© 2015. American Geophysical Union. All Rights Reserved. The 1992 M < inf > w < /inf > 7.3 Landers and 1999 M < inf > w < /inf > 7.1 Hector Mine earthquakes struck the Eastern California Shear Zone (ECSZ) in the Mojave Desert, Southern California. Coseismic and postseismic deformation from these events affect efforts to use Global Positioning System (GPS) observations collected since these events to establish a secular surface velocity field, especially in the near field of the coseismic ruptures. We devise block motion models constrained by both historical pre-Landers triangulation and trilateration observations and post-Landers GPS measurements to recover the secular deformation field and differentiate the postseismic transients in the Mojave region. Postseismic transients are found to remain in the Southern California Earthquake Center Crustal Motion Map Version 4, Plate Boundary Observatory, and Scripps Orbit and Permanent Array Center GPS velocity solutions in the form of 2-3mm/yr excess right-lateral shear across the Landers and Hector Mine coseismic ruptures. The cumulative deformation rate across the Mojave ECSZ is 13.2-14.4mm/yr, at least twice the geologic rate since the late Pleistocene (≤6.2±1.9mm/yr). Postseismic GPS time series based on our secular velocity field reveal enduring late-stage transient motions in the near field of the coseismic ruptures that provide new constraints on the rheological structure of the lower crust and upper mantle.

Long-term landslide deformation is disruptive and costly in urbanized environments. We rely on TerraSAR-X satellite images (2009–2014) and an improved data processing algorithm (SqueeSAR™) to produce an exceptionally dense Interferometric Synthetic Aperture Radar ground deformation time series for the San Francisco East Bay Hills. Independent and principal component analyses of the time series reveal four distinct spatial and temporal surface deformation patterns in the area around Blakemont landslide, which we relate to different geomechanical processes. Two components of time-dependent landslide deformation isolate continuous motion and motion driven by precipitation-modulated pore pressure changes controlled by annual seasonal cycles and multiyear drought conditions. Two components capturing more widespread seasonal deformation separate precipitation-modulated soil swelling from annual cycles that may be related to groundwater level changes and thermal expansion of buildings. High-resolution characterization of landslide response to precipitation is a first step toward improved hazard forecasting.

The 1872 Owens Valley earthquake is the third largest known historical earthquake in California. Relatively sparse field data and a complex rupture trace, however, inhibited attempts to fully resolve the slip distribution and reconcile the total moment release. We present a new, comprehensive record of surface slip based on lidar and field investigation, documenting 162 new measurements of laterally and vertically displaced landforms for 1872 and prehistoric Owens Valley earthquakes. Our lidar analysis uses a newly developed analytical tool to measure fault slip based on cross-correlation of sublinear topographic features and to produce a uniquely shaped probability density function (PDF) for each measurement. Stacking PDFs along strike to form cumulative offset probability distribution plots (COPDs) highlights common values corresponding to single and multiple-event displacements. Lateral offsets for 1872 vary systematically from ∼1.0 to 6.0 m and average 3.3 ± 1.1 m (2σ). Vertical offsets are predominantly east-down between ∼0.1 and 2.4 m, with a mean of 0.8 ± 0.5 m. The average lateral-to-vertical ratio compiled at specific sites is ∼6:1. Summing displacements across subparallel, overlapping rupture traces implies a maximum of 7–11 m and net average of 4.4 ± 1.5 m, corresponding to a geologic Mw ∼7.5 for the 1872 event. We attribute progressively higher-offset lateral COPD peaks at 7.1 ± 2.0 m, 12.8 ± 1.5 m, and 16.6 ± 1.4 m to three earlier large surface ruptures. Evaluating cumulative displacements in context with previously dated landforms in Owens Valley suggests relatively modest rates of fault slip, averaging between ∼0.6 and 1.6 mm/yr (1σ) over the late Quaternary.

The evaluation of long-wavelength deformation associated with interseismic strain accumulation traditionally relies on spatially sparse GPS measurements, or on high spatial-resolution InSAR velocity fields aligned to a GPS-based model. In this approach the InSAR contributes only short-wavelength deformation and the two data sets are dependent, thereby challenging the evaluation of the InSAR uncertainties and the justification of atmospheric corrections. Here we present an analysis using 7 years of Envisat InSAR data to characterize interseismic deformation along the southern San Andreas Fault (SAF) and the San Jacinto Fault (SJF) in southern California, where the SAF bifurcates onto the Mission Creek (MCF) and the Banning (BF) fault strands. We outline the processing steps for using InSAR alone to characterize both the short- and long-wavelength deformation, and evaluate the velocity field uncertainties with independent continuous GPS data. InSAR line-of-sight (LOS) and continuous GPS velocities agree within ∼1-2 mm/yr in the study area, suggesting that multiyear InSAR time series can be used to characterize interseismic deformation with a higher spatial resolution than GPS. We investigate with dislocation models the ability of this mean LOS velocity field to constrain fault slip rates and show that a single viewing geometry can help distinguish between different slip-rate scenarios on the SAF and SJF (∼35 km apart) but multiple viewing geometries are needed to differentiate slip on the MCF and BF (<12 km apart). Our results demonstrate that interseismic models of strain accumulation used for seismic hazards assessment would benefit from the consideration of InSAR mean velocity maps.

The central section of the San Andreas Fault hosts tectonic tremor and low-frequency earthquakes (LFEs) similar to subduction zone environments. LFEs are often interpreted as persistent regions that repeatedly fail during the aseismic shear of the surrounding fault allowing them to be used as creepmeters. We test this idea by using the recurrence intervals of individual LFEs within LFE families to estimate the timing, duration, recurrence interval, slip, and slip rate associated with inferred slow slip events. We formalize the definition of a creepmeter and determine whether this definition is consistent with our observations. We find that episodic families reflect surrounding creep over the interevent time, while the continuous families and the short time scale bursts that occur as part of the episodic families do not. However, when these families are evaluated on time scales longer than the interevent time these events can also be used to meter slip. A straightforward interpretation of episodic families is that they define sections of the fault where slip is distinctly episodic in well-defined slow slip events that slip 16 times the long-term rate. In contrast, the frequent short-term bursts of the continuous and short time scale episodic families likely do not represent individual creep events but rather are persistent asperities that are driven to failure by quasi-continuous creep on the surrounding fault. Finally, we find that the moment-duration scaling of our inferred creep events are inconsistent with the proposed linear moment-duration scaling. However, caution must be exercised when attempting to determine scaling with incomplete knowledge of scale.

The Hayward and Calaveras Faults, two strike-slip faults of the San Andreas System located in the East San Francisco Bay Area, are commonly considered independent structures for seismic hazard assessment. We use Interferometric Synthetic Aperture RADAR to show that surface creep on the Hayward Fault continues 15 km farther south than previously known, revealing new potential for rupture and damage south of Fremont. The extended trace of the Hayward Fault, also illuminated by shallow repeating micro-earthquakes, documents a surface connection with the Calaveras Fault. At depths greater than 3-5 km, repeating micro-earthquakes located 10 km north of the surface connection highlight the 3-D wedge geometry of the junction. Our new model of the Hayward and Calaveras Faults argues that they should be treated as a single system with potential for earthquake ruptures generating events with magnitudes greater than 7, posing a higher seismic hazard to the East San Francisco Bay Area than previously considered.

Thirty horizontal displacement time series from GPS sites in the area around the 2005 Kashmir earthquake show lateral spatial variations in displacement magnitude and relaxation time for the postseismic interval from 2005 to 2012. The observed spatial pattern of surface displacements can only be reproduced by finite element models of postseismic deformation in elastic over viscoelastic crust that include lateral differences in both the thickness of the elastic layer and the viscosity of the viscoelastic layer. Solutions reproducing the sign of horizontal displacements everywhere in the epicentral region also require afterslip on the portion of the fault dislocation in the viscoelastic layer but not in the elastic lid. Although there are substantial tradeoffs among contributions to postseismic displacements of the surface, the observations preclude both crustal homogeneity and shallow afterslip. In the best family of solutions, the thickness of the elastic upper crust differs by a factor of 5 and the viscosity of the middle and lower crust by an order of magnitude between domains north and south of a suture zone containing the Main Boundary Thrust and Main Mantle Thrust.

Interseismic strain accumulation and fault creep is usually estimated from GPS and alignment arrays data, which provide precise but spatially sparse measurements. Here we use interferometric synthetic aperture radar to resolve the interseismic deformation associated with the Hayward and Calaveras Faults (HF and CF) in the East San Francisco Bay Area. The large 1992-2011 SAR data set permits evaluation of short- and long-wavelength deformation larger than 2 mm/yr without alignment of the velocity field to a GPS-based model. Our time series approach in which the interferogram selection is based on the spatial coherence enables deformation mapping in vegetated areas and leads to refined estimates of along-fault surface creep rates. Creep rates vary from 0 ± 2 mm/yr on the northern CF to 14 ± 2 mm/yr on the central CF south of the HF surface junction. We estimate the long-term slip rates by inverting the long-wavelength deformation and the distribution of shallow slip due to creep by inverting the remaining velocity field. This distribution of slip reveals the locations of locked and slowly creeping patches with potential for a M6.8 ± 0.3 on the HF near San Leandro, a M6.6 ± 0.2 on the northern CF near Dublin, a M6.5 ± 0.1 on the HF south of Fremont, and a M6.2 ± 0.2 on the central CF near Morgan Hill. With cascading multisegment ruptures the HF rupturing from Berkeley to the CF junction could produce a M6.9 ± 0.1, the northern CF a M6.6 ± 0.1, the central CF a M6.9 ± 0.2 from the junction to Gilroy, and a joint rupture of the HF and central CF could produce a M7.1 ± 0.1.