Abstract

The mbLg 4.2 Vonore, Tennessee, earthquake of March 27, 1987 — although of modest size — is the second largest instrumentally located event in the southern Appalachians and the largest in the region to occur within an existing regional seismic network. An aggressive portable network deployment yielded tightly constrained hypocentral locations and focal mechanisms for the main shock and a 21-event aftershock sequence. This high quality data set has yielded several estimates of fault characteristics, heretofore unavailable, that improve the understanding of seismogenesis in the southern Appalachians. For example, we obtained excellent agreement between fault plane orientation from focal mechanisms with that from aftershock distribution; nearly pure right lateral strike-slip faulting on a near-vertical, north-south plane is indicated. Fault rupture area from aftershocks is constrained to 4.0–9.9 km2, with a best value of 6.6 km2; seismic moment obtained from a robust regression on mbLg is M0=6.7(±1.5)×1021 dyne-cm. These values yield an average slip of 23–61 mm (best value, 31 mm) and a stress drop of approximately 1 bar. Rupture was probably unilateral, initiating at a depth of 18.1(±1) km and propagating up-dip as well as laterally. The entire rupture plane is within crystalline basement rock at least 10 km beneath the Appalachian decollement, in agreement with previous models of the southern Appalachian seismogenic zone (e.g., Johnston et al., 1985). Finally we discuss two unusual phenomena and their probable association with the Vonore earthquake: surface fissuring and peripheral aftershocks.

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