The Mw 5.8 Virginia Earthquake of August 23, 2011 and its Aftershocks: A Shallow High Stress Drop Event
Abstract
We analyze the hypocentral distribution and source parameters of the aftershocks of the Virginia Earthquake of August 23, 2011 using a temporary array of telemetered instruments deployed within 20 km of the main shock. Our data come from four USGS NetQuakes accelerometers and seven IRIS/PASSCAL seismometers that were established within a few days of the earthquake. Aftershock seismograms at these near-source stations are characterized by impulsive, high-frequency P and S phases at most sites. In addition, we use the five closest permanent stations (60 - 310 km distance) to analyze the main shock. Hypocenters, crustal velocity model and station corrections were determined using the program VELEST (Kissling, et al, 1994). The aftershocks define a 10-km-long, N 30 degree E striking, 45 degree ESE dipping fault. This fault plane agrees well with the USGS moment tensor solutions for the main shock. Aftershock depths range from 2.5 to 8 km, placing the sequence in the Cambrian metamorphic rocks of the Eastern Piedmont thrust sheet. We relocated the main shock relative to a well-located Mw 3.5 aftershock using the P-wave arrival times at the five permanent stations. The main shock epicenter lies in the middle of the aftershock zone. Its focal depth, although not well constrained, is similar to the aftershocks. A crustal-scale seismic reflection profile was acquired by the USGS in 1981 along I-64 just 4 km southwest of the nearest aftershocks. This profile runs nearly parallel to the dip direction of the aftershock zone and has been interpreted to contain many ESE-dipping reverse faults in the allochthonous upper crust (Harris et al., 1986; Pratt, et al., 1988). When projected onto the reflection profile the aftershocks locate within a relatively non-reflective zone bounded above and below by prominent bands of more shallowly dipping reflectors reported by Pratt et al. (1988) raising the question whether or not the earthquake reactivated a pre-existing fault. Seismic moments and corner frequencies for the aftershocks were determined using the 200 sps NetQuakes data. Both P and S corner frequencies were determined using the Multi-Window Spectral Ratio (MWSR) method of Imanishi and Ellsworth (2006) for pairs of nearly co-located events. Stress drops as high as 100 MPa were measured, and all values are higher than the "normal" range of 3-5 MPa commonly found in tectonically active areas. We also analyzed the main shock using the MWSR method with the five permanent stations and larger aftershock records. We determined a stress drop of 50-75 MPa for the main shock. This corresponds to a circular rupture 1.6-2.0 km in radius with an average slip of 1.5-2.5 m. A circular source with these dimensions fits snugly in the lightly populated center of the aftershock zone where it is bracketed by dense concentrations of aftershocks to either side and above. The high stress drop values for the main shock and aftershock require a nearly complete stress drop for a crust in frictional equilibrium with a coefficient of friction of 0.6.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S14B..05E
- Keywords:
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- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology;
- 7215 SEISMOLOGY / Earthquake source observations;
- 7230 SEISMOLOGY / Seismicity and tectonics;
- 8110 TECTONOPHYSICS / Continental tectonics: general