Publications

Export 3 results:
Sort by: [ Author  (Asc)] Title Type Year
A B C D E F G H I [J] K L M N O P Q R S T U V W X Y Z   [Show ALL]
J
Jacobs, A, Sandwell D, Fialko Y, Sichoix L.  2002.  The 1999 (M-w 7. 1) Hector Mine, California, earthquake: Near-field postseismic deformation from ERS interferometry. Bulletin of the Seismological Society of America. 92:1433-1442.   10.1785/0120000908   AbstractWebsite

Interferometric synthetic aperture radar (InSAR) data over the area of the Hector Mine earthquake (M-w 7.1, 16 October 1999) reveal postseismic deformation of several centimeters over a spatial scale of 0.5 to 50 km. We analyzed seven SAR acquisitions to form interferograms over four time periods after the event. The main deformations seen in the line-of-sight (LOS) displacement maps are a region of subsidence (60 mm LOS increase) on the northern end of the fault, a region of uplift (45 mm LOS decrease) located to the northeast of the primary fault bend, and a linear trough running along the main rupture having a depth of up to 15 mm and a width of about 2 km. We correlate these features with a double left-bending, right-lateral, strike-slip fault that exhibits contraction on the restraining side and extension along the releasing side of the fault bends. The temporal variations in the near-fault postseismic deformation are consistent with a characteristic time scale of 135 + 42 or - 25 days, which is similar to the relaxation times following the 1992 Landers earthquake. High gradients in the LOS displacements occur on the fault trace, consistent with afterslip on the earthquake rupture. We derive an afterslip model by inverting the LOS data from both the ascending and descending orbits. Our model indicates that much of the afterslip occurs at depths of less than 3 to 4 km.

Johnson, CL, Sandwell DT.  1992.  Joints in Venusian Lava Flows. Journal of Geophysical Research-Planets. 97:13601-13610. AbstractWebsite

Venusian plains regions, as imaged by the Magellan spacecraft, display many styles of tectonic and volcanic deformation. Radar images of several areas of the volcanic plains reveal polygonal patterns of bright lineations, Intersection geometries of the lineations defining the polygonal patterns are typical of those found in tensile networks. In addition, the polygonal patterns generally exhibit no preferred orientation, implying that they are the result of horizontally isotropic stress fields. Such stress fields usually arise on the Earth as a consequence of desiccation, freeze-thaw cycles, or cooling and produce mud cracks, ice-wedge polygons, and columnar joints, respectively. We propose that the polygonal patterns seen in the Magellan images of some of the volcanic plains are the result of thermal stresses. We consider two alternative scenarios which would generate sufficient tensile thermal stresses Lo cause failure. The first scenario is that of a cooling lava flow; the residual thermal stress which would develop (assuming no failure of the rock) is tensional and of the order of 400 MPa. This is much greater than the strength of unfractured terrestrial basalt (approximately 10 MPa), so we can expect joints to form during cooling of Venusian lava flows. However, the spacing of the polygonal lineations seen in Magellan images is typically 1-2 km, much larger than the largest spacings of decimeters for joints in terrestrial lavas. The second scenario involves an increased heat flux to the base of the lithosphere; the resulting thermal stresses cause the upper lithosphere to be in tension and the lower lithosphere to be in compression. Brittle tensile failure occurs near the surface due to the finite yield strength of the lithosphere. The maximum depth to which failure occurs increases with increasing elevation of the temperature gradient. For an initially 25-km-thick lithosphere and temperature gradient of ll-degrees/km, this maximum depth varies from 0.5 km to 2 km as the temperature gradient is increased to 12-degrees/km and 22-degrees/km, respectively. Both the cooling flow scenario and the heated lithosphere scenario produce isotropic tensile surface stress patterns, but the heated lithosphere model is more compatible with the kilometer scale of the polygonal patterns seen in Magellan images.

Johnson, CL, Sandwell DT.  1994.  Lithospheric Flexure on Venus. Geophysical Journal International. 119:627-647.   10.1111/j.1365-246X.1994.tb00146.x   AbstractWebsite

Topographic flexural signatures on Venus are generally associated with the outer edges of coronae, with some chasmata and with rift zones. Using Magellan altimetry profiles and grids of venusian topography, we identified 17 potential flexure sites. Both 2-D cartesian, and 2-D axisymmetric, thin-elastic plate models were used to establish the flexural parameter and applied load/bending moment. These parameters can be used to infer the thickness, strength and possibly the dynamics of the venusian lithosphere. Numerical simulations show that the 2-D model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter. However, an axisymmetric model must be used to obtain a reliable estimate of load/bending moment. 12 of the 17 areas were modelled with a 2-D thin elastic plate model, yielding best-fit effective elastic thicknesses in the range 12 to 34 km. We find no convincing evidence for flexure around smaller coronae, though five possible candidates have been identified. These five features show circumferential topographic signatures which, if interpreted as flexure, yield mean elastic thicknesses ranging from 6 to 22 km. We adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on Venus are similar to, or lower than, strain rates on Earth. Many of the flexural signatures correspond to relatively high plate-bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow, respectively. For areas where the curvatures are not too extreme, the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere. The large amplitude flexures in Aphrodite Terra predict complete failure of the plate, rendering mechanical thickness estimates from these features unreliable. One smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data. Reliable mechanical thicknesses found by forward modelling in this study are 21 km-37 km, significantly greater than the 13 km-20 km predictions based on heat-flow scaling arguments and chondritic thermal models. If the modelled topography is the result of lithospheric flexure, then our results for mechanical thickness, combined with the lack of evidence for flexure around smaller features, are consistent with a venusian lithosphere somewhat thicker than predicted. Dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere, also consistent with low temperature gradients. Recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed. Corresponding time-scales for gravitational relaxation are 10(8)-10(9) yr, making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features. If dry olivine is also found to be stronger than previously believed, the mechanical thickness estimates for Venus will be reduced, and will be more consistent with the predictions of global heat scaling models.