SPATIAL AND TEMPORAL COMPLEXITY OF THE AUGUST 23, 2011 VIRGINIA EARTHQUAKE RUPTURE PROCESS

The kinematic aspects of the M 5.7, August 23, 2011 Virginia earthquake source process were studied using local and teleseismic recordings. The earthquake was a shallow reverse faulting event in the central Virginia seismic zone. The epicenter was at 37.905°N, 77.975°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 deployment of stations. The aftershocks define a tabular zone oriented in close 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 a complex rupture, and were used to locate two large subevents relative to a small initial subevent. The initial slip event had moment of roughly 2.5 x 10¹⁶ N∙m, and was followed 0.75 seconds later by a subevent with a moment of approximately 2.3 x10¹⁷ N∙m that amounted to approximately 60% of the total moment release. A third subevent with moment 1.2 x10¹⁷ N∙m occurred 1.57 seconds after rupture initiation. Rupture initiated near the southwestern corner of the aftershock zone and proceeded to the northeast along strike and up-dip. The mainshock rupture occurred at the base of the early aftershock zone. The three subevents involved a compact fault area: the estimated distance between the initial and final subevent is only 2.0 km. However, the total rise time of the earthquake is large in comparison to the fault rupture area and the origin time difference between subevents indicates a slow rupture velocity of 1.3 to 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. The characteristics of the rupture in time and space have a significant effect on the nature of strong-motion near the epicenter, the most noteworthy aspect being a strongly modulated amplitude spectrum.