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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 ∼6 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.

The original kinematic plate tectonic model proposed that the outer shell (lithosphere) of the Earth is divided into a small number of nearly rigid plates that slide over the weak asthenosphere. The plates are the surface thermal boundary layer of upper-mantle or global-scale mantle convection, and the descending slabs are the primary active components of the convective system. The plate boundaries are generally narrow and are characterized by earthquakes and volcanoes. The plates, however, are not really rigid or undeformable, and the plate boundaries need not be localized (Dewey and Bird, 1970; Gordon, 2000; Anderson, 2001, 2002; Steinberger et al., 2004). Not only is the plate-slab system the main driver of plate tectonics and mantle convection, but much of the energy dissipation may be in this part of the system as well, rather than in mantle viscosity (Conrad and Hager, 2001). Plates may be held together by lateral compression-or absence of extension-rather than by strength or rigidity; rocks are strong in compression but have little resistance to extension.

Crustal movements and deformation within the diffuse Pacifi c‐North America (Pa-NA) plate boundary are dominated by the right-lateral motion between the two plates. By using the Pa-NA pole of rotation (PoR) spherical coordinate system, we decompose observed crustal movements into parallel and normal components to the Pa-NA plate motion. We transformed the 840 velocity vectors of the Southern California Earthquake Center (SCEC) 3.0 velocity fi eld into the Pa-NA PoR system in order to characterize the interseismic velocity across the plate boundary. Our results show that despite the very different deformation styles occurring across the San Andreas fault, the fault trace follows the half plate motion contour. Deviation occurs in the southern section, where the half motion contour correlates with the San Jacinto and Imperial fault segments. Our analysis yields interesting asymmetric patterns in both parallel and normal components. The parallel component shows asymmetrical velocity gradients across the San Andreas fault, and the normal component indicates compression southwest of the Big Bend, but not northeastward. The observations are compared with viscoelastic modeling results, which show a similar velocity fi eld. The main disagreements between the observations and the model are in a narrow band along the San Andreas fault and in the Mojave block, suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models.

Gravity maps derived from satellite altimeter measurements provide unprecedented me. Dium-resolution coverage of sparsely surveyed mid-ocean ridges in the southern oceans. A spectral analysis of 76 000 km of coincident shipboard and satellite gravity measurements shows that satellite altimeters can accurately resolve features with half-wavelengths as short as 13 km. The coverage and resolution of these gravity data allow us to determine accurately both the location of poorly charted ridge axes and the variation in axial anomaly character along the ridge axis, although their detailed morphology is not resolved. The results of this study support earlier studies that showed a transition from spreading-rate-dependent axial gravity lows to rate-independent axial highs with increasing spreading rate. Four such transitions are imaged on the Southeast Indian Ridge and Pacific Antarctic Ridge. We expect that these transitions are the result of a temperature-sensitive threshold phenomenon and may be influenced by nearby hot spots.