THE SLOW DECAY OF AFTERSHOCKS TRIGGERED BY THE AUGUST 2011, MINERAL, VIRGINIA EARTHQUAKE

On August 23, 2011, a M_w 5.8 earthquake, one of the larger earthquakes in the eastern U.S. in over a century, struck near the town of Mineral, Virginia. A multi-institution deployment of seismometers in the epicentral region of the Mineral earthquake yielded the best recorded aftershock sequence in the eastern U.S. and has offered a rare opportunity to study the decay of aftershocks in an intraplate setting. The Mineral earthquake was a reverse faulting event, and comparison with two reverse faulting earthquakes in California (the 1985 M_w 6.1 Kettleman Hills and 1987 M_w 5.9 Whittier Narrows earthquakes) has revealed variations in the rate and duration of aftershock sequences. The rate in the California aftershock sequences decreased to two or fewer events per day 20 days after the mainshock. Aftershocks of the Mineral earthquake decreased in a power law decay fashion for the first 10 days after the mainshock, but then increased to more than two events per day at about ~25 and 100 days after the mainshock. Each catalog was constrained using a M_c = 2.2 and to events located <15 km from the mainshock. Modified Omori’s law curves fit to each sequence yielded a low p-value of 0.76 for the Mineral earthquake compared to p-values of 1.13 and 1.25 for the Kettleman Hills and Whittier Narrows earthquakes (respectively), indicating a significantly slower decay of aftershocks from the Mineral earthquake. This slow decay rate can be attributed in part to the delayed occurrence of earthquakes along the aftershock-delineated Fredericks Hall (~25 days after the mainshock) and late northwest (~100 days after the mainshock) faults. Structural heterogeneities, stress, and temperature in the crust are all cited as factors responsible for causing variations in p-value. The crust in the eastern U.S. is older and colder than in many tectonically active regions. These characteristics could explain its prolonged aftershock decay rate. The slow decay of aftershocks from the Mineral earthquake may also support the hypothesis that aftershock duration is inversely proportional to fault stress rate, according to which aftershocks in active tectonic margins may last only a few years whereas aftershocks in intraplate regions may endure much longer.