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