Deformation on nearby faults induced by the 1999 Hector Mine earthquake

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.     

Evidence of shallow fault zone strengthening after the 1992 M7.5 landers, california, earthquake

Repeated seismic surveys of the Landers, California, fault zone that ruptured in the magnitude (M) 7.5 earthquake of 1992 reveal an increase in seismic velocity with time. P, S, and fault zone trapped waves were excited by near-surface explosions in two locations in 1994 and 1996, and were recorded on two linear, three-component seismic arrays deployed across the Johnson Valley fault trace. The travel times of P and S waves for identical shot-receiver pairs decreased by 0.5 to 1.5 percent from 1994 to 1996, with the larger changes at stations located within the fault zone. These observations indicate that the shallow Johnson Valley fault is strengthening after the main shock, most likely because of closure of cracks that were opened by the 1992 earthquake. The increase in velocity is consistent with the prevalence of dry over wet cracks and with a reduction in the apparent crack density near the fault zone by approximately 1.0 percent from 1994 to 1996.     

The 2002 Denali fault earthquake,Alaska: a large magnitude,slip-partitioned event

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.     

Viscoelastic flow in the lower crust after the 1992 landers, california, earthquake

Space geodesy showed that broad-scale postseismic deformation occurred after the 1992 Landers earthquake. Three-dimensional modeling shows that afterslip can only explain one horizontal component of the postseismic deformation, whereas viscoelastic flow can explain the horizontal and near-vertical displacements. The viscosity of a weak, about 10-km-thick layer, in the lower crust beneath the rupture zone that controls the rebound is about 10(18) pascal seconds. The viscoelastic behavior of the lower crust may help to explain the extensional structures observed in the Basin and Range province and it may be used for the analysis of earthquake hazard.     

Estimation of fault strength: reconstruction of stress before the 1995 Kobe earthquake

We have estimated the stress field before the 1995 Kobe, Japan, earthquake (moment magnitude 6.9) using in situ post-shock stress measurements obtained from hydraulic fracturing experiments near the fault. We reconstructed the pre-shock stress field using a kinematic source model inverted from seismic waveforms and geodetic deformations. We found that at the center of the fault, two sides of the fault surface coupled completely before the earthquake, with a coefficient of friction of 0.6, which is equivalent to strong crust. At the edge of the fault, a possible aseismic slip is expected to occur from the pre-shock stress orientation.     

Transient uplift after a 17th-century earthquake along the Kuril subduction zone

In eastern Hokkaido, 60 to 80 kilometers above a subducting oceanic plate, tidal mudflats changed into freshwater forests during the first decades after a 17th-century tsunami. The mudflats gradually rose by a meter, as judged from fossil diatom assemblages. Both the tsunami and the ensuing uplift exceeded any in the region's 200 years of written history, and both resulted from a shallow plate-boundary earthquake of unusually large size along the Kuril subduction zone. This earthquake probably induced more creep farther down the plate boundary than did any of the region's historical events.     

Overpressure and fluid flow in the new jersey continental slope: implications for slope failure and cold seeps

Miocene through Pleistocene sediments on the New Jersey continental slope (Ocean Drilling Program Site 1073) are undercompacted (porosity between 40 and 65%) to 640 meters below the sea floor, and this is interpreted to record fluid pressures that reach 95% of the lithostatic stress. A two-dimensional model, where rapid Pleistocene sedimentation loads permeable sandy silt of Miocene age, successfully predicts the observed pressures. The model describes how lateral pressure equilibration in permeable beds produces fluid pressures that approach the lithostatic stress where overburden is thin. This transfer of pressure may cause slope failure and drive cold seeps on passive margins around the world.     

Interaction of the san jacinto and san andreas fault zones, southern california: triggered earthquake migration and coupled recurrence intervals

Two lines of evidence suggest that large earthquakes that occur on either the San Jacinto fault zone (SJFZ) or the San Andreas fault zone (SAFZ) may be triggered by large earthquakes that occur on the other. First, the great 1857 Fort Tejon earthquake in the SAFZ seems to have triggered a progressive sequence of earthquakes in the SJFZ. These earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1.7 kilometers per year along the SJFZ after the 1857 earthquake. Second, the similarity in average recurrence intervals in the SJFZ (about 150 years) and in the Mojave segment of the SAFZ (132 years) suggests that large earthquakes in the northern SJFZ may stimulate the relatively frequent major earthquakes on the Mojave segment. Analysis of historic earthquake occurrence in the SJFZ suggests little likelihood of extended quiescence between earthquake sequences.     

Swimming against the flow: a mechanism of zooplankton aggregation

Zooplankton reside in a constantly flowing environment. However, information about their response to ambient flow has remained elusive, because of the difficulties of following the individual motions of these minute, nearly transparent animals in the ocean. Using a three-dimensional acoustic imaging system, we tracked >375,000 zooplankters at two coastal sites in the Red Sea. Resolution of their motion from that of the water showed that the animals effectively maintained their depth by swimming against upwelling and downwelling currents moving at rates of up to tens of body lengths per second, causing their accumulation at frontal zones. This mechanism explains how oceanic fronts become major feeding grounds for predators and targets for fishermen.     

A Fluorite Isotype of SnO2 and a New Modification of TiO2: Implications for the Earth's Lower Mantle

The existence of a cubic fluorite-type SnO(2) and a hexagonal TiO(2) (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO(2) (fluorite) is a = 4.925 +/- 0.005 angstroms and those for TiO(2) (fluorite-related) are a = 9.22 +/- 0.01 angstroms and c = 5.685 +/- 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about -8 percent for SnO(2) and -10.5 percent for TiO(2) at transition. Upon release of pressure, both the fluorite-type SnO(2) and the TiO(2) reverted to the alpha-PbO(2) structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO(2) to possess the fluorite structure or its related forms at high pressure, as shown for SnO(2) and TiO(2) in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here.     

Footwall refrigeration along a detachment fault: implications for the thermal evolution of core complexes

Oxygen isotope compositions of epidote and quartz from chloritic breccias that underlie the detachment fault in the metamorphic core complex of the Whipple Mountains yielded quartz-epidote fractionations that range from 4.1 to 6.4 per mil and increase systematically toward the fault. These fractionations give mean temperatures that decrease from approximately 432 degrees C at 50 meters below the fault to approximately 350 degrees C at 12 meters below the fault. This extreme thermal gradient of 82 degrees C over 38 meters (2160 degrees C per kilometer) is best explained by advective heat extraction by means of circulating surface-derived fluids. Models of lithospheric extension consider only conductive cooling resulting from tectonic denudation and thus require revision to include fluid-induced fault-zone refrigeration.     

Deep-seated gas emission induced by the Earth tide: a basic observation for geochemical earthquake prediction

Variations of the helium/argon ratio of gas bubbles in a mineral spring along a fault zone coincide with fluctuations of areal dilation induced by the earth tide. This observation suggests that deep-seated gases characterized by higher heliumlargon ratios are squeezed out by stress preceding an earthquake.     

Melting of (Mg, Fe)SiO3-Perovskite to 625 Kilobars: Indication of a High Melting Temperature in the Lower Mantle

The melting curves of two compositions of (Mg,Fe) SiO3-perovskite, the likely dominant mineral phase in the lower mantle, have been measured in a C02 laser-heated diamond cell with direct temperature measurements and in situ detection of melting. At 625 kilobars, the melting temperature is 5000 +/- 200 kelvin, independent of composition. Extrapolation to the core-mantle boundary pressure of 1.35 megabar with three different melting relations yields melting temperatures between 7000 and 8500 kelvin. Thus, the temperature at the base of the lower mantle, accepted to lie between 2550 and 2750 kelvin, is only at about one-third of the melting temperature. The large difference between mantle temperature and corresponding melting temperature has several important implications; particularly the temperature sensitivity of the viscosity is reduced thus allowing large lateral temperature variations inferred from seismic tomographic velocity anomalies and systematics found in measured velocity-density functions. Extensive melting of the lower mantle can be ruled out throughout the history of the Earth.     

Earthquake in a maze: compressional rupture branching during the 2012 M(w) 8.6 Sumatra earthquake

Seismological observations of the 2012 moment magnitude 8.6 Sumatra earthquake reveal unprecedented complexity of dynamic rupture. The surprisingly large magnitude results from the combination of deep extent, high stress drop, and rupture of multiple faults. Back-projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system, with relatively slow rupture speed. The east-southeast-west-northwest ruptures add a new dimension to the seismotectonics of the Wharton Basin, which was previously thought to be controlled by north-south strike-slip faulting. The rupture turned twice into the compressive quadrant, against the preferred branching direction predicted by dynamic Coulomb stress calculations. Orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure-insensitive strength of the deep oceanic lithosphere.     

Crustal Spreading in Southern California: The Imperial Valley and the Gulf of California formed by the rifting apart of a continental plate

The current excitement among geologists and geophysicists stemming from the "new global tectonics" has led to a widespread, speculative reinterpretation of continental geology. The Gulf of California and its continuation into the Imperial Valley provide an excellent opportunity for studying the border zone between the North American and Pacific plates, and an interface of continental and oceanic tectonics. The Salton trough, the landward extension of the gulf, is a broad structural depression, comparable in size with the deeper marine basins of the southern part of the gulf, but here partially filled with sediments deposited by the Colorado River.     

Magnitude of shear stress on the san andreas fault: implications of a stress measurement profile at shallow depth

A profile of measurements of shear stress perpendicular to the San Andreas fault near Palmdale, California, shows a marked increase in stress with distance from the fault. The pattern suggests that shear stress on the fault increases slowly with depth and reaches a value on the order of the average stress released during earthquakes. This result has important implications for both long- and shortterm prediction of large earthquakes.