![]() ![]() We suggest that these earthquakes could be the seismogenic response of the crust to active foundering of mafic-ultramafic lithosphere and resultant asthenospheric upwelling beneath the central Sierra Nevada.ĭeep intraplate earthquakes occur within the lower crust and upper mantle of the western United States ( Wong and Chapman, 1990) in areas with a wide range of geologic and tectonic histories, including multiple regions within the Colorado Plateau, near zones of Sevier-Laramide deformation in the Rocky Mountains and Wyoming craton ( Frohlich et al., 2014 O’Rourke et al., 2016), and across a plate boundary system in California. The events overlie a significant variation in the character of the Moho, and two long-period events occur near the seismically imaged Moho at nearly 40 km depth. From first arrivals, we calculate a one-dimensional model of crustal P-wavespeeds, which resolves a gradational increase from 5.8 km/s near the surface to 6.7 km/s at 35 km depth. We calculate focal mechanisms for 52 of these events, and about half exhibit reverse faulting, which represents a state of horizontal compressional stress that is distinct from the regional stress field. In addition, some of the events appear to be repeating due to the similarity of their waveforms and locations. Most of the events occur at depths of 20–35 km and cluster into two distinct groups. We detect more events, at greater depths, than are present in the Northern California Seismic Network catalog during this period. We locate 131 earthquakes, which occurred from 3.1 to 47.1 km deep during June 2005 to May 2006. Using a network of temporarily deployed broadband seismometers, we characterize an unusual region of crustal earthquakes in the west-central Sierra Nevada, California (USA).
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