Publications

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2014
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

2013
Tong, X, Sandwell DT, Smith-Konter B.  2013.  High-resolution interseismic velocity data along the San Andreas Fault from GPS and InSAR. Journal of Geophysical Research-Solid Earth. 118:369-389.   10.1029/2012jb009442   AbstractWebsite

We compared four interseismic velocity models of the San Andreas Fault based on GPS observations. The standard deviations of the predicted secular velocity from the four models are larger north of the San Francisco Bay area, near the creeping segment in Central California, and along the San Jacinto Fault and the East California Shear Zone in Southern California. A coherence spectrum analysis of the secular velocity fields indicates relatively high correlation among the four models at longer wavelengths (>15-40 km), with lower correlation at shorter wavelengths. To improve the short-wavelength accuracy of the interseismic velocity model, we integrated interferometric synthetic aperture radar (InSAR) observations, initially from Advanced Land Observing Satellite (ALOS) ascending data (spanning from the middle of 2006 to the end of 2010, totaling more than 1100 interferograms), with GPS observations using a Sum/Remove/Filter/Restore approach. The final InSAR line of sight data match the point GPS observations with a mean absolute deviation of 1.5 mm/yr. We systematically evaluated the fault creep rates along major faults of the San Andreas Fault and compared them with creepmeters and alignment array data compiled in Uniform California Earthquake Rupture Forecast, Version 2 (UCERF2). Moreover, this InSAR line of sight dataset can constrain rapid velocity gradients near the faults, which are critical for understanding the along-strike variations in stress accumulation rate and associated earthquake hazard. Citation: Tong, X., D. T. Sandwell, and B. Smith-Konter (2013), High-resolution interseismic velocity data along the San Andreas Fault from GPS and InSAR, J. Geophys. Res. Solid Earth, 118, 369-389, doi:10.1029/2012JB009442.

2010
Wei, M, Sandwell D, Smith-Konter B.  2010.  Optimal combination of InSAR and GPS for measuring interseismic crustal deformation. Advances in Space Research. 46:236-249.   10.1016/j.asr.2010.03.013   AbstractWebsite

High spatial resolution measurements of interseismic deformation along major faults are critical for understanding the earthquake cycle and for assessing earthquake hazard. We propose a new remove/filter/restore technique to optimally combine GPS and InSAR data to measure interseismic crustal deformation, considering the spacing of GPS stations in California and the characteristics of interseismic signal and noise using InSAR. To constrain the longer wavelengths (>40 km) we use GPS measurements, combined with a dislocation model, and for the shorter wavelength information we rely on InSAR measurements. Expanding the standard techniques, which use a planar ramp to remove long wavelength error, we use a Gaussian filter technique. Our method has the advantage of increasing the signal-to-noise ratio, controlling the variance of atmosphere error, and being isotropic. Our theoretical analysis indicates this technique can improve the signal-to-noise ratio by up to 20%. We test this method along three segments of the San Andreas Fault (Southern section near Salton Sea, Creeping section near Parkfield and Mojave/Big Bend section near Los Angeles), and find improvements of 26%, 11% and 8% in these areas, respectively. Our data shows a zone of uplift to the west of the Creeping section of the San Andreas Fault and an area of subsidence near the city of Lancaster. This work suggests that after only 5 years of data collection, ALOS interferograms will provide a major improvement in measuring details of interseismic deformation. (C) 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

2009
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.

2002
Watson, KM, Bock Y, Sandwell DT.  2002.  Satellite interferometric observations of displacements associated with seasonal groundwater in the Los Angeles basin. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000470   AbstractWebsite

[1] The Newport-Inglewood fault zone (NIFZ) displays interferometric synthetic aperture radar (SAR) phase features along most of its length having amplitudes of up to 60 mm. However, interpretation in terms of right-lateral, shallow slip along the fault fails to match the range of geologic estimates of slip. Recently, Bawden et al. [2001] proposed that these phase features, as well as a broader deformation pattern in the Los Angeles basin, are due to vertical motion related to annual variations in the elevation of the water table. We confirm this hypothesis through the analysis of a longer span of data consisting of 26 SAR images collected by the ERS-1 and ERS-2 spacecraft between June 1992 and June 2000. Moreover, we use continuous GPS measurements from 1995 to the present to establish the amplitude and phase of the vertical deformation. The Los Angeles basin becomes most inflated one quarter of the way through the year, which is consistent with water table measurements as well as with the end of the rainy season when the aquifer should be at a maximum. The spatial pattern of the amplitude of the annual signal derived from continuous GPS measurements is consistent with the shape of the interferometric fringes. GPS sites both near the NIFZ and in a 20 by 40 km zone within the basin also show significant N-S annual variations that may be related to the differential expansion across the fault. Since these horizontal signals have peak-to-trough amplitudes of 6 mm, they mask the smaller tectonic signals and need to be taken into account when interpreting GPS time series of site position. Moreover, since the groundwater signal appears to have a long-term vertical trend which varies in sign depending on location, it will be difficult to distinguish interseismic tectonic slip along the NIFZ and within the affected areas in the basin.

1998
Sandwell, DT, Price EJ.  1998.  Phase gradient approach to stacking interferograms. Journal of Geophysical Research-Solid Earth. 103:30183-30204.   10.1029/1998jb900008   AbstractWebsite

The phase gradient approach is used to construct averages and differences of interferograms without phase unwrapping. Our objectives for change detection are to increase fringe clarity and decrease errors due to tropospheric and ionospheric delay by averaging many interferograms. The standard approach requires phase unwrapping, scaling the phase according to the ratio of the perpendicular baseline, and finally forming the average or difference; however, unique phase unwrapping is usually not possible. Since the phase gradient due to topography is proportional to the perpendicular baseline, phase unwrapping is unnecessary prior to averaging or differencing. Phase unwrapping may be needed to interpret the results, but it is delayed until all of the largest topographic signals are removed. We demonstrate the method by averaging and differencing six interferograms having a suite of perpendicular baselines ranging from 18 to 406 m. Cross-spectral analysis of the difference between two Tandem interferograms provides estimates of spatial resolution, which are used to design prestack filters. A wide range of perpendicular baselines provides the best topographic recovery in terms of accuracy and coverage. Outside of mountainous areas the topography has a relative accuracy of better than 2 m. Residual interferograms (single interferogram minus stack) have tilts across the unwrapped phase that are typically 50 mm in both range and azimuth, reflecting both orbit error and atmospheric delay. Smaller-scale waves with amplitudes of 15 mm are interpreted as atmospheric lee waves. A few Global Positioning System (GPS) control points within a Game could increase the precision to similar to 20 mm for a single interferogram; further improvements may be achieved by stacking residual interferograms.