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The eastern Tennessee earthquake sequence of October through December, 1973

DEPARTMENT OF GEOLOGICAL SCIENCES VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY, BLACKSBURG, VIRGINIA 24061
U.S. GEOLOGICAL SURVEY, BOX 25046, DFC, DENVER, COLORADO 80225

Abstract

An earthquake sequence consisting of one foreshock, a magnitude 4.6 main shock, and more than 30 aftershocks occurred south of Knoxville, Tennessee, during the latter part of 1973. The foreshock had a magnitude of 3.4, preceded the main shock by one month, and had a felt area of 2,100 km². The area had been inactive for the preceding 2 years.

A focal mechanism solution for the main shock indicated primarily dip-slip faulting, but the data were too sparse to accurately define the direction of the motion. However, based on other data (trend of epicenters, vertical distribution of hypocenters, regional in situ stress measurements), a northwest-striking reverse fault is interpreted as the most likely causal fault. A detailed intensity survey revealed a felt area of 65,000 km² that exhibited a rapid decrease of intensity to the north and unusually low decrease of macroseismic effects to the south. Topographic focusing of seismic energy is suggested by the intensity data for the town of Maryville, Tennessee.

A network of eight portable seismographs was operated in the epicentral area for a period of some 10 days following the main shock. Data from these stations allowed the determination of 18 aftershock hypocenters, whose depth ranged from approximately 1 to 10 km. A composite focal mechanism solution (CFMS) study was made of 14 of the better recorded aftershocks. The CFMS for the four shallow events ( 1 km) gave a northwest-trending reverse fault solution similar to that for the main shock. However, the majority of events were at greater depths ( 5 km), and their CFMS showed a near-vertical nodal plane (dip-slip motion) and a near-horizontal nodal plane (strike-slip motion). The number and quality of these data are such that the implied reorientation of stresses between the main shock and the deeper aftershocks is tentativc.

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