A Note on the Goochland County, Virginia, Earthquake of March 15, 1991

Abstract

The Goochland county, Virginia, earthquake of March 15, 1991 (mb (Lg) = 3.8), was the largest earthquake recorded in the central Virginia seismic zone (CVSZ) since the Cunningham, Virginia, earthquake of 1984 (mb(Lg) = 4.2). The 1991 event was felt over 23,000 km2 with a maximum MM V epicentral intensity. The preferred depth of focus was 12.5 km as determined graphically from the T2 vs X2 plot, whereas the computer program HYPOELLIPSE gave a more model-sensitive depth of 15 km. The average focal depth for shocks in this zone is 8.6 km, and the 90% quantile depth is 13.3 km. This earthquake is important because it is the largest shock that has occurred near the base of the CVSZ since network recording begun in 1978. P wave first motions and (SV/P)Z amplitude ratios define a focal mechanism exhibiting primarily strike-slip faulting with a north-south or east-west strike. The P axis trends northwest, similar to deeper focus earthquakes (>8 km) in the CVSZ. P wave spectral analyses indicate a corner frequency at 8 Hz and a low stress drop level of under 100 bars.