Publications

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2017
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.

2016
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.

2014
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

2013
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.

2011
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.

2010
Barbot, S, Fialko Y.  2010.  A unified continuum representation of post-seismic relaxation mechanisms: semi-analytic models of afterslip, poroelastic rebound and viscoelastic flow. Geophysical Journal International. 182:1124-1140.   10.1111/j.1365-246X.2010.04678.x   AbstractWebsite

P>We present a unified continuum mechanics representation of the mechanisms believed to be commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticity. The time-dependent relaxation that follows an earthquake, or any other static stress perturbation, is considered in a framework of a generalized viscoelastoplastic rheology whereby some inelastic strain relaxes a physical quantity in the material. The relaxed quantity is the deviatoric stress in case of viscoelastic relaxation, the shear stress in case of creep on a fault plane and the trace of the stress tensor in case of poroelastic rebound. In this framework, the instantaneous velocity field satisfies the linear inhomogeneous Navier's equation with sources parametrized as equivalent body forces and surface tractions. We evaluate the velocity field using the Fourier-domain Green's function for an elastic half-space with surface buoyancy boundary condition. The accuracy of the proposed method is demonstrated by comparisons with finite-element simulations of viscoelastic relaxation following strike-slip and dip-slip ruptures for linear and power-law rheologies. We also present comparisons with analytic solutions for afterslip driven by coseismic stress changes. Finally, we demonstrate that the proposed method can be used to model time-dependent poroelastic rebound by adopting a viscoelastic rheology with bulk viscosity and work hardening. The proposed method allows one to model post-seismic transients that involve multiple mechanisms (afterslip, poroelastic rebound, ductile flow) with an account for the effects of gravity, non-linear rheologies and arbitrary spatial variations in inelastic properties of rocks (e.g. the effective viscosity, rate-and-state frictional parameters and poroelastic properties).

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.

2009
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.

2008
Barbot, S, Fialko Y, Sandwell D.  2008.  Effect of a compliant fault zone on the inferred earthquake slip distribution. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005256   AbstractWebsite

We present a new semi-analytic method to evaluate the deformation due to a screw dislocation in arbitrarily heterogeneous and/or anisotropic elastic half plane. The method employs integral transformations to reduce the governing partial differential equations to the integral Fredholm equation of the second kind. Dislocation sources, as well as spatial perturbations in the elastic properties are modeled using equivalent body forces. The solution to the Fredholm equation is obtained in the Fourier domain using a method of successive over-relaxation, and is mapped into the spatial domain using the inverse Fast Fourier Transform. We apply this method to investigate the effect of a soft damage zone around an earthquake fault on the co-seismic displacement field, and on the earthquake slip distribution inferred from inversions of geodetic data. In the presence of a kilometer-wide damage zone with a reduction of the effective shear modulus of a factor of 2, inversions that assume a laterally homogeneous model tend to underestimate the amount of slip in the middle of the seismogenic layer by as much as 20%. This bias may accentuate the inferred maxima in the seismic moment release at depth between 3-6 km suggested by previous studies of large strike-slip earthquakes.

2006
Hamiel, Y, Katz O, Lyakhovsky V, Reches Z, Fialko Y.  2006.  Stable and unstable damage evolution in rocks with implications to fracturing of granite. Geophysical Journal International. 167:1005-1016.   10.1111/j.1365-246X.2006.03126.x   AbstractWebsite

We address the relation between the rock rigidity and crack density by comparing predictions of a viscoelastic damage rheology model to laboratory data that include direct microscopic mapping of cracks. The damage rheology provides a generalization of Hookean elasticity to a non-linear continuum mechanics framework incorporating degradation and recovery of the effective elastic properties, transition from stable to unstable fracturing, and gradual accumulation of irreversible deformation. This approach is based on the assumption that the density of microcracks is uniform over a length scale much larger than the length of a typical crack, yet much smaller than the size of the entire deforming domain. For a system with a sufficiently large number of cracks, one can define a representative volume in which the crack density is uniform and introduce an intensive damage variable for this volume. We tested our viscoelastic damage rheology against sets of laboratory experiments done on Mount Scott granite. Based on fitting the entire stress-strain records the damage variable is constrained, and found to be a linear function of the crack density. An advantage of these sets experiments is that they were preformed with different loading paths and explicitly demonstrated the existence of stable and unstable fracturing regimes. We demonstrate that the viscoelastic damage rheology provides an adequate quantitative description of the brittle rock deformation and simulates both the stable and unstable damage evolution under various loading conditions. Comparison between the presented data analysis of experiments with Mount Scott granite and previous results with Westerly granite and Berea sandstone indicates that granular or porous rocks have lower seismic coupling. This implies that the portion of elastic strain released during a seismic cycle as brittle deformation depends on the lithology of the region. Hence, upper crustal regions with thick sedimentary cover, or fault zones with high degree of damage are expected to undergo a more significant inelastic deformation in the interseismic period compared to 'intact' crystalline rocks.

2005
Fialko, Y, Sandwell D, Simons M, Rosen P.  2005.  Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit. Nature. 435:295-299.   10.1038/nature03425   AbstractWebsite

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4 - 5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths ( 4 - 10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.

2004
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.

2001
Fialko, Y, Simons M.  2001.  Evidence for on-going inflation of the Socorro magma body, New Mexico, from interferometric synthetic aperture radar imaging. Geophysical Research Letters. 28:3549-3552.   10.1029/2001gl013318   AbstractWebsite

Interferometric synthetic aperture radar (In-SAR) imaging of the central Rio Grande rift (New Mexico, USA) during 1992-1999 reveals a crustal uplift of several centimeters that spatially coincides with the seismologically determined outline of the Socorro magma body, one of the largest currently active magma intrusions in the Earth's continental crust. Modeling of interferograms shows that the observed deformation may be due to elastic opening of a sill-like intrusion at a rate of a few millimeters per year. Despite an apparent constancy of the geodetically determined uplift rate, thermodynamic arguments suggest that it is unlikely that the Socorro magma body has formed via steady state elastic inflation.

Fialko, Y, Simons M, Agnew D.  2001.  The complete (3-D) surface displacement field in the epicentral area of the 1999 M(w)7.1 Hector Mine earthquake, California, from space geodetic observations. Geophysical Research Letters. 28:3063-3066.   10.1029/2001gl013174   AbstractWebsite

We use Interferometric Synthetic Aperture Radar (InSAR) data to derive continuous maps for three orthogonal components of the co-seismic surface displacement field due to the 1999 M-w 7.1 Hector Mine earthquake in southern California. Vertical and horizontal displacements are both predominantly antisymmetric with respect to the fault plane, consistent with predictions of linear elastic models of deformation for a strike-slip fault. Some deviations from symmetry apparent in the surface displacement data may result from complexity in the fault geometry.