FAULT SLIP DISTRIBUTION ALONG THE SOUTHERN 15 KM OF THE M7.1 RIDGECREST EARTHQUAKE SURFACE RUPTURE

The M_w7.1 Ridgecrest earthquake of 5 July 2019 was the largest strike-slip earthquake in California since the 1999 Hector Mine earthquake. The earthquake occurred in the Eastern California Shear Zone-Walker Lane system, which accommodates roughly 10 mm/yr of the motion between the North American and Pacific plates. We mapped the southernmost 15 km of the rupture, which cut across Pleistocene lacustrine and Pleistocene-Holocene alluvial deposits. We followed the main rupture southeastwards from Highway 178, making field observations of fault orientations, slip sense, fault zone width, and displacement measurements where piercing lines were available. A total of 391 waypoints were collected over seven field days. Eighty-two offset measurements were made along the main (eastern) and central strands of the ruptured fault zone, including measurements made on cracks. A summary of our displacement data shows: (1) a 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 beyond that to the south as discontinuous fractures; (2) at this one-km mark, rupture stepped 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) 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.