North-westward looking view of the ERS interferometric map of the northern Mojave, California area drapped on USGS digital topography. The colors depict the movement of the ground averaged over the 1992-2000 time period. One color cycle represents 10 mm/yr of ground motion parallel to the radar line of sight. The clear contrast between the left (green) and right (blue) sides of the scene depicts the inter-seismic displacement along the Blackwater-Little lake fault system. The data allowed us to estimate a rate of creep of 7 mm/yr on the section of the fault below the depth of 5 km. This rate is 3 times faster than the long-term geologic rate estimated on the this fault and the creeping section anomalously shallow. These observations suggest that the Blackwater-Little Lake fault system is currently the locus of a transient deformation process, which has never been observed before. The article describing these results is in the November, 2001 issue of the journal Geology.

Satellite synthetic aperture radar interferometry revealed an undiscovered transient strain pattern along the Blackwater-Little Lake fault system within the Eastern California Shear Zone (See map). The surface strain map obtained by averaging eight years (1992-2000) of ERS (1) radar data shows a 120 km-long, ~20 km-wide zone of concentrated shear between the southern end of the 1872 Owens Valley earthquake surface break and the northern end of the 1992 Landers earthquake surface break. The observed shear zone is continuous through the Garlock fault, which does not show any evidence of localized left-lateral slip during the same time period. A dislocation model of the observed shear indicates that the Blackwater-Little Lake fault is currently creeping below the depth of ~5 km at a rate of 7±3 mm/yr in a right-lateral direction. This rate is about 3 times larger than the long-term geological rate estimated for the Blackwater fault(2) and takes up more than 50% of the entire right-lateral shear distributed across the Eastern California Shear Zone.

This transient slip rate observed in the 1992-2000 ERS radar data and the absence of resolvable slip on the Garlock fault during the same time period may be the manifestation of an oscillatory strain pattern between interacting, conjugate fault systems. Such a cycle provides a possible explanation for the observed clustering of large earthquakes in the ECSZ and on the Garlock fault. In this interpretation, the recent seismicity in the ECSZ (Owens Valley 1872, Landers 1992) may have been triggered by accelerated, localized strain accumulation within the shear zone in the last several hundred years as it is now observed along the Blackwater-Little Lake fault system.

Alternatively the fast, localized shear observed along the Blackwater-Little Lake fault system may have been triggered by the recent large earthquakes at both ends (Owens Valley, 1872 and Landers, 1992) but the mechanism by which these earthquakes may have triggered the observed shallow creep is not understood.

(1) Two Earth Resource Satellites (ERS1 and ERS2) launched by the European Space Agency  in 1991 and 1995.

(2) See Southern California Earthquake Center site:   http://www.scecdc.scec.org/mojfault.html

(3) ERS radar data (c) European Space Agency

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Figure 1. Tectonic map of southern California. Solid lines are active faults (Jennings, 1975). Yellow dots are relocated earthquakes between 1981 and 2000 (Hauksson, 2000). Dashed line box is area covered by ERS data used in this study. White dashed line shows location of concentrated shear observed in SAR data. Black stars indicate epicenters of recent earthquakes: OV, Owens Valley, 1872; JT, Joshua Tree, 1992; L, Landers, 1992; BB, Big Bear, 1992; N, Northridge, 1994; RC, Ridge Crest, 1994, 1995; HM, Hector Mine 1999. Heavy, solid lines depict surface ruptures of the Landers (Sieh et al., 1993), Hector Mine (Scientists from the U.S. geological Survey, 2000; Peltzer et al., 1999), and Owens Valley (Beanland and Clark, 1994; only southern half of rupture is shown) earthquakes. Black dots and arrows show locations and observed velocities of 11 stations of the Yucca GPS array (Gan et al., 2000). Fault labels are: SAF, San Andreas fault; GF, Garlock fault; BWF, Blackwater fault; CF, Calico fault; HMF, Hunter-Mountain fault; PVF, Panamint Valley fault; DFV, Death Valley fault; HF, Harper fault; LLF, Little Lake fault; SMF, Sierra Madre fault; SJF, San Jacinto fault.

Figure 2. Surface velocity map obtained by averaging 25 interferograms of the Mojave area. One colors cycle depicts 10 mm/yr of surface displacement along radar line of sight (at Latitude N34d, ERS descending track trends S13.6dW with a radar looking westward at 23d off vertical incidence angle in middle of imaged swath). Gray areas are zones of low phase coherence that have been masked before phase unwrapping.  Black lines are active faults (Jennings, 1975). Note conspicuous shear strain along Blackwater-Little Lake fault system. Large ground subsidence in NW corner of frame is related to the Coso volcanic and geothermal field activity (Fig.1). Surface displacement associated with the 1994 and 1995 Ridgecrest earthquakes is visible south of the Coso area. Other patterns of surface deformation include ground subsidence due to ground water level change near Lancaster and around dry lakes. See map (Figure 1)  or figure below for location.

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