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The 2010Mw7.2ElMayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the coldMesozoic arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the Salton Trough. We use five years of global positioning system measurements to invert for afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ~65 km below the Peninsular Ranges and ~32 km beneath the Salton Trough. These depths are shallower than the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm.

The 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake in Baja, California, and Sonora, Mexico, had primarily right-lateral strike-slip motion and a minor normal-slip component. The surface rupture extended about 120 km in a NW-SE direction, west of the Cerro Prieto fault. Here we use geodetic measurements including near- to far-field GPS, interferometric synthetic aperture radar (InSAR), and subpixel offset measurements of radar and optical images to characterize the fault slip during the EMC event. We use dislocation inversion methods and determine an optimal nine-segment fault geometry, as well as a subfault slip distribution from the geodetic measurements. With systematic perturbation of the fault dip angles, randomly removing one geodetic data constraint, or different data combinations, we are able to explore the robustness of the inferred slip distribution along fault strike and depth. The model fitting residuals imply contributions of early postseismic deformation to the InSAR measurements as well as lateral heterogeneity in the crustal elastic structure between the Peninsular Ranges and the Salton Trough. We also find that with incorporation of near-field geodetic data and finer fault patch size, the shallow slip deficit is reduced in the EMC event by reductions in the level of smoothing. These results show that the outcomes of coseismic inversions can vary greatly depending on model parameterization and methodology.

Consideration of the three-dimensional heterogeneity of mantle rheology allows models of viscoelastic relaxation following the 2004 Sumatra-Andaman earthquake to simultaneously fit both the observed far-field and near-field postseismic deformation. We use horizontal and vertical campaign and continuous GPS observations from the Andaman, Nicobar, and Sumatran forearc islands, mainland Sumatra, Thailand, the Malay Peninsula, the Indian Ocean, and southern India, spanning the first five years of postseismic deformation. The postseismic relaxation models consider contributions from the 2004 Mw 9.2 Sumatra-Andaman, the 2005 Mw 8.7 Nias, and 2007 Mw 8.4 Bengkulu earthquakes. Far-field motions to the east of the ruptures are equally well fit by homogeneous or laterally variable earth models. However, only models with contrasting rheology across the subducting slab, a ten-times higher mantle viscosity under the Indian Ocean lithosphere than the backarc mantle, can also produce the observed enduring postseismic uplift along the forearc and lack of far-field transient displacements in southern India. While postseismic uplift of forearc stations can also be produced by rapid and enduring down-dip afterslip, the inferred rheology structure is consistent with the distribution of mantle temperature inferred from seismic tomography.

Author(s): Andrew M. Freed (corresponding author) [1]; Roland Bürgmann [2] Studies of the Earth's response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress [...]

A sequence of four [M.sub.w] > 6 earthquakes, including the 1992 [M.sub.w] = 7.3 Landers and [M.sub.w] = 7.1 Hector Mine earthquakes, occurred in the Mojave Desert in the 1990s in close proximity to the southern San Andreas fault, inducing stress changes on several of its segments. We calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and/or upper mantle has led to a Coulomb stress increase of 2.3-3.5 bar on the San Bernardino Mountain segment of the southern San Andreas fault between 1992 and 2001, with a projected increase of 3.6-4.9 bar by the year 2020. In comparison, the calculated coseismic stress increase is 1.8 bar for this segment. This accelerated buildup of stresses is predicted to bring the San Bernardino Mountain segment, which last ruptured more than 190 yr ago, closer to a potentially major rupture. Meanwhile we project a net stress decrease of as much as -3.5 bar between 1992 and 2020 for the western Coachella Valley segment if the fault is governed by low effective friction, or an increase of 1.5 bar if the fault is governed by high effective friction. Coulomb stresses are calculated to decrease on the Mojave segment by as much as -1 bar between 1992 and 2020. Accelerated stress buildup is also predicted to occur on parts of the San Jacinto, Elsinore, and Calico faults. The pattern of the observed post-Landers aftershock clustering and the calculated Coulomb stress buildup on the Calico fault is similar to that noted in the Hector Mine region prior to the 1999 [M.sub.w] = 7.1 earthquake. These results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust, many years later through viscoelastic processes. Keywords: earthquake stress triggering, viscoelastic deformation, Landers earthquake, San Andreas fault, southern California.

Author(s): Andrew M. Freed (corresponding author) [1]; Jian Lin [2] Stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences [...]

The 2010 Mw7.2 El Mayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the cold Mesozoic arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the Salton Trough. We use five years of global positioning system measurements to invert for afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ~65 km below the Peninsular Ranges and ~32 km beneath the Salton Trough. These depths are shallower than the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm. [ABSTRACT FROM AUTHOR]

Orbital images of Mercury obtained by the MESSENGER spacecraft have revealed families of troughs, interpreted to be graben, on volcanic plains material that largely or completely buried preexisting craters and basins. The graben are partially to fully encircled by rings of contractional wrinkle ridges localized over the rims of the buried impact features to form systems of associated contractional and extensional landforms. Most of the buried craters and basins with graben identified to date are located in the extensive volcanic plains that cover much of Mercury's northern high latitudes. The distinctive relationship between wrinkle ridges and graben in buried craters and basins on Mercury is interpreted to be the result of a combination of extensional stresses from cooling and thermal contraction of thick lava flow units and compressional stresses from cooling and contraction of the planet's interior. [ABSTRACT FROM AUTHOR]