GROUND SURFACE RUPTURE MAPPING OF FAULTS IN THE RIDGECREST-TRONA FAULT COMPLEX AND EARTHQUAKE SEQUENCE

The 2019 Ridgecrest-Trona Fault Complex and Earthquake Sequence resulted in rupture of the ground surface in the Indian Wells, Salt Wells, and Searles Valleys. The Ridgecrest-Trona Fault Complex and Earthquake Sequence started on July 4^th, 2019 with an M_W 6.4 foreshock and peaked with the July 5^th, 2019, M_W 7.1 mainshock. Both of these epicenters were located in the extreme northwest corner of San Bernardino County and on the northern portion of the United States Navy’s China Lake Naval Air Weapons Station. Both of these major earthquakes originated from depths of approximately 8 km.

SCEC (2019) issued an updated fault map showing the foreshock as having occurred along the “South Little Lake” fault and the main shock as having ruptured the “East Little Lake” fault. San Bernardino County informally recognized the “South Little Lake” fault as the “Salt Wells Valley” fault zone and the “East Little Lake” as the “China Lake/Paxton Ranch” fault zone. Ground surface rupture mapping by San Bernardino County, California State University, San Bernardino, and volunteers of the Searles Valley Working Group in the days and months following the earthquakes mapped portions of the ”Salt Wells Valley” and “China Lake/Paxton Ranch” fault zones.

Detailed mapping of surface ruptures in the eastern Searles Valley by the volunteers of the Searles Valley Working Group confirmed the presence of northeast-striking faults in the Trona, Argus, Panorama Point, and Windy Acres areas. The County informally refers to these faults as the “Trona” and “Aguirre Ranch” faults in the eastern Searles Valley, as well as several previously unmapped splays of the “Salt Wells Valley” fault zone in the Windy Acres area. The “Trona” and “Aguirre Ranch” faults are aligned along strike with the projection of the northeast-striking “Salt Wells Valley” fault into the Searles Valley.

Evidence for primary fault ruptures within the fault complex were observed in the Trona and West Searles Valley areas based on review of stacked phase gradient radar imagery made available by the IGPP (Xu, 2019). Fault ruptures were preliminarily identified as linear tonal deformations on the radar imagery. Ground truthing identified en echelon ground fractures in zones up to a mile in length trending generally perpendicular to topographic contours.