earthquake faults, material
sciences, and physics of spatially extended dissipative systems.
The problems include dynamic rupture along an interface separating
different elastic solids, dynamic rupture on a planar surface
governed by strongly velocity-weakening friction, and elastodynamic
calculations of long deformation history on a smooth fault in
an elastic continuum. These separate problems share a number
of methodological and conceptual issues that form recurring themes
in the paper. An important methodological issue for computational
schemes is dependency of numerical results on the used grid size.
This arises inevitably in computer simulations when the assumed
constitutive laws do not include a length scale (e.g., of shear
or extensional displacement) over which material properties evolve.
Such simulations do not have a stable underlying solution, to
which they may converge with sufficient grid refinement. However,
they may provide rough approximations -- lacking at present a
rigorous foundation -- to the behavior of systems containing
elements of discreteness (associated with abrupt fluctuations)
at scales relevant to observations of interest. Related important
conceptual issues are connections between, or when appropriate
separation of, small scale phenomena (e.g., nucleation of rupture,
processes at rupture front) and large scale features of the response
(e.g., overall space-time dimensions of rupture, statistics of
many events). Additional recurring conceptual topics are crack
vs. pulse modes of dynamic rupture, the stress under which earthquake
faults slip, and the origin of spatio-temporal complexities of
earthquakes. These seemingly different issues probably have one
or more common origins. Dynamic rupture on an interface between
different solids, strongly velocity-weakening friction on a homogeneous
fault, and strong fault zone heterogeneities can all produce
narrow self-healing slip pulses with low dynamic stress (and
low associated frictional heat) during the active part of slip.
Strong fault heterogeneities probably play the dominant role
in producing the observed earthquake complexities. Improved understanding
of the discussed problems will require establishing connections
between discrete and continuum descriptions of mechanical failure
processes, generalization of current models to realistic three-dimensional
dynamic models, and high-resolution laboratory and in-situ observations
over broad scales of space and time. These challenging problems
provide by their subject matter and involved great difficulties
important targets for multi-disciplinary research by engineers,
earth scientists, and physicists.
578. Nikolaidis, R.M., Y. Bock,
P.J. de Jonge, P. Shearer, D.C. Agnew, and M. Van Domselaar,
Seismic wave observations with the global positioning system,
Journal of Geophysical Research, 106, pp. 21897-21916,
We describe the direct measurement of ground
displacement caused by the Hector Mine earthquake in southern
California (Mw 7.1, October 16, 1999). We use a new method of
instantaneous positioning, which estimates site coordinates from
only a single epoch of Global Positioning System (GPS) data,
to measure dynamic as well as static displacements at 24 stations
of the Southern California Integrated GPS Network (SCIGN), with
epicentral distances from 50 to 200 km. For sites outside the
Los Angeles basin, the observed displacements are well predicted
by an elastic half-space model with a point shear dislocation;
within the sedimentary basin, we observe large displacements
with amplitudes up to several cm that last as long as 3-4 minutes.
Since we resolve the GPS phase ambiguities and determine site
coordinates independently at each epoch, the GPS data rate is
the same as the receiver sampling rate. For the SCIGN data, this
is 0.033Hz (once per 30 seconds), though sample rates up to 2
Hz are possible with the SCIGN receivers. Since the GPS phase
data are largely uncorrelated at 1 s, a higher sampling rate
would offer improved temporal resolution of ground displacement,
so that, in combination with inertial seismic data, instantaneous
GPS positioning would significantly increase the observable frequency
band for strong ground motions.
H., J. Schmittbuhl, J. R. Rice, and M. Cocco, Frictional response
induced by time-dependent fluctuations of the normal loading,
Journal of Geophysical Research, 106, no. 7, pp.
We study the effect of time variable normal
stress perturbations on a creeping fault which satisfies a velocity-weakening
rate-and state-dependent friction law and is slipping at constant
speed. We use the spring block model and include the effect of
inertia. To account for the variable normal stress, we use the
description introduced by Linker, 1992, which links normal stress
fluctuations to changes of the state variable. We consider periodic
perturbations of the normal stress in time (as caused for instance
by tides) and compare the behavior for two commonly used friction
laws (the "slip" and the "ageing" laws).
Their mechanical response is shown to be significantly different
for normal stress fluctuations. It could be used to probe these
two laws during laboratory friction experiments. We show that
there is a resonance phenomenon, involving strong amplification
of the shear and velocity response of the interface, when the
spring stiffness is modestly above its critical value (or when,
at a given stiffness, the normal stress is modestly below its
critical value). We show that such an amplification is also observed
when periodic fluctuations of the shear loading are considered,
making the resonance phenomenon a general feature of the response
of a near-critical creeping surface to periodic fluctuations
of the external loading. Analytical solutions are based on a
linear expansion for low amplitude of normal or shear stress
variations and are in very good agreement with numerical solutions.
A method to get the evolution of friction in the case of an arbitrary
perturbation of the normal stress is also presented. The results
show that a creeping fault may be destabilized and enter a stick-slip
regime owing to small normal stress oscillations. This may also
account for a mechanism for the generation of "creep bursts".
However, these phenomena require very specific parameter ranges,
to excite the resonance, which may not be met very generally
in nature. This study illustrates the importance of the normal
stress fluctuations on stable sliding and suggests further friction
P. and K.B. Olsen, Fault zone amplified waves as a possible seismic
hazard along the Calaveras fault in central California, Geophysical
Research Letters, 28, pp. 2533-2536, 2001.
The Calaveras fault is situated within
a low velocity zone (LVZ) 1-2 km wide near Gilroy, California.
Strong motion accelerographs G06, located in the LVZ 1.2 km from
the Calaveras fault, and G07, 4 km from G06, recorded both the
M 6.2 1984 Morgan Hill and the M 6.9 1989 Loma Prieta earthquakes.
Comparison of the ground motions shows that a large 0.6-1.0 Hz
velocity pulse observed at G06 during the Morgan Hill event may
be amplified by focusing caused by the LVZ. Such amplified waves
might be a mappable seismic hazard, and the zone of increased
hazard can extend as much as 1.2 km from the surface trace of
the fault. The discrepancy between the inferred wide LVZ on the
Calaveras and the narrow LVZs inferred on other faults might
be actual, making the Calaveras an unusual fault zone, or might
result from systematic differences of observational technique.
The observed zone of ground motion amplification is not contained
within the Special Studies Zone defined at this location.
Y. and C. Chen, Fault rupture process of the September 20, 1999
Chi Chi, Taiwan earthquake, Bulletin of the Seismological
Society of America, 91, pp. 1088-1098, 2001.
We modeled the source rupture process of
the September 20, 1999 Chi Chi, Taiwan earthquake (Mw7.6) using
an integrated dataset of near-field strong ground motion and
GPS observations. This large inland thrust earthquake produced
more than 80 km long surface break along the Chelungpu fault.
The event triggered over seven hundred strong motion instruments
on the Island of Taiwan. This large ground motion dataset provides
the best opportunity to date to understand earthquake source
processes in a major earthquake. First, we develop a 3-D curved
fault model consistent with the surface rupture data and aftershock
distribution. The subsurface fault plane has an average dip of
30 degrees to the east, and measures 96 km along strike and 40
km down-dip. Next we applied a genetic algorithm to reconstruct
the source rupture process. The inversion results indicate a
complex slip distribution with a generally broad triangular shaped
slip zone that points down-dip. Most of the displacement occurred
over a 15 km depth range in the northern segment of the fault,
with the maximum slip exceeding 20 m. The average slip is 3.8
m and the average rupture velocity is 2.6 km/sec. The mainshock
slip distribution clearly complements the aftershock location
distribution. Slip rise times range from 8 to 6 seconds over
most of the primary slip area and decrease slightly as rupture
propagates south to north. This relatively long rise time resulted
in a low dynamic stress drop and low peak ground accelerations
observed at surface stations. The rake angle rotates across the
rupture plane from predominantly dip-slip in the southern segment
to mostly left-lateral oblique slip in the north. We find that
geometrical irregularities over the fault plane played an important
role in controlling the temporal rupture behavior. At fault bends,
rupture tends to decelerate, the rake angle rotates to a near
dip-slip orientation and the shear stress increases as rupture
tries to overcome the bending barriers. The total seismic moment
from our inversion is 2.9x1027 dyne-cm, in good agreement with
the Harvard and USGS moment estimation.
Y., Viscoelastic stress-triggering of the 1999 Hector Mine earthquake
by the 1992 Landers Earthquake, Geophysical Research Letters,
28, pp. 3007-3010, 2001.
The 1999 Mw 7.1 Hector Mine earthquake
occurred only 20 km from the 1992 Mw 7.3 Landers earthquake fault.
The close spacing between the two earthquakes has lead to wide
speculation that the two events are related. The main question
is whether the Landers earthquake triggered the Hector Mine event.
The condition under which faults break is usually characterized
by the Coulomb failure criterion. A stress transfer calculation
by scientists from the USGS, SCEC and CDMG (2000) using an elastic
half-space model has shown, however, a negative Coulomb stress
change of -1.4 to -0.6 bar at the Hector Mine hypocenter due
to Landers. A negative Coulomb stress change would be inconsistent
with the hypothesis of triggering by static stress, suggesting
that other physical mechanisms have controlled the process of
the stress evolution after the Landers earthquake. In this paper,
I show evidence of stress triggering of the Hector Mine earthquake
by Landers, due to a process governed by viscoelastic flow in
the lower crust. This visoelastic flow has produced broad-scale
postseismic rebound observed by GPS and InSAR measurements (Deng
et al., 1998). The result of this study is that viscoelastic
flow has significantly modified the regional stress field in
the Mojave Desert after the Landers earthquake. The evolving
stress field, including viscoelastic flow, has gradually moved
the Coulomb stress change at the Hector Mine hypocenter to a
positive level. The increase in Coulomb stress exceeded 1 bar
right before the Hector Mine earthquake, bringing the Hector
Mine ruptures to the proximity of catastrophic failure.