IMPLICATIONS OF THE AUGUST 2011 MINERAL, VA, AND JULY 2010 GERMANTOWN, MD, EARTHQUAKES FOR SEISMIC HAZARD IN THE NATIONAL CAPITAL REGION

Spaced only ~1 year apart, the magnitude 3.4 Germantown, MD, (July 16, 2010) and magnitude 5.8 Mineral, VA, (August 23, 2011) earthquakes rocked the U.S. national capital region (Washington, DC) drawing attention to the occurrence of earthquakes along the East coast of the United States. We evaluate to what extent slip from the Mineral and Germantown earthquakes may have increased the risk of future failure on nearby faults by calculating Coulomb stress change on the DC fault zone, the Stafford fault system, and the Mountain Run fault zone. In general, the Mineral earthquake brought these faults farther away from failure. Only the southern portion of the Mountain Run fault zone appears to have been loaded as a result of the Mineral earthquake. Stress transfer from the Germantown earthquake was much smaller than from the Mineral earthquake, but also generally reduced the chance of failure on nearby faults. In intraplate regions, the USGS probabilistic seismic hazard map forecasts future seismicity well in regions where earthquakes have previously been recorded. For example, the Mineral earthquake occurred near the Central Virginia Seismic Zone. However, the Germantown earthquake occurred in a gap in the seismic hazard map. It is possible that long term seismicity along the Appalachians is essentially uniform and that seismic gaps only reflect the short duration of earthquake records. However, these gaps may also highlight recently inactive regions that now have an increased risk of future rupture.