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Luttrell, K, Sandwell D.  2006.  Strength of the lithosphere of the Galilean satellites. Icarus. 183:159-167.   10.1016/j.icarus.2006.01.015   AbstractWebsite

Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment Must be less than the saturation bending moment of the yield strength envelope derived front Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths ( similar to 10 km) but extremely smooth, and perhaps conforming to an equipotential Surface, at longer wavelengths ( > 100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere. requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength hand will provide tight constraints on lithospheric strength. (c) 2006 Elsevier Inc. All riahts reserved.

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Neves, MC, Cabral J, Luttrell K, Figueiredo P, Rockwell T, Sandwell D.  2015.  The effect of sea level changes on fault reactivation potential in Portugal. Tectonophysics. 658:206-220.   10.1016/j.tecto.2015.07.023   AbstractWebsite

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5-1 MPa along the Manteigas-Vilarica-Braganca (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas-Vilarica-Braganca fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies. (C) 2015 Elsevier B.V. All rights reserved.

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Price, EJ, Sandwell DT.  1998.  Small-scale deformations associated with the 1992 Landers, California, earthquake mapped by synthetic aperture radar interferometry phase gradients. Journal of Geophysical Research-Solid Earth. 103:27001-27016.   10.1029/98jb01821   AbstractWebsite

The Landers earthquake (M-w 7.3) occurred on June 28, 1992, and ruptured nearly 100 km of previously mapped and unmapped faults in the Mojave Desert. We use synthetic aperture radar interferometry (InSAR) to examine the cumulative surface deformation between April 24 and August 7, 1992, in a 100 x 100 km region surrounding the northern portion of the earthquake rupture. Also, we introduce a technique for manipulating SAR interferograms to extract short-wavelength displacement information. This technique involves computation and subsequent combination of interferometric phase gradient maps. The InSAR results show significant deformation signatures associated with faults, fractures, dry lake beds, and mountainous regions within 75-100 km of the main rupture. Using the phase gradient method, we are able to extract small-scale deformation patterns near the main rupture. Many of the preexisting, mapped faults within 50 km of the main rupture experienced triggered slip; these include the Old Woman, Lenwood, Johnson Valley, West Calico, and Calico Faults. The InSAR results also indicate right-lateral offsets along secondary fractures trending N-NE within the left-lateral zone of shear between the main rupture and the Johnson Valley Fault. Additionally, there are interesting interferogram fringe signatures surrounding Troy Dry Lake and Coyote Dry Lake that are related to deformation of dry lake beds.

S
Sandwell, D, Fialko Y.  2004.  Warping and cracking of the Pacific plate by thermal contraction. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003091   AbstractWebsite

Lineaments in the gravity field and associated chains of volcanic ridges are widespread on the Pacific plate but are not yet explained by plate tectonics. Recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere. We show that the large thermoelastic stress produced by top-down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks. Both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle. Cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite-derived gravity. Our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required.

Sandwell, D, Rosen P, Moore W, Gurrola E.  2004.  Radar interferometry for measuring tidal strains across cracks on Europa. Journal of Geophysical Research-Planets. 109   10.1029/2004je002276   AbstractWebsite

A major uncertainty in understanding the interaction between the surface of Europa and its ocean below is the present-day activity of fractures. Using well-constrained models for tidal strain and a force balance in a cracked shell, we estimate the shear and normal displacement of cracks that penetrate upward from the base of the shell. If more than half of the plate is fractured, then surface displacements having amplitudes of 3 to 30 cm will be localized in a band 1 to 100 km from the crack. Plate spreading will occur if more than similar to85% of the plate is fractured. The pattern of deformation is sensitive to both the percentage of plate that is cracked and the total thickness of the shell. Repeat-pass radar interferometry could easily detect and map the activity of the cracks during a short experiment from a variety of suitable orbits with repeating ground tracks.