[1] Loud, shallow microearthquakes (M < 3.0), occurring in the vicinity of granitic plutons represent a different category of seismicity compared to other recognized seismic sources in South Carolina. We demonstrate this difference by comparing the locations of microearthquakes in the vicinity of three granitic plutons in South Carolina, with the results of two-dimensional numerical modeling and analytical studies. The less rigid plutons, embedded in more rigid country rock, were loaded by applying ambient tectonic plate stresses along the direction of maximum horizontal compression. The results of modeling showed that regions of computed high stresses lie on the periphery of the plutons, and coincide with both the observed locations of seismicity and with lobes of elevated stresses obtained by analytical calculations for a weak pluton subjected to a homogenous stress field. The amplitude of the modeled stresses appears to be a function of the shape and size of the pluton. Citation:

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On 16 October 1983, at 19:40 (UTC), a magnitude 3.8 earthquake occurred near Lake Charles in southwestern Louisiana. The earthquake was felt over an area of 2600 km2 and had a maximum Modified Mercalli intensity of V. This was the first significant Louisiana Gulf Coast earthquake to be recorded and located by nearby microseismic networks. One possible foreshock and three aftershocks also were recorded and located using a velocity model developed for this study. The focal mechanism of the earthquake was determined based on P-wave first motions from 22 local and regional stations. The solution indicates a predominantly east-west trending, southeast-dipping normal fault with a small strike-slip component. The depth of this event (14+ km) provides the first significant evidence that normal faulting within the crystalline basement may control shallower growth faults along the Gulf Coast.

An earthquake swarm occurred near Lake Keowee, South Carolina in January to February 1978. The swarm was monitored by using portable seismographs. The shallow (&lt;3 km), low level (ML ≦ 2.2), intense (up to 200 events per day) activity occurred in a narrow elliptical epicentral region (2 by 3 km). This active area trends NW-SE, normal to local geologic grain, and appears to be related to the steeply dipping NE trending joints. A search for earlier seismicity in the area suggests that this swarm is possibly related to the Seneca earthquake of 1971 which followed the impounding of Lake Keowee.