FAULT PLANE SOLUTIONS AND RELOCATIONS OF RECENT EARTHQUAKES IN MIDDLETON PLACE SUMMERVILLE SEISMIC ZONE NEAR CHARLESTON, SOUTH-CAROLINA

The Middleton Place Summerville Seismic Zone (MPSSZ), located about 20 km northwest of Charleston is the most active seismic zone in South C arolina. Between 1980 and 1991, 58 events with M(d) 0.8 to 3.3 were re corded in MPSSZ. They lie in a diffuse area of 11 km by 14 km of which over two-thirds are located in a narrow 5 km by 6-km zone. The hypoce ntral depths range from 2 to 11 km with over 90% deeper than 4 km. Sin gle fault plane solutions were obtained for 35 events. Based on the fo cal mechanisms the earthquakes were grouped into five subsets. The mea n P-axis of all fault plane solutions is oriented N63-degrees-E, in ge neral agreement with the direction of S(Hmax) obtained from in situ st ress measurements. Of the 35 events, 18 are associated with reverse fa ulting on NW - SE striking and SW dipping fault planes. These events w ere inferred to be associated with the Ashley River fault zone, which is not a planar feature, but is composed of short segments of varying strikes (N20-degrees-W to N70-degrees-W) and dips (40-degrees to 70-de grees-SW). Eleven events were associated with strike-slip motion on NN E - SSW striking vertical faults and with thrust faulting on N - S ori ented faults dipping to the west, respectively. These two sets are ide ntified as being parts of the Woodstock fault zone. The concentrated z one of seismicity included events associated with both the ARF and WF zones suggesting that it is at the intersection of these two fault zon es.