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Published August 1, 2020 | public
Journal Article

Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment


Decadal scale variations in the seismicity rate in the Ridgecrest‐Coso region, part of the Eastern California Shear Zone, included seismic quiescence from the 1930s to the early 1980s, followed by increased seismicity until the 2019 M_w 6.4 and 7.1 Ridgecrest sequence. This sequence exhibited complex rupture on almost orthogonal faults and triggered aftershocks over an area of ∼90  km long by ∼5–10  km wide, which is a fraction of the area of the previously seismically active Indian Wells Valley and Coso range region. During the last 40 yr, the seismicity has been predominantly the result of strike‐slip motion, extending north from the Garlock fault, along the Little Lake and Airport Lake fault zones, and approaching the southernmost Owens Valley fault to the north. The Coso range forms an extensional stepover between these two strike‐slip fault systems. This evolution of a plate boundary zone is driven by the northwestward motion of the Sierra Nevada, and crustal extension along the southwestern edge of the Basin and Range Province. Stress inversion of focal mechanisms shows that the postseismic stress state consists of almost horizontal σ1 and vertical σ2⁠. The σ1 is spatially rotated across the Coso range stepover with σ1‐trending ∼N17° E to the north, whereas, along the M_w 7.1 mainshock rupture, the trend is ∼N6° E⁠. The friction angles as measured between fault strikes and the σ1 trends correspond to a frictional coefficient of 0.75, suggesting average fault strength. In comparison, the mature Garlock fault has a smaller frictional coefficient of 0.28, similar to weak faults like the San Andreas fault. Thus, it appears that the heterogeneously oriented and spatially distributed but strong Ridgecrest‐Coso faults accommodate seismicity at seemingly random places and times within the region and are in the process of self‐organizing to form a major throughgoing plate‐boundary segment.

Additional Information

© 2020 Seismological Society of America. Manuscript received 30 January 2020; Published online 2 June 2020. This research was supported by the U.S. Geological Survey/ National Earthquake Hazards Reduction Program (USGS/NEHRP) Grant Numbers G16AP00147 and G18AP00028; National Science Foundation Award Numbers EAR‐1550704 and EAR‐1818582; the Gordon and Betty Moore Foundation Grant Number 5229 to California Institute of Technology (Caltech), and by the Southern California Earthquake Center (SCEC, Contribution Number 10075). SCEC is funded by NSF Cooperative Agreement EAR‐1600087 and USGS Cooperative Agreement G17AC00047. The authors used Generic Mapping Tools (GMT) from Wessel et al. (2013) to make the figures. The Southern California Seismic Network (SCSN) is partially funded by USGS Cooperative Agreements G19AS00034, G20AP00037, and CalOES Agreement 6012‐2017 with Caltech. The authors appreciate comprehensive reviews by I. Wong and R. Weldon, and the support provided by more than 20 SCSN and Southern California Earthquake Data Center (SCEDC) staff members who maintain stations and communications systems, as well as data flow, processing, and archiving. The authors thank H. Kanamori for assistance with the magnitude calculations in the Appendix. Data and Resources: We have used waveforms and parametric data from the California Institute of Technology/U.S. Geological Survey (Caltech/USGS) Southern California Seismic Network (SCSN; doi: 10.7914/SN/CI); stored at the Southern California Earthquake Data Center (doi: 10.7909/C3WD3xH1). Caltech began operating a seismic network in the late 1920s and seismic stations in the Ridgecrest area in 1947. The hypocenters and magnitudes from 1930 to 1980 are from the Caltech/USGS southern California earthquake catalog (Hutton et al., 2010). The seismicity parameters from 1981 to the end of 2019 are from the waveform‐relocated catalog as described by Hauksson et al. (2012). However, we use GrowClust for relocating the most recent version of this catalog (Trugman and Shearer, 2017). All of the Ridgecrest‐Coso seismicity was detected by the SCSN automated picker and reviewed by data analysts, except for some of the events in the 2019 Ridgecrest sequence that have not yet been reviewed. In some of the figures, we also included the 1980 seismicity from the SCSN catalog to include the beginning of the 1980–1981 Indian Wells Valley swarms. More information about Global Centroid Moment Tensors (Global CMT) can be found at https://www.globalcmt.org (last accessed January 2020). Earthquake information is also available in National Earthquake Information Center at https://earthquake.usgs.gov (last accessed April 2020).

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