Fault slip distribution and structural style along the southern 15 km of the M7.1 Ridgecrest earthquake surface rupture: Initial observations
Abstract
The M7.1 Ridgecrest earthquake of 5 July 2019 was the largest strike-slip earthquake in California since the 1999 Hector Mine earthquake. The earthquake epicenter was within a complicated zone of conjugate NNW-trending dextral and WSW-trending sinistral faulting and the main rupture propagated bilaterally to the NW and SE. The southernmost 15 km of the rupture cutting across Pleistocene lacustrine and alluvial deposits and pre-Tertiary plutonic rocks, was not previously mapped. We followed the main rupture from Highway 178 southeastwards, making field observations of fault orientations, slip sense, fault zone width, and displacement measurements where piercing lines were available. Our most noteworthy observations are as follows: (1) Minimum of 70 cm of right-lateral slip occurred along the main fault rupture, but the displacement rapidly decreased to <2cm within one km of Highway 178 and continued to the south as discontinuous fractures. (2) At this one-km mark, rupture steps left across a 500-m-wide zone, before consolidating along a dominantly left-stepping zone of en-echelon faults and fault splays, with a typical width of ~ 20-50 m. (3) South of the stepover area, surface fracturing with measurable offsets occurred along three sub-parallel faults. The eastern strand exhibited right-lateral displacements of 70 - 120 cm, the central strand accommodated a maximum of ~30 cm of dextral slip, and the western strand is mainly a zone of distributed fracturing with <5 cm displacements. (4) Dislodged boulders and cobbles observed at the fault endpoints within this restraining stepover region indicate high accelerations that locally exceeded Earth's gravity. (5) Nearly 100 cm right-lateral slip along the eastern strand extended to within 500 m of the southern end of the rupture trace, which is characterized by a >650-m-wide zone of complicated cross-fracturing that extended to within 5 km of the Garlock fault to the south. (6) The central strand surface trace ruptured an ~400-m-wide right step with a toppled tufa tower near its southern end. This is only the completely collapsed tufa mound we observed, and its position is consistent with high accelerations associated with a stopping phase generated by rupture deceleration at the stepover (Elliott et al., 2009).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S34C..04A
- Keywords:
-
- 7299 General or miscellaneous;
- SEISMOLOGY