GPS-DERIVED STRAIN AND STRAIN ZONATION NEAR CHARLESTON, SOUTH CAROLINA

TRENKAMP, R. AND TALWANI, P., Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, trenkamp@geol.sc.edu, pradeep@sc.edu.

Although the average strain rates within stable continental regions (SCR) are 2-3 orders of magnitude lower than at plate boundaries, pockets of higher strain rates might be expected where large SCR earthquakes have occurred in the past. Reoccupation in February 1999 and November 2000 of a 20 station grid near the location of current seismicity (and the inferred location of the 1886 Charleston, South Carolina earthquake) have refined and are consistent with previous shear strain estimates. These refined estimates suggest an average shear strain rate over the study area that is at least an order of magnitude higher than the average intraplate strain rate for the North American Plate east of the Rocky Mountains. Further, subnet (GPS-GPS) strain analysis suggests strain zonation within the study area. The SCR, within which the seismogenic zone is located, has a shear strain rate of ~10⁹ rad yr^-1 while the area (~60km x 100 km) immediatley surrounding the seismogenic zone has a shear strain rate of ~5.5 x 10^-8 ± 3.1 x 10^-8 rad

yr^-1. Within the approximately 20km x 30km region of current, active seismicity, the strain is associated with a shear strain rate of ~2.1 x 10^-7 ± 1.0 x 10⁷ rad yr¹, two orders of magnitude larger than the backround (SCR) strain-rate. The average orientation of the "direction of minimum extension" is consistent with the orientation of measured SH_max (~N60^oE).