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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.

Baer, G, Shamir G, Sandwell D, Bock Y.  2001.  Crustal deformation during 6 years spanning the M (sub w) = 7.2 1995 Nuweiba earthquake, analyzed by Interferometric Synthetic Aperture Radar. Israel Journal of Earth-Sciences. 50( Baer G, Wdowinski S, Eds.).:9-22., Jerusalem, Israel (ISR): Laser Pages Publishing, Jerusalem AbstractWebsite

The November 22, 1995, M (sub w) = 7.2 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of Elat (Aqaba). We examine the surface deformation patterns in the region by Interferometric Synthetic Aperture Radar (InSAR) for the period 1993 to 1999, which includes the end of one seismic cycle and the beginning of the next. Because the main rupture was under water, ERS coverage is limited to distances of approximately 5 km or more away from the rupture. Pre-earthquake interferograms do not show any detectable deformation along the Gulf. Coseismic interferograms show deformation at distances of up to 50 km from the main rupture, with the highest fringe rate (strain) NW of the rupture termination. Coseismic phase gradient maps show triggered slip along faults parallel to the main rupture (sinistral or normal with the Gulf side down) along the western shore of the Gulf, and in a belt of extensional faults along the eastern shore, striking at angles of about 30 degrees to the major rupture. Postseismic deformation is observed only in a time window of up to 6 months following the mainshock. It was concentrated in the region of the high coseismic strain, and seems to be related to the M (sub L) <4.5 aftershocks in the respective time window.

Baer, G, Schattner U, Wachs D, Sandwell D, Wdowinski S, Frydman S.  2002.  The lowest place on Earth is subsiding - An InSAR (interferometric synthetic aperture radar) perspective. Geological Society of America Bulletin. 114:12-23.   10.1130/0016-7606(2002)114<0012:tlpoei>;2   AbstractWebsite

Since the early 1990s, sinkholes and wide, shallow subsidence features (WSSFs) have become major problems along the Dead Sea shores in Israel and Jordan. Sinkholes are readily observed in the field, but their locations and timing are unpredictable. WSSFs are often difficult to observe in the field. However, once identified, they delineate zones of instability and increasing hazard. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the European Remote Sensing ERS-1 and ERS-2 satellites. The interferograms span periods of between 2 and 71 months. WSSFs are observed in the Lisan Peninsula and along the Dead Sea shores, in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt-diapir margins. Phase differences measured in our interferograms correspond to subsidence rates generally in the range of 0-20 mm/yr within the studied period, with exceptional high rates that exceed 60 mm/yr in two specific regions. During the study period, the level of the Dead Sea and of the associated ground water has dropped by similar to6 m. This water-level drop within an aquifer overlying fine-grained, marly layers, would be expected to have caused aquifer-system consolidation resulting in gradual subsidence. Comparison of our InSAR observations with calculations of the expected consolidation shows that in areas where marl layers are known to compose part of the upper 30 m of the profile, estimated consolidation settlements are of the order of the measured subsidence. Our observations also show that in certain locations, subsidence appears to be structurally controlled by faults, seaward landslides, and salt domes. Gradual subsidence is unlikely to be directly related to the sinkholes, excluding the use of the WSSFs features as predictable precursors to sinkhole formation.

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.

Bassett, D, Sandwell DT, Fialko Y, Watts AB.  2016.  Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake. Nature. 531:92-96.: Nature Publishing Group   10.1038/nature16945   Abstract

The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south). The megathrust north of the Median Tectonic Line is interseismically locked, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake. In contrast, the megathrust south of this boundary has higher rates of interseismic creep, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 20111. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.

Becker, JJ, Sandwell DT, Smith WHF, Braud J, Binder B, Depner J, Fabre D, Factor J, Ingalls S, Kim SH, Ladner R, Marks K, Nelson S, Pharaoh A, Trimmer R, Von Rosenberg J, Wallace G, Weatherall P.  2009.  Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy. 32:355-371.   10.1080/01490410903297766   AbstractWebsite

A new 30-arc second resolution global topography/bathymetry grid (SRTM30_PLUS) has been developed from a wide variety of data sources. Land and ice topography comes from the SRTM30 and ICESat topography, respectively. Ocean bathymetry is based on a new satellite-gravity model where the gravity-to-topography ratio is calibrated using 298 million edited soundings. The main contribution of this study is the compilation and editing of the raw soundings, which come from NOAA, individual scientists, SIO, NGA, JAMSTEC, IFREMER, GEBCO, and NAVOCEANO. The gridded bathymetry is available for ftp download in the same format as the 33 tiles of SRTM30 topography. There are 33 matching tiles of source identification number to convey the provenance of every grid cell. The raw sounding data, converted to a simple common format, are also available for ftp download.

Becker, JJ, Sandwell DT.  2008.  Global estimates of seafloor slope from single-beam ship soundings. Journal of Geophysical Research-Oceans. 113   10.1029/2006jc003879   AbstractWebsite

Rough topography on the ocean floor is a source of ocean mixing which is of interest to both physical oceanography and climate science. Most mixing has been attributed to high slopes of the large-scale structures of the deep ocean floor such as seamounts, continental margins, and mid-ocean ridge axes. In this paper, we show the small-scale but ubiquitous abyssal hills and fracture zones dominate the global map of rough topography. Much of this rugged seafloor occurs in the Southern Ocean on the flanks of the Pacific-Antarctic Rise and Southwest Indian Ridge. We present our results as a global map of the mean slope of the ocean floor, and as a global map of the ocean floor above the M(2) critical slope. We compare our results to multibeam and satellite bathymetry data to show that satellite bathymetry is not a valid proxy for multibeam measurements, but edited single-beam sonar data are adequate to provide a global perspective on features with horizontal wavelengths as small as 2 km.

Brooks, BA, Foster J, Sandwell D, Wolfe CJ, Okubo P, Poland M, Myer D.  2008.  Magmatically triggered slow slip at Kilauea volcano, Hawaii. Science. 321:1177-1177.   10.1126/science.1159007   AbstractWebsite

We demonstrate that a recent dike intrusion probably triggered a slow fault-slip event (SSE) on Kilauea volcano's mobile south flank. Our analysis combined models of Advanced Land Observing Satellite interferometric dike-intrusion displacement maps with continuous Global Positioning System (GPS) displacement vectors to show that deformation nearly identical to four previous SSEs at Kilauea occurred at far-field sites shortly after the intrusion. We model stress changes because of both secular deformation and the intrusion and find that both would increase the Coulomb failure stress on possible SSE slip surfaces by roughly the same amount. These results, in concert with the observation that none of the previous SSEs at Kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation, suggest that both extrinsic (intrusion-triggering) and intrinsic (secular fault creep) fault processes can lead to SSEs.