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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.

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.

Crowell, BW, Bock Y, Sandwell DT, Fialko Y.  2013.  Geodetic investigation into the deformation of the Salton Trough. Journal of Geophysical Research-Solid Earth. 118:5030-5039.   10.1002/jgrb.50347   AbstractWebsite

The Salton Trough represents a complex transition between the spreading center in Baja California and the strike-slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high-resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008-2009 and previous surveys between 2000 and 2005. We also investigate small-scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15mm/yr of left-lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10 degrees/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.

Lindsey, EO, Fialko Y.  2013.  Geodetic slip rates in the southern San Andreas Fault system: Effects of elastic heterogeneity and fault geometry. Journal of Geophysical Research-Solid Earth. 118:689-697.   10.1029/2012jb009358   AbstractWebsite

We use high resolution interferometric synthetic aperture radar and GPS measurements of crustal motion across the southern San Andreas Fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths. Geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern San Andreas and San Jacinto faults. Two possibilities have been proposed to explain this observation: large contrasts in crustal rigidity across the faults, or an alternate fault geometry such as a dipping San Andreas fault or a blind segment of the San Jacinto Fault. We evaluate these possibilities using a two-dimensional elastic model accounting for heterogeneous structure computed from the Southern California Earthquake Center crustal velocity model CVM-H 6.3. The results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates. However, we find that small changes in the location of faults at depth can strongly impact the results. Our preferred model includes a San Andreas Fault dipping northeast at 60 degrees, and two active branches of the San Jacinto fault zone. In this case, we infer nearly equal slip rates of 18 +/- 1 and 19 +/- 2 mm/yr for the San Andreas and San Jacinto fault zones, respectively. These values are in good agreement with geologic measurements representing average slip rates over the last 10(4)-10(6) years, implying steady long-term motion on these faults. Citation: Lindsey, E. O., and Y. Fialko (2013), Geodetic slip rates in the southern San Andreas Fault system: Effects of elastic heterogeneity and fault geometry, J. Geophys. Res. Solid Earth, 118, 689-697, doi:10.1029/2012JB009358.

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.

LaBonte, AL, Brown KM, Fialko Y.  2009.  Hydrologic detection and finite element modeling of a slow slip event in the Costa Rica prism toe. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005806   AbstractWebsite

We investigate transient fluid flux through the seafloor recorded near the Costa Rica trench during the 2000 Costa Rica Seismogenic Zone Experiment using a 2-D fully coupled poroelastic finite element model. We demonstrate that the observed hydrologic anomalies are consistent with a model of propagating slow slip at the subduction interface between the frontal prism and downgoing plate. There are two sources of volumetric strain that drive fluid flux at the seafloor in response to fault slip at depth: (1) compression and dilation in the vicinity of the tips of a slipping patch and (2) extension and compression due to flexure of the seafloor. The superposition of these two effects results in distinctive spatial and temporal patterns of fluid flow through the seafloor. In a forward modeling approach, time series from shear ruptures with a range of fault length-to-depth ratios in a heterogeneous crust are generated and compared with flow rate observations. Assuming a constant propagation rate and an elliptical profile for the distribution of slip along the decollement, the set of model predictions enables us to infer the probable rupture location, extent, propagation velocity, and duration from a single flow rate time series. The best fit model suggests that the slow slip event initiated within the toe at a depth of less than 4 km and propagated bilaterally at an average rate of 0.5 km d(-1). This interpretation implies that stress in the shallow subduction zone is relieved episodically. Furthermore, the Costa Rica data suggest that episodic slow slip events may initiate in the prism toe without being triggered by a seismic event further downdip.

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.

Kaneko, Y, Fialko Y, Sandwell DT, Tong X, Furuya M.  2013.  Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties. Journal of Geophysical Research-Solid Earth. 118:316-331.   10.1029/2012jb009661   AbstractWebsite

We present high-resolution measurements of interseismic deformation along the central section of the North Anatolian Fault (NAF) in Turkey using interferometric synthetic aperture radar data from the Advanced Land Observing Satellite and Envisat missions. We generated maps of satellite line-of-sight velocity using five ascending Advanced Land Observing Satellite tracks and one descending Envisat track covering the NAF between 31.2 degrees E and 34.3 degrees E. The line-of-sight velocity reveals discontinuities of up to similar to 5 mm/yr across the Ismetpasa segment of the NAF, implying surface creep at a rate of similar to 9 mm/yr; this is a large fraction of the inferred slip rate of the NAF (21-25 mm/yr). The lateral extent of significant surface creep is about 75 km. We model the inferred surface velocity and shallow fault creep using numerical simulations of spontaneous earthquake sequences that incorporate laboratory-derived rate and state friction. Our results indicate that frictional behavior in the Ismetpasa segment is velocity strengthening at shallow depths and transitions to velocity weakening at a depth of 3-6 km. The inferred depth extent of shallow fault creep is 5.5-7 km, suggesting that the deeper locked portion of the partially creeping segment is characterized by a higher stressing rate, smaller events, and shorter recurrence interval. We also reproduce surface velocity in a locked segment of the NAF by fault models with velocity-weakening conditions at shallow depth. Our results imply that frictional behavior in a shallow portion of major active faults with little or no shallow creep is mostly velocity weakening. Citation: Kaneko, Y., Y. Fialko, D. T. Sandwell, X. Tong, and M. Furuya (2013), Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties, J. Geophys. Res. Solid Earth, 118, 316-331, doi: 10.1029/2012JB009661.

Fialko, Y.  2006.  Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system. Nature. 441:968-971.   10.1038/nature04797   AbstractWebsite

The San Andreas fault in California is a mature continental transform fault that accommodates a significant fraction of motion between the North American and Pacific plates. The two most recent great earthquakes on this fault ruptured its northern and central sections in 1906 and 1857, respectively. The southern section of the fault, however, has not produced a great earthquake in historic times ( for at least 250 years). Assuming the average slip rate of a few centimetres per year, typical of the rest of the San Andreas fault, the minimum amount of slip deficit accrued on the southern section is of the order of 7 - 10 metres, comparable to the maximum co-seismic offset ever documented on the fault(1,2). Here I present high-resolution measurements of interseismic deformation across the southern San Andreas fault system using a well-populated catalogue of space-borne synthetic aperture radar data. The data reveal a nearly equal partitioning of deformation between the southern San Andreas and San Jacinto faults, with a pronounced asymmetry in strain accumulation with respect to the geologically mapped fault traces. The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle.

Lindsey, E, Sahakian V, Fialko Y, Bock Y, Barbot S, Rockwell T.  2013.  Interseismic strain localization in the San Jacinto Fault Zone. Pure and Applied Geophysics. :1-18.: Springer Basel   10.1007/s00024-013-0753-z   AbstractWebsite

We investigate interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2–3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2–1.6 as imaged tomographically by Allam and Ben-Zion (Geophys J Int 190:1181–1196, 2012) can explain about 50 % of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14–18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to explain fully the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

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

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.

Brown, K, Fialko Y.  2012.  "Melt welt" mechanism of extreme weakening of gabbro at seismic slip rates. Nature. 488:638-641.   10.1038/nature11370   Abstract

Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (~0.1–1 m s−1), and at moderate normal stresses (1–10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting. The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood. The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt (‘melt welts’), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1–0.4 MW m−2, while the average temperature of the shear zone is well below the solidus (250–450 °C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.

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.

Fialko, YA, Rubin AM.  1997.  Numerical simulation of high-pressure rock tensile fracture experiments: Evidence of an increase in fracture energy with pressure? Journal of Geophysical Research-Solid Earth. 102:5231-5242.   10.1029/96jb03859   AbstractWebsite

High confining pressure fracture tests of Indiana limestone [Abou-Sayed, 1977] and Iidate granite [Hashida et al., 1993] were simulated using boundary element techniques and a Dugdale-Barenblatt (tension-softening) model of the fracture process zone. Our results suggest a substantial (more than a factor of 2) increase in the fracture energy of Indiana limestone when the confining pressure was increased from zero to only 6-7 MPa. While Hashida es al. [1993] concluded that there was no change in the fracture energy of Iidate granite at confining pressures up to 26.5 MPa, we find that data from one series of experiments (''compact-tension'' tests in their terminology) are also consistent with a significant (more than a factor of 2) increase in fracture energy. Data from another set of their experiments (thick-walled cylinder tests) seem to indicate a decrease in the fracture energy of Iidate granite at confining pressures,of 6-8 MPa, but these may be biased due to the very small specimen size. To our knowledge these results are the first reliable indication from laboratory experiments that rock tensile fracture energy varies with confining pressure. Based on these results, some possible mechanisms of pressure sensitive fracture are discussed. We suggest that the inferred increase in fracture energy results from more extensive inelastic deformation near the crack tip that increases the effective critical crack opening displacement. Such deformation might have occurred due to the large deviatoric stress in the vicinity of the crack tip in the Abou-Sayed experiments, and due to the enlarged region of significant tensile stress near the crack tip in the Hashida et al. compact tension tests. These results also highlight the fact that at confining pressures that exceed the tensile strength of the material, tensile fracture energy will in general depend upon the crack size and the distribution of loads within it, as well as the ambient stress.

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).

Lin, GQ, Shearer P, Fialko Y.  2006.  Obtaining absolute locations for quarry seismicity using remote sensing data. Bulletin of the Seismological Society of America. 96:722-728.   10.1785/0120050146   AbstractWebsite

We obtain absolute locations for 19 clusters of mining-induced seismicity in southern California by identifying quarries using remote sensing data, including optical imagery and differential digital elevation models. These seismicity clusters contain 16,574 events from the Southern California Seismic Network from 1984 to 2002, which are flagged as quarry blasts but without any -round-truth location constraints. Using georeferenced airphotos and satellite radar topography data, we identify the likely sources of these events as quarries that are clearly visible within 1 to 2 km of the seismically determined locations. We then shift the clusters to align with the airphoto images, obtaining an estimated absolute location accuracy of similar to 200 m for the cluster centroids. The improved locations of these explosions should be helpful for constraining regional 3D velocity models.

Fialko, Y.  2001.  On origin of near-axis volcanism and faulting at fast spreading mid-ocean ridges. Earth and Planetary Science Letters. 190:31-39.   10.1016/s0012-821x(01)00376-4   AbstractWebsite

At fast and superfast spreading mid-ocean ridges, such as the East Pacific Rise, a plate boundary is defined by a narrow (tens to hundreds of meters wide) neovolcanic zone within which the bulk of the upper oceanic crust is created. However, detailed near-bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge axis. It has been proposed that off-axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis, or spatial migration of magmatic sources in the crust and upper mantle. We demonstrate that off-axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space, and the magma delivery is perfectly focussed at the ridge axis. Theoretical modeling and field observations indicate that off-axis volcanisin may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth. Volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills, arguably the most abundant relief form on Earth. (C) 2001 Elsevier Science B.V. All rights reserved.

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.

Barbot, S, Fialko Y, Bock Y.  2009.  Postseismic deformation due to the M(w) 6.0 2004 Parkfield earthquake: Stress-driven creep on a fault with spatially variable rate-and-state friction parameters. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005748   AbstractWebsite

We investigate the coseismic and postseismic deformation due to the M(w) 6.0 2004 Parkfield, California, earthquake. We produce coseismic and postseismic slip models by inverting data from an array of 14 continuous GPS stations from the SCIGN network. Kinematic inversions of postseismic GPS data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip, predominantly at a depth of similar to 5 km. Inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane. The kinetics of afterslip is consistent with a velocity-strengthening friction generalized to include the case of infinitesimal velocities. We performed simulations of stress-driven creep using a numerical model that evaluates the time-dependent deformation due to coseismic stress changes in a viscoelastoplastic half-space. Starting with a coseismic slip distribution, we compute the time-dependent evolution of afterslip on a fault plane and the associated displacements at the GPS stations. Data are best explained by a rate-strengthening model with frictional parameter (a - b) = 7 x 10(-3), at a high end of values observed in laboratory experiments. We also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks. The rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip, suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep. The San Andreas fault around Parkfield is deduced to have large along-strike variations in rate-and-state frictional properties. Velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity-weakening patches after the 2004 rupture.

Fialko, Y.  2004.  Probing the mechanical properties of seismically active crust with space geodesy: Study of the coseismic deformation due to the 1992 M(w)7.3 Landers (southern California) earthquake. Journal of Geophysical Research-Solid Earth. 109   10.1029/2003jb002756   AbstractWebsite

[1] The coseismic deformation due to the 1992 M(w)7.3 Landers earthquake, southern California, is investigated using synthetic aperture radar (SAR) and Global Positioning System (GPS) measurements. The ERS-1 satellite data from the ascending and descending orbits are used to generate contiguous maps of three orthogonal components ( east, north, up) of the coseismic surface displacement field. The coseismic displacement field exhibits symmetries with respect to the rupture plane that are suggestive of a linear relationship between stress and strain in the crust. Interferometric synthetic aperture radar (InSAR) data show small-scale deformation on nearby faults of the Eastern California Shear Zone. Some of these faults ( in particular, the Calico, Rodman, and Pinto Mountain faults) were also subsequently strained by the 1999 M(w)7.1 Hector Mine earthquake. I test the hypothesis that the anomalous fault strain represents essentially an elastic response of kilometer-scale compliant fault zones to stressing by nearby earthquakes [Fialko et al., 2002]. The coseismic stress perturbations due to the Landers earthquake are computed using a slip model derived from inversions of the InSAR and GPS data. Calculations are performed for both homogeneous and transversely isotropic half-space models. The compliant zone model that best explains the deformation on the Calico and Pinto Mountain faults due to the Hector Mine earthquake successfully predicts the coseismic displacements on these faults induced by the Landers earthquake. Deformation on the Calico and Pinto Mountain faults implies about a factor of 2 reduction in the effective shear modulus within the similar to 2 km wide fault zones. The depth extent of the low-rigidity zones is poorly constrained but is likely in excess of a few kilometers. The same type of structure is able to explain high gradients in the radar line of sight displacements observed on other faults adjacent to the Landers rupture. In particular, the Lenwood fault north of the Soggy Lake has likely experienced a few centimeters of left-lateral motion across < 1-km-wide compliant fault zone having the rigidity reduction of more than a factor of 2. The inferred compliant fault zones are interpreted to be a result of extensive damage due to past earthquakes.

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.

Jiang, JL, Fialko Y.  2016.  Reconciling seismicity and geodetic locking depths on the Anza section of the San Jacinto fault. Geophysical Research Letters. 43:10663-10671.   10.1002/2016gl071113   AbstractWebsite

Observations from the Anza section of the San Jacinto Fault in Southern California reveal that microseismicity extends to depths of 15-18km, while the geodetically determined locking depth is less than similar to 10km. This contrasts with observations from other major faults in the region and also with predictions of fault models assuming a simple layered distribution of frictional properties with depth. We suggest that an anomalously shallow geodetic fault locking may result from a transition zone at the bottom of seismogenic layer with spatially heterogeneous frictional properties. Numerical models of faults that incorporate stochastic heterogeneity at transitional depths successfully reproduce the observed depth relation between seismicity and geodetic locking, as well as complex spatiotemporal patterns of microseismicity with relatively scarce repeating earthquakes. Our models predict propagation of large earthquakes to the bottom of the transition zone, and ubiquitous aseismic transients below the locked zone, potentially observable using high-precision geodetic techniques.

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.

Cochran, ES, Li YG, Shearer PM, Barbot S, Fialko Y, Vidale JE.  2009.  Seismic and geodetic evidence for extensive, long-lived fault damage zones. Geology. 37:315-318.   10.1130/g25306a.1   AbstractWebsite

During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%-50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These findings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.