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Wei, M, Sandwell D, Fialko Y, Bilham R.  2011.  Slip on faults in the Imperial Valley triggered by the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed by InSAR. Geophysical Research Letters. 38   10.1029/2010gl045235   AbstractWebsite

Radar interferometry (InSAR), field measurements and creepmeters reveal surface slip on multiple faults in the Imperial Valley triggered by the main shock of the 4 April 2010 El Mayor-Cucapah M(w) 7.2 earthquake. Co-seismic offsets occurred on the San Andreas, Superstition Hills, Imperial, Elmore Ranch, Wienert, Coyote Creek, Elsinore, Yuha, and several minor faults near the town of Ocotillo at the northern end of the mainshock rupture. We documented right-lateral slip (<40 mm) on northwest-striking faults and left-lateral slip (<40 mm) on southwest-striking faults. Slip occurred on 15-km- and 20-km-long segments of the San Andreas Fault in the Mecca Hills (<= 50 mm) and Durmid Hill (<= 10 mm) respectively, and on 25 km of the Superstition Hills Fault (<= 37 mm). Field measurements of slip on the Superstition Hills Fault agree with InSAR and creepmeter measurements to within a few millimeters. Dislocation models of the InSAR data from the Superstition Hills Fault confirm that creep in this sequence, as in previous slip events, is confined to shallow depths (<3 km). Citation: Wei, M., D. Sandwell, Y. Fialko, and R. Bilham (2011), Slip on faults in the Imperial Valley triggered by the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed by InSAR, Geophys. Res. Lett., 38, L01308, doi:10.1029/2010GL045235.

Wei, M, Sandwell D, Fialko Y.  2009.  A silent M-w 4.7 slip event of October 2006 on the Superstition Hills fault, southern California. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006135   AbstractWebsite

During October 2006, the 20-km-long Superstition Hills fault (SHF) in the Salton Trough, southern California, slipped aseismically, producing a maximum offset of 27 mm, as recorded by a creepmeter. We investigate this creep event as well as the spatial and temporal variations in slip history since 1992 using ERS-1/2 and Envisat satellite data. During a 15-year period, steady creep is punctuated by at least three events. The first two events were dynamically triggered by the 1992 Landers and 1999 Hector Mine earthquakes. In contrast, there is no obvious triggering mechanism for the October 2006 event. Field measurements of fault offset after the 1999 and 2006 events are in good agreement with the interferometric synthetic aperture radar data indicating that creep occurred along the 20-km-long fault above 4 km depth, with most of the slip occurring at the surface. The moment released during this event is equivalent to a M-w 4.7 earthquake. This event produced no detectable aftershocks and was not recorded by the continuous GPS stations that were 9 km away. Modeling of the long-term creep from 1992 to 2007 creep using stacked ERS-1/2 interferograms also shows a maximum creep depth of 2-4 km, with slip tapering with depth. Considering that the sediment thickness varies between 3 km and 5 km along the SHF, our results are consistent with previous studies suggesting that shallow creep is controlled by sediment depth, perhaps due to high pore pressures in the unconsolidated sediments.

Wang, K, Fialko Y.  2018.  Observations and modeling of coseismic and postseismic deformation due to the 2015 M-w 7.8 Gorkha (Nepal) earthquake. Journal of Geophysical Research-Solid Earth. 123:761-779.   10.1002/2017jb014620   AbstractWebsite

We use space geodetic data to investigate coseismic and postseismic deformation due to the 2015 M-w 7.8 Gorkha earthquake that occurred along the central Himalayan arc. Because the earthquake area is characterized by strong variations in surface relief and material properties, we developed finite element models that explicitly account for topography and 3-D elastic structure. We computed the line-of-sight displacement histories from three tracks of the Sentinel-1A/B Interferometric Synthetic Aperture Radar (InSAR) satellites, using persistent scatter method. InSAR observations reveal an uplift of up to approximate to 70mm over approximate to 20months after the main shock, concentrated primarily at the downdip edge of the ruptured asperity. GPS observations also show uplift, as well as southward movement in the epicentral area, qualitatively similar to the coseismic deformation pattern. Kinematic inversions of GPS and InSAR data and forward models of stress-driven creep suggest that the observed postseismic transient is dominated by afterslip on a downdip extension of the seismic rupture. A poroelastic rebound may have contributed to the observed uplift and southward motion, but the predicted surface displacements are small. We also tested a wide range of viscoelastic relaxation models, including 1-D and 3-D variations in the viscosity structure. Models of a low-viscosity channel previously invoked to explain the long-term uplift and variations in topography at the plateau margins predict opposite signs of horizontal and vertical displacements compared to those observed. Our results do not preclude a possibility of deep-seated viscoelastic response beneath southern Tibet with a characteristic relaxation time greater than the observation period (2years).

Wang, K, Fialko Y.  2015.  Slip model of the 2015 M-w 7.8 Gorkha (Nepal) earthquake from inversions of ALOS-2 and GPS data. Geophysical Research Letters. 42:7452-7458.   10.1002/2015gl065201   AbstractWebsite

We use surface deformation measurements including Interferometric Synthetic Aperture Radar data acquired by the ALOS-2 mission of the Japanese Aerospace Exploration Agency and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 M-w 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the Main Frontal Thrust (MFT) fault between 84.34 degrees E and 86.19 degrees E, the best fitting model suggests a dip angle of 7 degrees. The moment calculated from the slip model is 6.08 x 10(20)Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150km long zone 50 to 100km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of approximate to 5.8m at a depth of approximate to 8km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar-Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the east.

Wang, K, Fialko Y.  2014.  Space geodetic observations and models of postseismic deformation due to the 2005 M7.6 Kashmir (Pakistan) earthquake. Journal of Geophysical Research-Solid Earth. 119:7306-7318.   10.1002/2014jb011122   AbstractWebsite

We use the L-band Advanced Land Observing Satellite (ALOS) and C-band Envisat interferometric synthetic aperture data and campaign GPS observations to study the postseismic deformation due to the 2005 magnitude 7.6 Kashmir (Pakistan) earthquake that occurred in the northwestern Himalaya. Envisat data are available from both the descending and ascending orbits and span a time period of similar to 4.5years immediately following the earthquake (2005-2010), with nearly monthly acquisitions. However, the Envisat data are highly decorrelated due to high topography and snow cover. ALOS data are available from the ascending orbit and span a time period of similar to 2.5years between 2007 and 2009, over which they remain reasonably well correlated. We derive the mean line-of-sight (LOS) postseismic velocity maps in the epicentral area of the Kashmir earthquake using persistent scatterer method for Envisat data and selective stacking for ALOS data. LOS velocities from all data sets indicate an uplift (decrease in radar range), primarily in the hanging wall of the earthquake rupture over the entire period of synthetic aperture radar observations (2005-2010). Models of poroelastic relaxation predict uplift of both the footwall and the hanging wall, while models of viscoelastic relaxation below the brittle-ductile transition predict subsidence (increase in radar range) in both the footwall and the hanging wall. Therefore, the observed pattern of surface velocities indicates that the early several years of postseismic deformation were dominated by afterslip on the fault plane, possibly with a minor contribution from poroelastic rebound. Kinematic inversions of interferometric synthetic aperture radar and GPS data confirm that the observed deformation is consistent with afterslip, primarily downdip of the seismic asperity. To place constraints on the effective viscosity of the ductile substrate in the study area, we subtract the surface deformation predicted by stress-driven afterslip model from the mean LOS velocities and compare the residuals to models of viscoelastic relaxation for a range of assumed viscosities. We show that in order to prevent surface subsidence, the effective viscosity has to be greater than 10(19)Pas. ations are negligible

Wang, K, Xu XH, Fialko Y.  2017.  Improving Burst Alignment in TOPS Interferometry With Bivariate Enhanced Spectral Diversity. Ieee Geoscience and Remote Sensing Letters. 14:2423-2427.   10.1109/lgrs.2017.2767575   AbstractWebsite

Terrain observation by progressive scans (TOPS)mode synthetic aperture radar interferometry requires high accuracy of burst alignments. Geometrical burst alignment relying on precise orbits and digital topography is not always sufficient for Sentinel-1A TOPS-mode interferometry. Enhanced spectral diversity (ESD) method was proposed to estimate a constant azimuth shift between radar images that minimizes phase discontinuities across the bursts. In some cases, however, the ESD refinement fails to align the bursts in Sentinel-1 interferograms, possibly because of ionospheric propagation effects. Here, we show that in such cases, a bivariate shift (that depends on both azimuth and range) can efficiently remove phase discontinuities across the bursts. The bivariate shift can be derived from the double-differenced radar phase in the burst overlap regions.

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Ujiie, K, Tsutsumi A, Fialko Y, Yamaguchi H.  2009.  Experimental investigation of frictional melting of argillite at high slip rates: Implications for seismic slip in subduction-accretion complexes. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006165   AbstractWebsite

Discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite-rich, argillite-derived slip zones during subduction earthquakes. We conducted high-velocity friction experiments on argillite at a slip rate of 1.13 m/s and normal stresses of 2.67-13.33 MPa. Experiments show slip weakening followed by slip strengthening. Slip weakening is associated with the formation and shearing of low-viscosity melt patches. The subsequent slip strengthening occurred despite the reduction in shear strain rate due to the growth (thickening) of melt layer, suggesting that the viscosity of melt layer increased with slip. Microstructural and chemical analyses suggest that the viscosity increase during the slip strengthening is not due to an increase in the volume fraction of solid grains and bubbles in the melt layer but could be caused primarily by dehydration of the melt layer. Our experimental results suggest that viscous braking can be efficient at shallow depths of subduction-accretion complexes if substantial melt dehydration occurs on a timescale of seismic slip. Melt lubrication can possibly occur at greater depths within subduction-accretion complexes because the ratio of viscous shear to normal stress decreases with depth. Argillite-derived natural pseudotachylytes formed at seismogenic depths in subduction-accretion complexes are more hydrous than the experimentally generated pseudotachylytes and may be evidence of nearly complete stress drop.

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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.

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Simons, M, Fialko Y, Rivera L.  2002.  Coseismic deformation from the 1999 M-w 7.1 Hector Mine, California, earthquake as inferred from InSAR and GPS observations. Bulletin of the Seismological Society of America. 92:1390-1402.   10.1785/0120000933   AbstractWebsite

We use interferometric synthetic aperture radar (InSAR) and Global Positioning System (GPS) observations to Investigate static deformation due to the 1999 M-w 7.1 Hector Mine earthquake, that occurred in the eastern California shear zone. Interferometric decorrelation, phase, and azimuth offset measurements indicate regions of surface and near-surface slip, which we use to constrain the geometry of surface rupture. The inferred geometry is spatially complex, with multiple strands. The southern third of the rupture zone consists of three subparallel segments extending about 20 km in length in a N45degreesW direction. The central segment is the simplest, with a single strand crossing the Bullion Mountains and a strike of N10degreesW. The northern third of the rupture zone is characterized by multiple splays, with directions subparallel to strikes in the southern and central. The average strike for the entire rupture is about N30degreesW. The interferograms indicate significant along-strike variations in strain which are consistent with variations in the ground-based slip measurements. Using a variable resolution data sampling routine to reduce the computational burden, we invert the InSAR and GPS data for the fault geometry and distribution of slip. We compare results from assuming an elastic half-space and a layered elastic space. Results from these two elastic models are similar, although the layered-space model predicts more slip at depth than does the half-space model. The layered model predicts a maximum coseismic slip of more than 5 In at a depth of 3 to 6 km. Contrary to preliminary reports, the northern part of the Hector Mine rupture accommodates the maximum slip. Our model predictions for the surface fault offset and total seismic moment agree with both field mapping results and recent seismic models. The inferred shallow slip deficit is enigmatic and may suggest that distributed inelastic yielding occurred in the uppermost few kilometers of the crust during or soon after the earthquake.

Sandwell, D, Fialko Y.  2004.  Warping and cracking of the Pacific plate by thermal contraction. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003091   AbstractWebsite

Lineaments in the gravity field and associated chains of volcanic ridges are widespread on the Pacific plate but are not yet explained by plate tectonics. Recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere. We show that the large thermoelastic stress produced by top-down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks. Both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle. Cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite-derived gravity. Our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required.

Samsonov, SV, Feng WP, Fialko Y.  2017.  Subsidence at Cerro Prieto Geothermal Field and postseismic slip along the Indiviso fault from 2011 to 2016 RADARSAT-2 DInSAR time series analysis. Geophysical Research Letters. 44:2716-2724.   10.1002/2017gl072690   AbstractWebsite

We present RADARSAT-2 Differential Interferometric Synthetic Aperture Radar (DInSAR) observations of deformation due to fluid extraction at the Cerro Prieto Geothermal Field (CPGF) and afterslip on the 2010 M7.2 El Mayor-Cucapah (EMC) earthquake rupture during 2011-2016. Advanced multidimensional time series analysis reveals subsidence at the CPGF with the maximum rate greater than 100mm/yr accompanied by horizontal motion (radial contraction) at a rate greater than 30mm/yr. During the same time period, more than 30mm of surface creep occurred on the Indiviso fault ruptured by the EMC earthquake. We performed inversions of DInSAR data to estimate the rate of volume changes at depth due to the geothermal production at the CPGF and the distribution of afterslip on the Indiviso fault. The maximum coseismic slip due to the EMC earthquake correlates with the Coulomb stress changes on the Indiviso fault due to fluid extraction at the CPGF. Afterslip occurs on the periphery of maximum coseismic slip areas. Time series analysis indicates that afterslip still occurs 6years after the earthquake.

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Pearse, J, Fialko Y.  2010.  Mechanics of active magmatic intraplating in the Rio Grande Rift near Socorro, New Mexico. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006592   AbstractWebsite

We investigate long-term deformation due to the Socorro Magma Body (SMB), one of the largest active intrusions in the Earth's continental crust, using interferometric synthetic aperture radar (InSAR) observations and finite element simulations. InSAR data spanning 15 years (1992-2007) indicate that the magma body is associated with a steady crustal uplift at a rate of about 2 mm yr(-1). Previous work showed that while the pattern of surface uplift is consistent with an elastic inflation of a large sill-like magma body, the SMB could not have formed via steady elastic inflation because the latter would be outpaced by magma solidification. We resolve this problem using coupled thermovisco-elastic models, and place constraints on the intrusion history as well as the rheology of the ambient crustal rocks. We demonstrate that observations rule out the linear Maxwell response of the ductile crust, but are consistent with laboratory-derived power law rheologies. Our preferred model suggests that the age of the SMB is of the order of 10(3) years, and that the apparent constancy of the present-day uplift may be due to slow heat transfer and ductile deformation in a metamorphic aureole of a giant sill-like magma intrusion, rather than due to a steady increase in the magma overpressure. The SMB is a contemporaneous example of "magmatic intraplating," a process by which large volumes of mafic melt stall and spread at midcrustal depths due to density or rheology contrasts.

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Nof, R, Ziv A, Doin M-P, Baer G, Fialko Y, Wdowinski S, Eyal Y, Bock Y.  2012.  Rising of the lowest place on Earth due to Dead Sea water-level drop: Evidence from SAR interferometry and GPS. J. Geophys. Res.. 117:B05412.   10.1029/2011JB008961   Abstract

The Dead Sea water-level has been dropping at an exceedingly increasing rate since 1960, and between 1993 and 2001, the interval of the InSAR data examined in this study, it has dropped at an average rate of 0.88 m per year. Such a water-level change could potentially give rise to a resolvable lithospheric rebound and regional uplift, with spatial extent and amplitude that are controlled by the effective mechanical properties of the crust and upper mantle combined. We measure that deformation for the years 1993 to 2001, using 149 short baseline interferograms made of 31 ERS-1 and ERS-2 Synthetic Aperture Radar (SAR) images and continuous GPS data from the Survey of Israel recorded between 1997 and 2011. The uplift rate at the Dead Sea is small (up to 4 mm/year), and the basin topography is almost a mirror of the displacement, introducing a strong trade-off between uplift and stratified atmosphere noise. To overcome this complication, we impose a linearity constraint on the satellite to ground Line Of Sight (LOS) phase changes based on the steady uplift observed by a continuous GPS station in the area of interest, and simultaneously solve for the LOS change rate, Digital Elevation Model (DEM) errors and the elevation-phase correlation. While the LOS rate and DEM errors are solved for each pixel independently, the elevation-phase correlation is solved for each SAR acquisition independently. Using this approach we separated the stratified atmospheric delay from the ground displacement. We observed a regional uplift around the Dead Sea northern basin, with maximum uplift close to the shorelines, and diminishing to zero by the Mediterranean coast. We modeled the effect of water load changes using a homogeneous elastic half-space, and found a good agreement between modeled and observed ground displacements using elastic properties that are compatible with seismic and gravity data down to a depth of 15 km below the Dead Sea basin, suggesting that the response of the crust to the sea level drop is controlled mainly by the elastic properties of the upper-crust immediately below the Dead Sea basin.

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Mitchell, EK, Fialko Y, Brown KM.  2015.  Frictional properties of gabbro at conditions corresponding to slow slip events in subduction zones. Geochemistry Geophysics Geosystems. 16:4006-4020.   10.1002/2015gc006093   AbstractWebsite

We conducted a series of experiments to explore the rate and state frictional properties of gabbro at conditions thought to be representative of slow slip events (SSEs) in subduction zones. The experiments were conducted using a heated direct shear apparatus. We tested both solid and simulated gouge samples at low effective normal stress (5-30 MPa) over a broad range of temperatures (20-600 degrees C) under dry and hydrated conditions. In tests performed on dry solid samples, we observed stable sliding at low temperatures (20-150 degrees C), stick slip at high temperatures (350-600 degrees C), and a transitional "episodic slow slip'' behavior at intermediate temperatures (200-300 degrees C). In tests performed on dry gouge samples, we observed stable sliding at all temperatures. Under hydrated conditions, the gouge samples exhibited episodic slow slip and stick-slip behavior at temperatures between 300 and 500 degrees C. Our results show a decrease in the rate parameter (a - b) with temperature for both solid and gouge samples; friction transitions from velocity strengthening to velocity weakening at temperature of about 150 degrees C for both solid and gouge samples. We do not observe transition to velocity-strengthening friction at the high end of the tested temperature range. Our results suggest that the occurrence of slow slip events and the downdip limit of the seismogenic zone on subduction megathrusts cannot be solely explained by the temperature dependence of frictional properties of gabbro. Further experimental studies are needed to evaluate the effects of water fugacity and compositional heterogeneity (e. g., the presence of phyllosilicates) on frictional stability of subduction megathrusts.

Mitchell, EK, Fialko Y, Brown KM.  2013.  Temperature dependence of frictional healing of Westerly granite: Experimental observations and numerical simulations. Geochemistry Geophysics Geosystems. 14:567-582.   10.1029/2012gc004241   AbstractWebsite

Temperature is believed to have an important control on frictional properties of rocks, yet the amount of experimental observations of time-dependent rock friction at high temperatures is rather limited. In this study, we investigated frictional healing of Westerly granite in a series of slide-hold-slide experiments using a direct shear apparatus at ambient temperatures between 20 degrees C and 550 degrees C. We observed that at room temperature coefficient of friction increases in proportion to the logarithm of hold time at a rate consistent with findings of previous studies. For a given hold time, the coefficient of friction linearly increases with temperature, but temperature has little effect on the rate of change in static friction with hold time. We used a numerical model to investigate whether time-dependent increases in real contact area between rough surfaces could account for the observed frictional healing. The model incorporates fractal geometry and temperature-dependent viscoelasoplastic rheology. We explored several candidate rheologies that have been proposed for steady state creep of rocks at high stresses and temperatures. None of the tested laws could provide an agreement between the observed and modeled healing behavior given material properties reported in the bulk creep experiments. An acceptable fit to the experimental data could be achieved with modified parameters. In particular, for the power-law rheology to provide a reasonable fit to the data, the stress exponent needs to be greater than 40. Alternative mechanisms include time-dependent gouge compaction and increases in bond strength between contacting asperities.

Mitchell, EK, Fialko Y, Brown KM.  2016.  Velocity-weakening behavior of Westerly granite at temperature up to 600 degrees C. Journal of Geophysical Research-Solid Earth. 121:6932-6946.   10.1002/2016jb013081   AbstractWebsite

The deep limit to seismicity in continental crust is believed to be controlled by a transition from velocity-weakening to velocity-strengthening friction based on experimental measurements of the rate dependence of friction at different temperatures. Available experimental data on granite suggest a transition to stable creep at about 350 degrees C (approximate to 15km depth). Here we present results from unconfined experiments on Westerly granite at both dry and hydrated conditions that show increasingly unstable slip (velocity-weakening behavior) at temperature up to 600 degrees C. A comparison of previously published experimental results with those presented in this study suggests that the rate and state friction parameters strongly depend on normal stress and pore pressure at high (>400 degrees C) temperature, which may help explain regional variations in the depth distribution of earthquakes in continental crust. Temperature dependence of the rate and state friction parameters may also contribute to strong dynamic weakening observed in high-speed friction experiments on crystalline rocks such as granite and gabbro.

McHone, G, Anderson D, Beutel E, Fialko Y.  2005.  Giant dikes, rifts, flood basalts, and plate tectonics: A contention of mantle models. GSA Special Papers. 388:401-420.   10.1130/0-8137-2388-4.401   Abstract

Giant dike swarms, often hundreds of kilometers long, have produced flood basalts in large igneous provinces since the early Proterozoic. Dike patterns described as radiating from a central source are actually syntectonic swarms that curve and diverge according to lithospheric stress regimes, but they are similar in origin to smaller swarms with parallel dikes. Giant radiating patterns of dikes do not characterize most hotspots or large igneous provinces, and they are not always linked to crustal uplift swells. These mafic intrusions and the fractures they follow are essentially features of plate tectonics, not products of indeterminable deep mantle plumes. As a compelling example, the Early Jurassic central Atlantic magmatic province and its associated Pangaean rift zone are evidential products of subducted materials and convection in the upper mantle beneath the insulating Pangaean plate. Giant dike swarms were formed along lithospheric structures through plate tectonics, not by a coincidental deep mantle plume.

Manzo, M, Fialko Y, Casu F, Pepe A, Lanari R.  2012.  A quantitative assessment of DInSAR measurements of interseismic deformation: the Southern San Andreas Fault case study. Pure and Appl. Geophys.. 169(8):1463-1482.   10.1007/s00024-011-0403-2   AbstractWebsite

We investigate the capabilities and limitations of the Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques, in particular of the Small BAseline Subset (SBAS) approach, to measure surface deformation in active seismogenetic areas. The DInSAR analysis of low-amplitude, long-wavelength deformation, such as that due to interseismic strain accumulation, is limited by intrinsic trade-offs between deformation signals and orbital uncertainties of SAR platforms in their contributions to the interferometric phases, the latter being typically well approximated by phase ramps. Such trade-offs can be substantially reduced by employing auxiliary measurements of the long-wavelength velocity field. We use continuous Global Positioning System (GPS) measurements from a properly distributed set of stations to perform a pre-filtering operation of the available DInSAR interferograms. In particular, the GPS measurements are used to estimate the secular velocity signal, approximated by a spatial ramp within the azimuth-range radar imaging plane; the phase ramps derived from the GPS data are then subtracted from the available set of DInSAR interferograms. This pre-filtering step allows us to compensate for the major component of the long-wavelength range change that, within the SBAS procedure, might be wrongly interpreted and filtered out as orbital phase ramps. With this correction, the final results are obtained by simply adding the pre-filtered long-wavelength deformation signal to the SBAS retrieved time series. The proposed approach has been applied to a set of ERS-1/2 SAR data acquired during the 1992–2006 time interval over a 200 × 200 km area around the Coachella Valley section of the San Andreas Fault in Southern California, USA. We present results of the comparison between the SBAS and the Line Of Sight (LOS)—projected GPS time series of the USGC/PBO network, as well as the mean LOS velocity fields derived using SBAS, GPS and stacking techniques. Our analysis demonstrates the effectiveness of the presented approach and provides a quantitative assessment of the accuracy of DInSAR measurements of interseismic deformation in a tectonically active area.

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Lindsey, EO, Fialko Y.  2016.  Geodetic constraints on frictional properties and earthquake hazard in the Imperial Valley, Southern California. Journal of Geophysical Research-Solid Earth. 121:1097-1113.   10.1002/2015jb012516   AbstractWebsite

We analyze a suite of geodetic observations across the Imperial Fault in southern California that span all parts of the earthquake cycle. Coseismic and postseismic surface slips due to the 1979 M 6.6 Imperial Valley earthquake were recorded with trilateration and alignment surveys by Harsh (1982) and Crook et al. (1982), and interseismic deformation is measured using a combination of multiple interferometric synthetic aperture radar (InSAR)-viewing geometries and continuous and survey-mode GPS. In particular, we combine more than 100 survey-mode GPS velocities with InSAR data from Envisat descending tracks 84 and 356 and ascending tracks 77 and 306 (149 total acquisitions), processed using a persistent scatterers method. The result is a dense map of interseismic velocities across the Imperial Fault and surrounding areas that allows us to evaluate the rate of interseismic loading and along-strike variations in surface creep. We compare available geodetic data to models of the earthquake cycle with rate- and state-dependent friction and find that a complete record of the earthquake cycle is required to constrain key fault properties including the rate-dependence parameter (a - b) as a function of depth, the extent of shallow creep, and the recurrence interval of large events. We find that the data are inconsistent with a high (>30mm/yr) slip rate on the Imperial Fault and investigate the possibility that an extension of the San Jacinto-Superstition Hills Fault system through the town of El Centro may accommodate a significant portion of the slip previously attributed to the Imperial Fault. Models including this additional fault are in better agreement with the available observations, suggesting that the long-term slip rate of the Imperial Fault is lower than previously suggested and that there may be a significant unmapped hazard in the western Imperial Valley.

Lindsey, EO, Fialko Y, Bock Y, Sandwell DT, Bilham R.  2014.  Localized and distributed creep along the southern San Andreas Fault. Journal of Geophysical Research-Solid Earth. 119:7909-7922.   10.1002/2014jb011275   AbstractWebsite

We investigate the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey-mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003-2010 were used to separate the pattern of horizontal and vertical motion, providing a high-resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates. Key PointsShallow creep is pervasive along the southernmost 50 km of the San Andreas FaultCreep is localized only along transpressional fault segmentsIn transtensional areas, creep is distributed over a 1-2 km wide fault zone