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Sandwell, D, Smith-Konter B.  2018.  Maxwell: A semi-analytic 4D code for earthquake cycle modeling of transform fault systems. Computers & Geosciences. 114:84-97.   10.1016/j.cageo.2018.01.009   AbstractWebsite

We have developed a semi-analytic approach (and computational code) for rapidly calculating 3D time-dependent deformation and stress caused by screw dislocations imbedded within an elastic layer overlying a Maxwell viscoelastic half-space. The maxwell model is developed in the Fourier domain to exploit the computational advantages of the convolution theorem, hence substantially reducing the computational burden associated with an arbitrarily complex distribution of force couples necessary for fault modeling. The new aspect of this development is the ability to model lateral variations in shear modulus. Ten benchmark examples are provided for testing and verification of the algorithms and code. One final example simulates interseismic deformation along the San Andreas Fault System where lateral variations in shear modulus are included to simulate lateral variations in lithospheric structure.

Tong, X, Sandwell DT, Schmidt DA.  2018.  Surface creep rate and moment accumulation rate along the Aceh segment of the Sumatran Fault from L-band ALOS-1/PALSAR-1 observations. Geophysical Research Letters. 45:3404-3412.   10.1002/2017gl076723   AbstractWebsite

We analyzed the interferometric synthetic aperture radar data from the ALOS-1/PALSAR-1 satellite to image the interseismic deformation along the Sumatran fault. The interferometric synthetic aperture radar time series analysis reveals up to similar to 20 mm/year of aseismic creep on the Aceh segment along the Northern Sumatran fault. This is a large fraction of the total slip rate across this fault. The spatial extent of the aseismic creep extends for similar to 100 km. The along-strike variation of the aseismic creep has an inverse "U" shape. An analysis of the moment accumulation rate shows that the central part of the creeping section accumulates moment at approximately 50% of the rate of the surrounding locked segments. An initial analysis of temporal variations suggests that the creep rate may be decelerating with time, suggesting that the creep rate is adjusting to a stress perturbation from nearby seismic activity. Our study has implications to the earthquake hazard along the northern Sumatran fault.

Sandwell, DT, Wessel P.  2016.  Interpolation of 2-D vector data using constraints from elasticity. Geophysical Research Letters. 43:10703-10709.   10.1002/2016gl070340   AbstractWebsite

We present a method for interpolation of sparse two-dimensional vector data. The method is based on the Green's functions of an elastic body subjected to in-plane forces. This approach ensures elastic coupling between the two components of the interpolation. Users may adjust the coupling by varying Poisson's ratio. Smoothing can be achieved by ignoring the smallest eigenvalues in the matrix solution for the strengths of the unknown body forces. We demonstrate the method using irregularly distributed GPS velocities from southern California. Our technique has been implemented in both the Generic Mapping Tools and MATLAB (R).

Howell, S, Smith-Konter B, Frazer N, Tong XP, Sandwell D.  2016.  The vertical fingerprint of earthquake cycle loading in southern California. Nature Geoscience. 9:611-+.   10.1038/ngeo2741   AbstractWebsite

The San Andreas Fault System, one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the crust in response to large-scale plate motions(1-8). Horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Here we show that when carefully treated for spatial consistency, global positioning system-derived vertical velocities expose a small-amplitude (+/- 2mmyr(-1)), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the fault system in southern California. We employ the statistical method of model selection to isolate this vertical velocity field fromnon-tectonic signals that induce velocity variations in both magnitude and direction across small distances (less than tens of kilometres; ref. 9), and find remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle models spanning the past few centuries(6,10). We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability. Understanding this critical component of interseismic deformation at a complex strike-slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.

Neves, MC, Cabral J, Luttrell K, Figueiredo P, Rockwell T, Sandwell D.  2015.  The effect of sea level changes on fault reactivation potential in Portugal. Tectonophysics. 658:206-220.   10.1016/j.tecto.2015.07.023   AbstractWebsite

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5-1 MPa along the Manteigas-Vilarica-Braganca (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas-Vilarica-Braganca fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies. (C) 2015 Elsevier B.V. All rights reserved.

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.

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 DT.  2010.  Decorrelation of L-Band and C-Band Interferometry Over Vegetated Areas in California. IEEE Transactions on Geoscience and Remote Sensing. 48:2942-2952.   10.1109/tgrs.2010.2043442   AbstractWebsite

Temporal decorrelation is one of the main limitations for recovering interseismic deformation along the San Andreas Fault system using interferometric synthetic aperture radar. To assess the improved correlation properties of L-band with respect to C-band, we analyzed L-band Advanced Land Observation Satellite (ALOS) interferograms with a range of temporal and spatial baselines over three vegetated areas in California and compared them with corresponding C-band European Remote Sensing Satellite (ERS) interferograms. Over the highly vegetated Northern California forests in the Coast Range area, ALOS remains remarkably well correlated over a 2-year period, whereas an ERS interferogram with a similar temporal and spatial baseline lost correlation. In Central California near Parkfield, we found a similar pattern in decorrelation behavior, which enabled the recovery of a fault creep and a local uplifting signal at L-band that was not apparent at C-band. In the Imperial Valley in Southern California, both ALOS and ERS have low correlation over farmlands. ALOS has lower correlation over some sandy surfaces than ERS, probably due to low signal-to-noise ratio. In general, L-band interferograms with similar seasonal acquisitions have higher correlation than those with dissimilar season. For both L-and C-band, correlation over vegetated areas decreases with time for intervals less than 1 year and then remains relatively constant at longer time intervals. The decorrelation time for L-band is more than 2 years in the forest in California whereas that for C-band is less than 6 months. Overall, these results suggest that L-band interferograms will reveal near-fault interseismic deformation once sufficient data become available.

Luttrell, K, Sandwell D.  2010.  Ocean loading effects on stress at near shore plate boundary fault systems. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006541   AbstractWebsite

Changes in eustatic sea level since the Last Glacial Maximum create a differential load across coastlines globally. The resulting plate bending in response to this load alters the state of stress within the lithosphere within a half flexural wavelength of the coast. We calculate the perturbation to the total stress tensor due to ocean loading in coastal regions. Our stress calculation is fully 3-D and makes use of a semianalytic model to efficiently calculate stresses within a thick elastic plate overlying a viscoelastic or fluid half-space. The 3-D stress perturbation is resolved into normal and shear stresses on plate boundary fault planes of known orientation so that Coulomb stress perturbations can be calculated. In the absence of complete paleoseismic indicators that span the time since the Last Glacial Maximum, we investigate the possibility that the seismic cycle of coastal plate boundary faults was affected by stress perturbations due to the change in sea level. Coulomb stress on onshore transform faults, such as the San Andreas and Alpine faults, is increased by up to 1-1.5 MPa, respectively, promoting failure primarily through a reduction in normal stress. These stress perturbations may perceptibly alter the seismic cycle of major plate boundary faults, but such effects are more likely to be observed on nearby secondary faults with a lower tectonic stress accumulation rate. In the specific instance of rapid sea level rise at the Black Sea, the seismic cycle of the nearby North Anatolian fault was likely significantly advanced.

Barbot, S, Fialko Y, Sandwell D.  2009.  Three-dimensional models of elastostatic deformation in heterogeneous media, with applications to the Eastern California Shear Zone. Geophysical Journal International. 179:500-520.   10.1111/j.1365-246X.2009.04194.x   AbstractWebsite

P>We present a semi-analytic iterative procedure for evaluating the 3-D deformation due to faults in an arbitrarily heterogeneous elastic half-space. Spatially variable elastic properties are modelled with equivalent body forces and equivalent surface traction in a 'homogenized' elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar inferferometric (InSAR) data. Our results confirm that the CZ models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone (ECSZ) can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50 per cent reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (similar to 1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60 per cent reduction in shear modulus, with no change in Poisson's ratio. We find that the required effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. These observations suggest pervasive and widespread damage around active crustal faults.

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.

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.

Smith, B, Sandwell D.  2004.  A three-dimensional semianalytic viscoelastic model for time-dependent analyses of the earthquake cycle. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003185   AbstractWebsite

[ 1] Exploring the earthquake cycle for large, complex tectonic boundaries that deform over thousands of years requires the development of sophisticated and efficient models. In this paper we introduce a semianalytic three-dimensional (3-D) linear viscoelastic Maxwell model that is developed in the Fourier domain to exploit the computational advantages of the convolution theorem. A new aspect of this model is an analytic solution for the surface loading of an elastic plate overlying a viscoelastic half-space. When fully implemented, the model simulates ( 1) interseismic stress accumulation on the upper locked portion of faults, ( 2) repeated earthquakes on prescribed fault segments, and ( 3) the viscoelastic response of the asthenosphere beneath the plate following episodic ruptures. We verify both the analytic solution and computer code through a variety of 2-D and 3-D tests and examples. On the basis of the methodology presented here, it is now possible to explore thousands of years of the earthquake cycle along geometrically complex 3-D fault systems.

Lyons, SN, Bock Y, Sandwell DT.  2002.  Creep along the imperial fault, southern California, from GPS measurements. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000763   AbstractWebsite

[1] In May of 1999 and 2000, we surveyed with Global Positioning System (GPS) 46 geodetic monuments established by Imperial College, London, in a dense grid (half-mile spacing) along the Imperial Fault, with three additional National Geodetic Survey sites serving as base stations. These stations were previously surveyed in 1991 and 1993. The Imperial College sites were surveyed in rapid-static mode (15-20 min occupations), while the NGS sites continuously received data for 10 h d(-1). Site locations were calculated using the method of instantaneous positioning, and velocities were determined relative to one of the NGS base stations. Combining our results with far-field velocities from the Southern California Earthquake Center (SCEC), we fit the data to a simple elastic dislocation model with 35 mm yr(-1) of right-lateral slip below 10 km and 9 mm yr(-1) of creep from the surface down to 3 km. The velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault.