The 9 December 2003 central Virginia earthquake sequence: A compound earthquake in the central Virginia seismic zone

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Bulletin of the Seismological Society of America
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The 9 December 2003 central Virginia earthquake sequence was a compound earthquake consisting of two nearly identical events occurring about 12 see apart. The second event is separated by about 300 m from the first event along the azimuth of approximately 195 (+/- 10)degrees. The source mechanism determined from regional waveform inversion indicates predominantly thrust faulting at a depth of approximately 10 (+/- 2) km. The sequence with two events attained the combined seismic moment of M-0 2.64 (+/- 1.01) X 10(15) N m (M-w 4.3). The focal mechanism indicates a subhorizontal P axis trending 301 degrees and plunging 19 degrees. A regional stress model for the central Virginia seismic zone (CVSZ) derived from the 9 December 2003 events and 11 previous earthquakes indicates a thrust-faulting stress regime with a, trending 133 ( 12)degrees and plunging 14 ( 3)degrees. The least principal stress axis (sigma(3)) trends 25 (+/- 10)degrees and plunges 52 (+/- 3)degrees. The sigma(1) axis is rotated approximately 68 degrees clockwise relative to the average maximum horizontal compressional stress (S-Hmax) direction for eastern North America (approximately 65 degrees). The 9 December 2003 earthquake sequence occurred among the systems of Paleozoic and Mesozoic faults above the southern Appalachian decollement, which is at depths from 12 to 19 km in the Piedmont geologic province of central Virginia.


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Doi 10.1785/0120040207