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Lindsey, EO, Fialko Y, Bock Y, Sandwell DT, Bilham R.  2014.  Localized and distributed creep along the southern San Andreas Fault. Journal of Geophysical Research-Solid Earth. 119:7909-7922.   10.1002/2014jb011275   AbstractWebsite

We investigate the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey-mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003-2010 were used to separate the pattern of horizontal and vertical motion, providing a high-resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates. Key PointsShallow creep is pervasive along the southernmost 50 km of the San Andreas FaultCreep is localized only along transpressional fault segmentsIn transtensional areas, creep is distributed over a 1-2 km wide fault zone

Tong, XP, Smith-Konter B, Sandwell DT.  2014.  Is there a discrepancy between geological and geodetic slip rates along the San Andreas Fault System? Journal of Geophysical Research-Solid Earth. 119:2518-2538.   10.1002/2013jb010765   AbstractWebsite

Previous inversions for slip rate along the San Andreas Fault System (SAFS), based on elastic half-space models, show a discrepancy between the geologic and geodetic slip rates along a few major fault segments. In this study, we use an earthquake cycle model representing an elastic plate over a viscoelastic half-space to demonstrate that there is no significant discrepancy between long-term geologic and geodetic slip rates. The California statewide model includes 41 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known historical ruptures and geologic recurrence intervals. The slip rates are constrained by 1981 secular velocity measurements from GPS and L-band intereferometric synthetic aperture radar. A model with a thick elastic layer (60 km) and half-space viscosity of 10(19)Pa s is preferred because it produces the smallest misfit to both the geologic and the geodetic data. We find that the geodetic slip rates from the thick plate model agrees to within the bounds of the geologic slip rates, while the rates from the elastic half-space model disagree on specific important fault segments such as the Mojave and the North Coast segment of the San Andreas Fault. The viscoelastic earthquake cycle models have generally higher slip rates than the half-space model because most of the faults along the SAFS are late in the earthquake cycle, so today they are moving slower than the long-term cycle-averaged velocity as governed by the viscoelastic relaxation process.

Kaneko, Y, Fialko Y, Sandwell DT, Tong X, Furuya M.  2013.  Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties. Journal of Geophysical Research-Solid Earth. 118:316-331.   10.1029/2012jb009661   AbstractWebsite

We present high-resolution measurements of interseismic deformation along the central section of the North Anatolian Fault (NAF) in Turkey using interferometric synthetic aperture radar data from the Advanced Land Observing Satellite and Envisat missions. We generated maps of satellite line-of-sight velocity using five ascending Advanced Land Observing Satellite tracks and one descending Envisat track covering the NAF between 31.2 degrees E and 34.3 degrees E. The line-of-sight velocity reveals discontinuities of up to similar to 5 mm/yr across the Ismetpasa segment of the NAF, implying surface creep at a rate of similar to 9 mm/yr; this is a large fraction of the inferred slip rate of the NAF (21-25 mm/yr). The lateral extent of significant surface creep is about 75 km. We model the inferred surface velocity and shallow fault creep using numerical simulations of spontaneous earthquake sequences that incorporate laboratory-derived rate and state friction. Our results indicate that frictional behavior in the Ismetpasa segment is velocity strengthening at shallow depths and transitions to velocity weakening at a depth of 3-6 km. The inferred depth extent of shallow fault creep is 5.5-7 km, suggesting that the deeper locked portion of the partially creeping segment is characterized by a higher stressing rate, smaller events, and shorter recurrence interval. We also reproduce surface velocity in a locked segment of the NAF by fault models with velocity-weakening conditions at shallow depth. Our results imply that frictional behavior in a shallow portion of major active faults with little or no shallow creep is mostly velocity weakening. Citation: Kaneko, Y., Y. Fialko, D. T. Sandwell, X. Tong, and M. Furuya (2013), Interseismic deformation and creep along the central section of the North Anatolian Fault (Turkey): InSAR observations and implications for rate-and-state friction properties, J. Geophys. Res. Solid Earth, 118, 316-331, doi: 10.1029/2012JB009661.

Crowell, BW, Bock Y, Sandwell DT, Fialko Y.  2013.  Geodetic investigation into the deformation of the Salton Trough. Journal of Geophysical Research-Solid Earth. 118:5030-5039.   10.1002/jgrb.50347   AbstractWebsite

The Salton Trough represents a complex transition between the spreading center in Baja California and the strike-slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high-resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008-2009 and previous surveys between 2000 and 2005. We also investigate small-scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15mm/yr of left-lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10 degrees/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.

Smith, BR, Sandwell DT.  2006.  A model of the earthquake cycle along the San Andreas Fault System for the past 1000 years. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003703   AbstractWebsite

[1] We simulate 1000 years of the earthquake cycle along the San Andreas Fault System by convolving best estimates of interseismic and coseismic slip with the Green's function for a point dislocation in an elastic plate overlying a viscoelastic half-space. Interseismic slip rate is based on long-term geological estimates while fault locking depths are derived from horizontal GPS measurements. Coseismic and postseismic deformation is modeled using 70 earthquake ruptures, compiled from both historical data and paleoseismic data. This time-dependent velocity model is compared with 290 present-day geodetic velocity vectors to place bounds on elastic plate thickness and viscosity of the underlying substrate. Best fit models (RMS residual of 2.46 mm/yr) require an elastic plate thickness greater than 60 km and a substrate viscosity between 2 x 10(18) and 5 x 10(19) Pa s. These results highlight the need for vertical velocity measurements developed over long time spans (> 20 years). Our numerical models are also used to investigate the 1000-year evolution of Coulomb stress. Stress is largely independent of assumed rheology, but is very sensitive to the slip history on each fault segment. As expected, present-day Coulomb stress is high along the entire southern San Andreas because there have been no major earthquakes over the past 150 - 300 years. Animations S1 and S2 of the time evolution of vector displacement and Coulomb stress are available as auxiliary material.

Lyons, SN, Bock Y, Sandwell DT.  2002.  Creep along the imperial fault, southern California, from GPS measurements. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000763   AbstractWebsite

[1] In May of 1999 and 2000, we surveyed with Global Positioning System (GPS) 46 geodetic monuments established by Imperial College, London, in a dense grid (half-mile spacing) along the Imperial Fault, with three additional National Geodetic Survey sites serving as base stations. These stations were previously surveyed in 1991 and 1993. The Imperial College sites were surveyed in rapid-static mode (15-20 min occupations), while the NGS sites continuously received data for 10 h d(-1). Site locations were calculated using the method of instantaneous positioning, and velocities were determined relative to one of the NGS base stations. Combining our results with far-field velocities from the Southern California Earthquake Center (SCEC), we fit the data to a simple elastic dislocation model with 35 mm yr(-1) of right-lateral slip below 10 km and 9 mm yr(-1) of creep from the surface down to 3 km. The velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault.