On the Rupture Process of the 23 August 2011 Virginia Earthquake

The 23 August 2011 M-w 5.7 Virginia earthquake was studied using local and teleseismic recordings. The earthquake was a shallow reverse rupture in the central Virginia seismic zone. The epicenter was at 37.905 degrees N, 77.975 degrees W, with focal depth 8.0 km. A few local stations recorded both the mainshock and several of the larger aftershocks. This allowed location of the mainshock epicenter relative to the accurate locations of aftershocks recorded by a temporary local deployment of stations. The aftershocks define a planar zone oriented in agreement with the mainshock focal mechanism nodal plane. The mainshock focal depth was determined by comparing teleseismic waveforms with synthetics. Local and teleseismic recordings show evidence of a complex rupture, and were used to locate two large subevents relative to a small initial subevent. The initial slip episode had moment of roughly 2.5 x 10(16) N.m, and was followed 0.75 s later by a subevent with a moment of approximately 2.3 x 10(17) N.m that amounted to approximately 60% of the total moment release. A third subevent with moment approximately 1.2 x 10(17) N.m occurred 1.57 s after rupture initiation. The mainshock rupture occurred at the base of the early aftershock zone. Rupture initiated near the southwestern corner of the aftershock zone and proceeded to the northeast along strike and up- dip. The three subevents may have involved a small fault area: the estimated distance between the initial and final subevent is only 2.0 km. However, the total rise time of the earthquake was comparatively large. The estimated distance between the subevents and the origin time difference between subevents suggests a slow rupture velocity of 1.3-1.7 km/s. This was a consequence of the rupture being comprised of two short-duration energetic slip events that were well separated in time along with a small, possibly low stress- drop, initiation event.