On December 11, 1969, an earthquake occurred north of Richmond in the central Virginia seismic zone. Microseismic and macroseismic studies of this shock were made at V.P.I.
Epicentral location and origin time were found using arrival time data from six regional stations. The epicenter as calculated by V.P.I. and by the USC&GS differed by about 30 miles. These differences are probably due to (1) lack of azimuthal station control, (2) inadequacy of the present knowledge of variations in crustal velocity and/or structure for the area, and (3) differences in the models used in calculations. The epicenters were 37.9°N–77.7°W (VPI) and 37.8°N–77.4°W (USC&GS). Origin times were 23:44:38.7 (VPI) and 23:44:39.2 (USC&GS). A magnitude (meus) of 3.5 was found using the BLA record.
Results of an intensity questionnaire survey indicated a maximum (MM) intensity of IV and a felt area of approximately 6,500 square miles in central Virginia. Also, a possible “halo” effect was implied by several isolated felt reports.