RECENT SEISMIC MONITORING RESULTS FROM THE CENTRAL VIRGINIA SEISMIC ZONE: IMPLICATIONS FOR FAULT RUPTURE AREA - MOMENT RELATIONS

A 24-station seismic network has been operating in the central Virginia seismic zone (CVSZ) since April, 2017, supported by the USGS. The network will operate for two years. The objective is to collect the data necessary to study the on-going aftershock sequence of the Mineral, Virginia earthquake and to examine the CVSZ for seismicity in areas where strongly felt earthquakes occurred in the past. The network can detect and accurately locate earthquakes as small as magnitude -2 in parts of the CVSZ. This capability exceeds that of the network temporarily deployed in the epicentral area of the Mineral earthquake in 2011, which operated approximately 1 year. After 8 months of operation, we have located 183 earthquakes.

The aftershock sequence of the M 5.7 Mineral event is continuing, after more than 6 years. The earthquakes are occurring with the same spatial distribution as the 2011 immediate aftershocks. Significantly, the area where the mainshock rupture occurred was, and remains, largely devoid of aftershocks. The aftershocks surround that area at shallower depths. A half-circle with a diameter of 2.6 km, centered between the locations of the second and third mainshock subevents matches the geometry of the gap in the aftershock distribution. The small dimensions of this aftershock gap are consistent with the findings of Wu and Chapman that the mainshock stress drop was approximately 70 MPa or 700 bars, making it one of the largest well-constrained stress drop estimates in the eastern U.S. The aftershock behavior (lack of events in the rupture zone) points to a total stress drop.

Our observations indicate that intraplate shocks may have tiny rupture areas compared to plate boundary shocks of the same moment. If Mineral is representative, the practice of inferring fault rupture dimensions from the dimensions of aftershock distributions will result in gross over-estimates of the rupture areas. The observations here suggest that intraplate shocks occur on small fault patches and are intensely energetic ruptures with large displacements.