Microearthquake spectra in the southeastern United States

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Abstract
Displacement spectra from microearthquakes in the Clark Hill Reservoir Area and the Jocassee Reservoir Area, both in the Piedmont Crystalline Province, showed similar spectral properties typified by a sharp spectral corner, ω-cubic, high-frequency decay and ratios of P- and S-wave corner frequencies predominantly greater than unity. Displacement spectra from microearthquakes in the Maryville Tennessee Area in the Folded Appalachian Province showed transitional spectral corners, ω-square or less high-frequency decay and a ratio of P- to S-wave corner frequencies less than or equal to unity. These spectral characteristics are interpreted as evidence for a possible regional variation in the earthquake mechanism. The Clark Hill and Jocassee spectral characteristics are best explained by an earthquake mechanism typical of an equidimensional fault which nucleates rupture at a point of high resistance to slip and ruptures at a velocity greater than the S-wave velocity along an existing fracture. The Maryville spectral characteristics are best explained by rupture at velocities less than the S-wave velocity along faults which may show premature arrest of movement.