Publications

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2015
Tong, XP, Sandwell DT, Smith-Konter B.  2015.  An integral method to estimate the moment accumulation rate on the Creeping Section of the San Andreas Fault. Geophysical Journal International. 203:48-62.   10.1093/gji/ggv269   AbstractWebsite

Moment accumulation rate (also referred to as moment deficit rate) is a fundamental quantity for evaluating seismic hazard. The conventional approach for evaluating moment accumulation rate of creeping faults is to invert for the slip distribution from geodetic measurements, although even with perfect data these slip-rate inversions are non-unique. In this study, we show that the slip-rate versus depth inversion is not needed because moment accumulation rate can be estimated directly from surface geodetic data. We propose an integral approach that uses dense geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and the Global Positioning System (GPS) to constrain the moment accumulation rate. The moment accumulation rate is related to the integral of the product of the along-strike velocity and the distance from the fault. We demonstrate our methods by studying the Creeping Section of the San Andreas fault observed by GPS and radar interferometry onboard the ERS and ALOS satellites. Along-strike variation of the moment accumulation rate is derived in order to investigate the degree of partial locking of the Creeping Section. The central Creeping Segment has a moment accumulation rate of 0.25-3.1 x 10(15) Nm yr(-1) km(-1). The upper and lower bounds of the moment accumulation rates are derived based on the statistics of the noise. Our best-fitting model indicates that the central portion of the Creeping Section is accumulating seismic moment at rates that are about 5 per cent to 23 per cent of the fully locked Carrizo segment that will eventually be released seismically. A cumulative moment budget calculation with the historical earthquake catalogue (M > 5.5) since 1857 shows that the net moment deficit at present is equivalent to a M-w 6.3 earthquake.

2010
Marks, KM, Smith WHF, Sandwell DT.  2010.  Evolution of errors in the altimetric bathymetry model used by Google Earth and GEBCO. Marine Geophysical Research. 31:223-238.   10.1007/s11001-010-9102-0   AbstractWebsite

We analyze errors in the global bathymetry models of Smith and Sandwell that combine satellite altimetry with acoustic soundings and shorelines to estimate depths. Versions of these models have been incorporated into Google Earth and the General Bathymetric Chart of the Oceans (GEBCO). We use Japan Agency for Marine-Earth Science and Technology (JAMSTEC) multibeam surveys not previously incorporated into the models as "ground truth" to compare against model versions 7.2 through 12.1, defining vertical differences as "errors." Overall error statistics improve over time: 50th percentile errors declined from 57 to 55 to 49 m, and 90th percentile errors declined from 257 to 235 to 219 m, in versions 8.2, 11.1 and 12.1. This improvement is partly due to an increasing number of soundings incorporated into successive models, and partly to improvements in the satellite gravity model. Inspection of specific sites reveals that changes in the algorithms used to interpolate across survey gaps with altimetry have affected some errors. Versions 9.1 through 11.1 show a bias in the scaling from gravity in milliGals to topography in meters that affected the 15-160 km wavelength band. Regionally averaged (> 160 km wavelength) depths have accumulated error over successive versions 9 through 11. These problems have been mitigated in version 12.1, which shows no systematic variation of errors with depth. Even so, version 12.1 is in some respects not as good as version 8.2, which employed a different algorithm.

1999
Baer, G, Sandwell D, Williams S, Bock Y, Shamir G.  1999.  Coseismic deformation associated with the November 1995, M-w=7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), detected by synthetic aperture radar interferometry. Journal of Geophysical Research-Solid Earth. 104:25221-25232.   10.1029/1999jb900216   AbstractWebsite

The November 22, 1995, M-w=7.1 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of flat (Aqaba). Although it was the largest earthquake along this fault in the last few centuries, little is yet known about the geometry of the rupture, the slip distribution along it, and the nature of postseismic deformation following the main shock. In this study we examine the surface deformation pattern during the coseismic phase of the earthquake in an attempt to better elucidate the earthquake rupture process. As the entire rupture zone was beneath the waters of the Gulf, and there is very little Global Positioning System (GPS) data available in the region for the period spanning the earthquake, interferometric synthetic aperture radar (INSAR) provides the only source of information of surface deformation associated with this earthquake. We chose four synthetic aperture radar (SAR) scenes of about 90x90 km each spanning the rupture area, imaged by the ERS-1 and ERS-2 satellites. The coseismic interferograms show contours of equal satellite-to-ground range changes that correspond to surface displacements due to the earthquake rupture. Interferograms that span the earthquake by 1 week show similar fringe patterns' as those that span the earthquake by 6 months, suggesting that postseismic deformation is minor or confined to the first week after the earthquake. A high displacement gradient is seen on the western side of the Gulf, 20-40 km south of flat and Aqaba, where the total satellite-to-ground range changes are at least 15 cm. The displacement gradient is relatively uniform on the eastern side of the Gulf and the range changes are less than 10 cm. To interpret these results, we compare them to synthetic interferograms generated by elastic dislocation models with a variety of fault parameters. Although selecting the best fit fault parameters is nonunique, we are able to generate a group of simplified model interferograms that provide a reasonable fit to the coseismic interferogram and serve to constrain the location of the fault. The present analysis shows that if the rupture reached the Gulf-bottom surface, the mean sinistral slip along the fault is constrained to about 1.4 m. If surface rupture did not occur, the average sinistral slip is constrained to the range of 1.4-3 m for a fault patch buried 0-4 km below the Gulf-bottom Surface, respectively, with a minor normal component.