Publications

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Journal Article
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.

Myer, D, Sandwell D, Brooks B, Foster J, Shimada M.  2008.  Inflation along Kilauea's Southwest Rift Zone in 2006. Journal of Volcanology and Geothermal Research. 177:418-424.   10.1016/j.jvolgeores.2008.06.006   AbstractWebsite

We report on InSAR and GPS results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20 years. Two independent interferograms (May 2-August 2, 2006 and June 22-Nov 7, 2006) from the ALOS PALSAR instrument reveal domal uplift located southwest of the main caldera. The uplift is bounded on the northeast by the caldera and follows the southwest rift zone for about 12 km. It is approximately 8 km wide. We use data derived from permanent GPS stations to calibrate the InSAR displacement data and estimate uplift of 7.7 cm during the first interferogram and 8.9 cm during the second with line-of-sight volumes of 2.8 x 10(6) m(3) and 3.0 X 10(6) m(3) respectively. The earthquake record for the periods before, during, and after inflation shows that a swarm of shallow earthquakes (z<5 km) signaled the beginning of the uplift and that elevated levels of shallow seismicity along the rift zones occurred throughout the uplift period. GPS data indicate that the inflation occurred steadily over nine months between mid-January and mid-October, 2006 making injection of a sill unlikely. We attribute the inflation to recharge of a shallow ductile area under the SWRZ. (c) 2008 Elsevier B.V. All rights reserved.

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).

DeSanto, JB, Chadwell CD, Sandwell DT.  2019.  Kinematic post-processing of ship navigation data using precise point positioning. Journal of Navigation. 72:795-804.   10.1017/s0373463318000887   AbstractWebsite

Seafloor geodetic studies such as Global Positioning System (GPS)-Acoustic experiments often require the measurement platform on the sea surface to be positioned accurately to within a few centimetres. In this paper, we test the utility of Precise Point Positioning (PPP) for this application with two experiments. The first fixed platform experiment is a comparison between three independent processing software packages: Positioning and Navigation Data Analyst (PANDA), Global Navigation Satellite System-Inferred Positioning System and Orbit Analysis Simulation Software (GIPSY-OASIS), and the Canadian Spatial Reference System (CSRS)) and a more accurate solution based on conventional differential processing of a remote GPS station in the Aleutian Islands. The second moving platform experiment is a comparison among the three PPP software packages using 40 hours of ship navigation data collected during the Roger Revelle RR1605 cruise 170 nautical miles southwest of Palau in May 2016. We found the PPP solutions were repeatable to 5 center dot 49 cm in the horizontal components and 12 center dot 4 cm in the vertical component. This demonstrates not only that PPP is a useful tool for positioning marine platforms in remote locations, but also that modern ship navigation instruments such as the Kongsberg Seapath 330 + are suitable for seafloor geodetic application.

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.

Xu, XH, Ward LA, Jiang JL, Smith-Konter B, Tymofyeyeva E, Lindsey EO, Sylvester AG, Sandwell DT.  2018.  Surface creep rate of the southern San Andreas Fault modulated by stress perturbations from nearby large events. Geophysical Research Letters. 45:10259-10268.   10.1029/2018gl080137   AbstractWebsite

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long-term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992-1999, ENVISAT 2003-2010, and Sentinel-1 2014-present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2-7 over the past few decades. We consider quasi-static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long-lasting effect on SSAF creep rates. Plain Language Summary There are two significant conclusions from this study. First, we analyzed 25 years of InSAR measurements over the Southern San Andreas Fault system to document a major increase in the average creep rate following the 1992 Mw 7.3 Landers Earthquake which is then followed by creep rate reductions after the 1999 Mw 7.1 Hector Mine Earthquake and the 2010 Mw 7.2 El Major Cucapah Earthquake. Second, we attribute all these creep rate changes to the Coulomb stress variations from these three major Earthquakes. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers and lower rates after the other two events. An implication of this study is that small but steady Coulomb stress changes have a larger impact on shallow creep than the larger dynamic stress changes associated with passing seismic waves. These results illuminate the significance of time scale-dependent complexity of shallow fault creep and how these behaviors are communicated by stress perturbations from regional earthquakes.

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.