abstract

The Sharpsburg, Kentucky, earthquake was the second largest earthquake to have occurred in the United States, east of the Continental Divide, in the past 20 yr, having a seismic moment of 4.1 × 1023 dyne-cm. A surface-wave focal mechanism study defines a nodal plane striking N30°E, dipping 50°SE, and a nearly vertical nodal plane striking N60°W. P-wave first motion data indicate right-lateral motion on the nodal plane striking N30°E, with the pressure axes oriented east-west. These angles can be varied by ±10° without affecting the fit to the surface-wave data. The surface-wave solution is reinforced by a modeling of long-period seismograms at regional distances.

The P, pP, and sP polarities and amplitudes from the short-period vertical component array stack at NORSAR are used together with six unambiguous short-period P-wave first motions recorded in North America to test whether it is possible to constrain focal mechanism solutions with such data. These solutions are compatible with the surface-wave solution. Waveform modeling of the NORSAR data suggests a source pulse duration of 1.0 sec and constrains the depth to 12.0 km. To match mb estimates from NORSAR and Canadian stations, t*, for teleseismic P, must be 0.7 and 0.5, respectively, when the synthetics are scaled using the surface-wave seismic moment.

In spite of extensive coverage of the epicentral zone, fewer than 70 aftershocks were recorded. The largest aftershock and an mbLg = 2.2. Aftershock locations suggest that the nodal plane striking N30°E is the fault plane. An aftershock area of 30 to 50 km2 implies a stress drop of 2.8 to 6 bars and a dislocation of 2.0 to 3.4 cm.

Because of the variety of studies performed, this earthquake is presently the best-studied eastern North American seismic event with well-constrained estimates of focal depth, focal mechanism and seismic moment, and indications of the duration of the source time function and upper mantle P-wave t*.

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