Order and timing of high-angle conjugate strike-slip faulting in Walker Lane sequences

Ken Smith, Rachel L. Hatch, Christine J. Ruhl, & Rachel E. Abercrombie

Published August 15, 2020, SCEC Contribution #10719, 2020 SCEC Annual Meeting Poster #009

With the 2019 Ridgecrest sequence displaying a conjugate strike slip faulting geometry it is worth noting other Walker Lane earthquake sequences that show a similar behavior. The initial July 4 M6.4 NE striking Left-Lateral (LL) Ridgecrest event was followed on July 5 by the Right-Lateral (RL) NW striking M7.1 main shock (Lomax, 2020 and Ross et al. 2020, have modeled additional complexities in the foreshocks). This order and geometry of faulting is observed in nearly all recent moderate sized (M>=~5.7) high-angle strike slip sequences in the Walker Lane region. A sequence of 4 M6 earthquakes in 1980 in the Long Valley area began with an M6 NE striking LL event south of the Caldera followed by an M6 WNW striking RL event in the south Caldera. The 1984 Round Valley sequence east of Long Valley began with the M5.9 NE striking LL mainshock followed by development of a NE dipping NW striking conjugate structure (Priestley et al., 1988). The 1986 Chalfant sequence, ~15 km east of Round Valley, began with an NE striking M5.7 LL event followed 24 hours by the M6.3 northwest striking RL mainshock, defining a conjugate geometry (Smith and Priestley, 1988, 2001). The 1987 Superstition Hills sequence also progressed from an initial LL to a RL high-angle event in a conjugate faulting geometry (Magistrale et al., 1989). The1994 M5.9 NE striking Double Springs Flat, Nevada LL strike slip event was followed by the development of an intersecting NW striking RL structure in a conjugate geometry identified in aftershock relocations (Ichinose et al., 1988). Three M5 events of the 2003-2004 Adobe Hills sequence east of Mono Lake, CA, included off-fault structures but less clear conjugate features in the closely spaced sets of high-angles faults in the Adobe Hills volcanic area. Three M5s (Mw5.9, Mw5.8 and Mw5.7) and aftershock relocations for the 2015 Nine-Mile Ranch, Nevada, sequence define a NE-SW striking conjugate fault geometry (Hatch et al, 2020) with the initial M5.8 most likely on the NE striking LL structure following immediately by an M5 NW striking high-angle RL event. With the prevailing T-axis orientated at a high-angle to an initiating NE striking structure suggests that sequences may be triggered by reducing normal stresses on the initiating faults yet primary long-term strain in the Walker Lane is oriented NW parallel to the plate boundary and accounted for in NW striking RL faulting.

Key Words
earthquake sequences, Walker Lane, conjugate strike-slip faulting

Citation
Smith, K., Hatch, R. L., Ruhl, C. J., & Abercrombie, R. E. (2020, 08). Order and timing of high-angle conjugate strike-slip faulting in Walker Lane sequences. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology