ACTIVE SEISMOGENIC SLIP ON A GENTLY DIPPING NORMAL FAULT BENEATH DEATH VALLEY, CALIFORNIA

To test our hypothesis that detachment faults in Death Valley are active, we deployed 10 portable seismographs, which continuously recorded 3-channel short-period data at 100 samples per second, for 18 months, from July 2012 through January 2014. We automatically detected, and manually examined, thousands of seismic sources in the dataset. A sub-set of 313 earthquakes, which lie within the footprint of the portable network, were relocated using a revised 1D velocity model with individual station corrections. The largest earthquake in our dataset was M ~2.5. Using a total least squares (TLS) solution, we determine that the best-fit plane to the earthquake hypocenters dips 7.8° to the NW (azimuth 326°) beneath central Death Valley and the eastern Panamint Mountains. This azimuth is approximately parallel to the strike of the northern Death Valley fault zone.

Our result is entirely compatible with: (1) GPS velocities, from sparse stations in and near Death Valley, of ca. 2 mm/yr NW; (2) recently published kinematic data indicating oblique dextral-normal slip on the Black Mountains fault system at Badwater and Mormon Point; and (3) earlier studies, based on stratigraphic evidence and regional structural restorations, hypothesizing ten’s of kilometers of late Cenozoic dextral transport of tectonic elements.

We infer that the Death Valley pull-apart basin is currently opening by oblique slip on (1) an active, but blind, detachment fault, and (2) the system of normal faults bounding the western front of the Black Mountains.