RESULTS FROM THE 2017-2018 SEISMIC NETWORK MONITORING EFFORT IN THE CENTRAL VIRGINIA SEISMIC ZONE

A temporary twenty-five station seismic network operated in central Virginia from April 2017 through November 2018. A total of 457 earthquakes were located, 400 of which are aftershocks of the M_w 5.7 August 23, 2011 Mineral earthquake. The aftershock sequence is persistent and stable. The general pattern of hypocenters is not appreciably different from that of the immediate (August 25 – December 31, 2011) aftershocks. The rupture zone of the mainshock remains an aftershock shadow, and most of the activity forms a distinctive halo at depths less than 6.2 km, above and to the north-northeast of the mainshock rupture zone. The majority of the 47 well-constrained focal mechanisms are reverse, and the nodal planes trend North to Northwest, consistent with a maximum horizontal stress trending approximately N70E, whereas the mainshock rupture plane (also reverse) strikes N30E. The great majority of these aftershocks are shallower than the mainshock rupture. The rate of aftershocks observed during the study period is within a factor of two of that predicted by Omori’s Law, using parameters determined from the early aftershock sequence in 2011. The results establish that the aftershock spatial distribution is temporally stationary, and that the stress field responsible for the early aftershocks at shallow depths is also temporally persistent. Currently, the most active part of the central Virginia seismic zone outside the 2011 Mineral aftershock area is centered approximately 60 km to the southwest, in Buckingham County, a historically active region, and possibly the site of a pre-historic shock of significant size.