Unusually deep earthquakes in the central sierra nevada (California, USA): Foundering ultramafic lithosphere?

Jamie Ryan, Andrew M. Frassetto, Owen Hurd, Craig H. Jones, Jeffrey Unruh, George Zandt, Hersh Gilbert, Thomas J. Owens

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Using a network of temporarily deployed broadband seismometers, we characterize an unusual region of crustal earthquakes in the west-central Sierra Nevada, California (USA). We locate 131 earthquakes, which occurred from 3.1 to 47.1 km deep during June 2005 to May 2006. We detect more events, at greater depths, than are present in the Northern California Seismic Network catalog during this period. Most of the events occur at depths of 20-35 km and cluster into two distinct groups. In addition, some of the events appear to be repeating due to the similarity of their waveforms and locations. 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. 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. 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. 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.

Original languageEnglish (US)
Pages (from-to)357-377
Number of pages21
Issue number1
StatePublished - Feb 1 2020

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

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