GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No.
Presentation Time: 8:00 AM-12:00 PM


LANGENHEIM, Victoria, U. S. Geological Survey, P O Box 158, Moffett Field, CA 94035

Gravity and magnetic data provide information on the geological setting and potential lithological controls of the 2019 M7.1 Ridgecrest and M6.4 Searles Valley earthquakes. The earthquakes produced numerous aftershocks and complex surface rupture north of the Garlock fault. The M6.4 earthquake produced an L-shaped zone of aftershocks, with surface rupture along the primary NE-trending aftershock zone. The NE-trending zone is aligned with a NE-trending magnetic gradient that must be sourced within the basement. This feature is only subtly expressed in the gravity, but the potential-field data suggest possible lithologic influence on the location of the NE-striking rupture. The M7.1 earthquake produced a NW-trending aftershock zone and surface rupture. Seismicity is more dispersed at the NW end of the zone where the aftershocks pass from a prominent gravity high to a region with gravity values that are 20-40 mGal lower. At the SE end of the aftershock zone, the earthquakes also are more dispersed, but along parallel, NW-trending zones, again reflecting passage from the prominent gravity high into a gravity low within the Garlock fault zone. These observations suggest lithologic control of the width of the aftershock zone. An assessment of magnetic anomalies along either side of the ruptures suggests that cumulative offset along these faults is less than several km. Finally, linear gradients in magnetic data filtered to enhance shallow sources can be used to extend southward mapped Quaternary faults west of the Ridgecrest earthquakes to the Garlock fault, where the anomalies appear to bend CCW into the fault zone.