T154 Central Virginia Earthquakes of 2011: Geology, Geophysics, and Significance for Seismic Hazards in Eastern North America

Tuesday, 6 November 2012
8:00 AM - 12:00 PM
207D

PRELIMINARY ANALYSIS OF MAGNITUDE 5.8 VIRGINIA EARTHQUAKE CAUSATIVE FAULT AND SUBSIDIARY FAULTS ILLUMINATED BY AFTERSHOCKS

Paper number: T154-11
Program: Technical Sessions
Time: 10:30 AM – 10:45 AM

Authors

Abstract

Rapid deployment of 46 portable seismographs by multiple institutions after the 23 August 2011, M5.8 Virginia earthquake produced the best recorded aftershock sequence in the eastern U.S. For the first time on the eastern seaboard, robust aftershock data illuminate the causative fault of a significant earthquake, and smaller aftershock clusters illuminate subsidiary faults. More than 450 well recorded aftershocks were located and analyzed, with moment tensors computed for the largest. The highest quality 394 aftershocks were relocated using hypocentral decomposition to study spatial distribution within the Central Virginia seismic zone. We compare distributions of well located aftershocks on maps, cross sections and 3D rotatable plots to previous geologic maps, aeromagnetic data, and geologic mapping in progress.

The majority of aftershocks define a tabular cluster named the Quail fault zone (QFZ). QFZ aftershocks range from 2-8 km in depth and lie in a 1 km thick zone striking ~N30°E and dipping ~45°SE, consistent with the N28°E, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends 10 km along strike and is centered at Yanceyville on the South Anna River beneath a convex-east bend in bedrock strike. The previously unknown QFZ is within Chopawamsic Fm. gneiss, which is locally mylonitic, and projects to the surface near and/or just east of the contact of Ellisville plutonic rocks. The deep, SE end of the QFZ aftershock cluster terminates near the surface trace of the steeply SE-dipping Long Branch fault zone and Quantico Fm. schist.

At least 3 clusters of more than ten shallow aftershocks illuminate other fault strands: (1) A linear cluster of early (<3 km deep) aftershocks, ~10 km east of the QFZ, extends 8 km from Fredericks Hall to Threemile Corner, strikes N39°E, and is nearly vertical. Linear magnetic anomalies suggest that the fault illuminated by these aftershocks, named the Fredericks Hall fault, is a strand of the Lakeside fault zone; (2) A cluster of later (<2 km deep) aftershocks, ~3 km northeast of Cuckoo, lies near the steeply dipping Ebenezer Church fault at the eastern contact of the Quantico Fm., just south of a small granitic body on the east; (3) A cluster of late (<2.5 km deep) aftershocks near an updip projection of the QFZ dips steeply NW suggesting movement along an antithetic fault.