Microseismicity of Death Valley, California: An Active Detachment Faulting Regime

Abstract:

Death Valley, a half-graben within the Eastern California Shear Zone at the western edge of the Basin and Range structural province, is being simultaneously extended east-west by normal faults seen in outcrop at its eastern edge, and sheared by strike-slip along the North and South Death Valley fault zones. Controversy surrounds the extent, mechanism, and activity of fault slip on the low-angle detachment fault that dips west beneath the valley. While geological evidence points to possibly active detachment faulting, no evidence of seismicity associated with a seismically active detachment fault has been reported. The valley lies between two regional seismic networks, and the precision of locations of the relatively few earthquakes cataloged by the networks is insufficient to address these details. To characterize microseismicity that accompanies active deformation, we deployed 12 portable seismographs for over a year, testing the hypothesis that the detachment fault is microseismically active, albeit at a level too low to have been usefully detected by the regional networks. Of the couple of thousand events recorded on our array, 313 with magnitudes between 0 and 2.5 are located within the central portion of Death Valley. We relocated these earthquakes using a velocity model and individual station delays derived using the VELEST program. Initial results reveal a westward-dipping pattern of hypocenters consistent with the geometry of the westward-dipping detachment fault, however with considerable scatter. The earthquakes were highly clustered; if they occurred on the detachment fault, large parts of it were silent during the experiment. First-motion focal mechanisms indicate normal fault slip on a mixture of low-angle and high-angle planes as well as some strike-slip earthquakes. We will also present an analysis of high-precision relative relocations. The field experiment was brought to an early end due to monsoon flooding in July of 2013.