Abstract

The 9 December 2003 central Virginia earthquake sequence was a compound earthquake consisting of two nearly identical events occurring about 12 sec apart. The second event is separated by about 300 m from the first event along the azimuth of approximately 195 (±10)°. 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 M0 2.64 (±1.01) × 1015 N m (Mw 4.3). The focal mechanism indicates a subhorizontal P axis trending 301° and plunging 19°. 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 σ1 trending 133 (±12)° and plunging 14 (±3)°. The least principal stress axis (σ3) trends 25 (±10)° and plunges 52 (±3)°. The σ1 axis is rotated approximately 68° clockwise relative to the average maximum horizontal compressional stress (SHmax) direction for eastern North America (approximately 65°). The 9 December 2003 earthquake sequence occurred among the systems of Paleozoic and Mesozoic faults above the southern Appalachian décollement, which is at depths from 12 to 19 km in the Piedmont geologic province of central Virginia.

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