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Tymofyeyeva, E, Fialko Y.  2018.  Geodetic evidence for a blind fault segment at the southern end of the San Jacinto Fault Zone. Journal of Geophysical Research-Solid Earth. 123:878-891.   10.1002/2017jb014477   AbstractWebsite

The San Jacinto Fault (SJF) splits into several active branches southeast of Anza, including the Clark fault and the Coyote Creek fault. The Clark fault, originally believed to terminate at the southern tip of the Santa Rosa Mountains, was suggested to extend further to the southeast to a junction with the Superstition Hills fault based on space geodetic observations and geologic mapping. We present new interferometric synthetic aperture radar and GPS data that confirm high deformation rates along the southeastern extent of the Clark fault. We derive maps of horizontal and vertical average velocities by combining data from the ascending and descending satellite orbits with an additional constraint provided by the azimuth of the horizontal component of secular velocities from GPS data. The resulting high-resolution surface velocities are differentiated to obtain a map of maximum shear strain rate. Joint inversions of InSAR and GPS data suggest that the hypothesized blind segment of the Clark fault and the Coyote Creek fault have slip rates of 13 3mm/yr and 5 4mm/yr, respectively. The blind southern segment of the Clark fault thus appears to be the main active strand of the SJF, posing a currently unrecognized seismic hazard.

Tymofyeyeva, E, Fialko Y.  2015.  Mitigation of atmospheric phase delays in InSAR data, with application to the eastern California shear zone. Journal of Geophysical Research-Solid Earth. 120:5952-5963.   10.1002/2015jb011886   AbstractWebsite

We present a method for estimating radar phase delays due to propagation through the troposphere and the ionosphere based on the averaging of redundant interferograms that share a common scene. Estimated atmospheric contributions can then be subtracted from the radar interferograms to improve measurements of surface deformation. Inversions using synthetic data demonstrate that this procedure can considerably reduce scatter in the time series of the line-of-sight displacements. We demonstrate the feasibility of this method by comparing the interferometric synthetic aperture radar (InSAR) time series derived from ERS-1/2 and Envisat data to continuous Global Positioning System data from eastern California. We also present results from several sites in the eastern California shear zone where anomalous deformation has been reported by previous studies, including the Blackwater fault, the Hunter Mountain fault, and the Coso geothermal plant.

Trugman, DT, Shearer PM, Borsa AA, Fialko Y.  2016.  A comparison of long-term changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields. Journal of Geophysical Research-Solid Earth. 121:225-247.   10.1002/2015jb012510   AbstractWebsite

Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.

Tong, XP, Sandwell DT, Fialko Y.  2010.  Coseismic slip model of the 2008 Wenchuan earthquake derived from joint inversion of interferometric synthetic aperture radar, GPS, and field data. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006625   AbstractWebsite

We derived a coseismic slip model for the M(w) 7.9 2008 Wenchuan earthquake on the basis of radar line-of-sight displacements from ALOS interferograms, GPS vectors, and geological field data. Available interferometric synthetic aperture radar (InSAR) data provided a nearly complete coverage of the surface deformation along both ascending (fine beam mode) and descending orbits (ScanSAR to ScanSAR mode). The earthquake was modeled using four subfaults with variable geometry and dip to capture the simultaneous rupture of both the Beichuan fault and the Pengguan fault. Our model misfits show that the InSAR and GPS data are highly compatible; the combined inversion yields a 93% variance reduction. The best fit model has fault planes that rotate from shallow dip in the south (35 degrees) to nearly vertical dip toward the north (70 degrees). Our rupture model is complex with variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<13 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike slip as it propagated northeast (with maximum offsets of 7 m). Our model suggests that most of the moment release was limited to the shallow part of the crust (depth less than 10 km). We did not find any "shallow slip deficit" in the slip depth distribution of this mixed mechanism earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically.

Takeuchi, CS, Fialko Y.  2012.  Dynamic models of interseismic deformation and stress transfer from plate motion to continental transform faults. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb009056   AbstractWebsite

We present numerical models of earthquake cycles on a strike-slip fault that incorporate laboratory-derived power law rheologies with Arrhenius temperature dependence, viscous dissipation, conductive heat transfer, and far-field loading due to relative plate motion. We use these models to explore the evolution of stress, strain, and thermal regime on "geologic" timescales (similar to 10(6)-10(7) years), as well as on timescales of the order of the earthquake recurrence (similar to 10(2) years). Strain localization in the viscoelastic medium results from thermomechanical coupling and power law dependence of strain rate on stress. For conditions corresponding to the San Andreas fault (SAF), the predicted width of the shear zone in the lower crust is similar to 3-5 km; this shear zone accommodates more than 50% of the far-field plate motion. Coupled thermomechanical models predict a single-layer lithosphere in case of "dry" composition of the lower crust and upper mantle, and a "jelly sandwich" lithosphere in case of "wet" composition. Deviatoric stress in the lithosphere in our models is relatively insensitive to the water content, the far-field loading rate, and the fault strength and is of the order of 10(2) MPa. Thermomechanical coupling gives rise to an inverse correlation between the fault slip rate and the ductile strength of the lithosphere. We show that our models are broadly consistent with geodetic and heat flow constrains from the SAF in Northern California. Models suggest that the regionally elevated heat flow around the SAF may be at least in part due to viscous dissipation in the ductile part of the lithosphere.

Takeuchi, CS, Fialko Y.  2013.  On the effects of thermally weakened ductile shear zones on postseismic deformation. Journal of Geophysical Research-Solid Earth. 118:6295-6310.   10.1002/2013jb010215   AbstractWebsite

We present three-dimensional (3-D) numerical models of postseismic deformation following repeated earthquakes on a vertical strike-slip fault. Our models use linear Maxwell, Burgers, and temperature-dependent power law rheology for the lower crust and upper mantle. We consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate-strengthening friction on a deep aseismic fault root. We find that the thermally activated shear zones have little effect on postseismic relaxation. In particular, the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. Stronger rheologies can give rise to an opposite polarity of vertical displacements, but the amplitude of the predicted transient deformation is generally negligible. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. We also investigate the discriminating power of models incorporating Burgers and power law rheology. These rheologies were proposed to explain postseismic transients following large (M7) earthquakes in the Mojave desert, Eastern California. Numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high-quality geodetic observations for observation periods less than a decade. Longer observations, however, may potentially allow discrimination between the competing models, as illustrated by the model comparisons with available GPS and interferometric synthetic aperture radar data.