Characterizing Seismicity with High-Precision Relocations of Recent Earthquake Sequences in Eastern California and Western Nevada

Rachel L. Hatch, Daniel T. Trugman, Ken D. Smith, Peter M. Shearer, & Rachel E. Abercrombie

Submitted August 14, 2016, SCEC Contribution #6799, 2016 SCEC Annual Meeting Poster #225

Recent earthquake sequences and swarms in eastern California and western Nevada (2010 to present) have caught the attention of the public and emergency responders. Our objective is to characterize this seismicity by computing high-quality event locations and source parameters, in order to resolve fault structures and assess hazard implications. We obtain high-precision relocations by applying the “GrowClust” algorithm (Trugman et al., 2016; see presentation at this meeting) to earthquake clusters at Herlong, CA (230+ events; 2016-present; largest event = Mw 4.5) and Thomas Creek, S. Reno (183 events; 2015-2016; largest event = Mw 4.3). Relocations provide evidence of complex structures within an overall transtensional stress field in the Northern Walker Lane. The August 3, 2016 Mw 4.5 Herlong earthquake occurred at 9:55 pm local time and was widely felt throughout the northeastern California and Reno-Tahoe area. Relocations suggest the event was on a steeply NE dipping structure striking ENE-WSW, possibly located on a segment of the NW-SE striking Honey Lake fault of the Northern Walker Lane. The regional surface-wave moment tensor solution for the mainshock shows a well-constrained strike-slip mechanism striking N50W (consistent with the orientation of the Honey Lake fault) and dipping slightly NE at 83 degrees. Additional short-period focal mechanisms also show evidence of strike-slip faulting in this sequence. The Mw 4.3 December 22, 2015 Thomas Creek earthquake was also widely felt throughout the Reno-Tahoe area. Relocations indicate a NNE-striking, west-dipping normal faulting mechanism, implying the event occurred along the Virginia Range frontal fault. This interpretation suggests active normal faulting bounds both the eastern and western Reno basin in the south Reno area in a graben structure. The regional surface-wave moment tensor solution for the main event shows a well-constrained normal-faulting mechanism striking N17E and dipping 53 degrees west, consistent with the dip of the aftershock distribution. Additional short-period focal mechanisms computed using the HASH program (Hardebeck and Shearer, 2002) show evidence for normal and oblique-normal faulting in the foreshock and aftershock periods. From the GrowClust relocation results, we estimate a rupture extent of 1.1 km during the Mw 4.3 mainshock, implying a stress drop of about 9 MPa. Our aim is to understand and characterize seismicity in the Northern Walker Lane region. Relocation results for other recent sequences generally show well-defined primary fault planes with additional, complex off-fault structures that are well resolved within the smaller sequences.

Key Words
Earthquake Relocation, Focal Mechanisms, hypocenters, source properties

Hatch, R. L., Trugman, D. T., Smith, K. D., Shearer, P. M., & Abercrombie, R. E. (2016, 08). Characterizing Seismicity with High-Precision Relocations of Recent Earthquake Sequences in Eastern California and Western Nevada. Poster Presentation at 2016 SCEC Annual Meeting.

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