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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.