The north-central Arkansas swarm, which began in January of 1982 and was still active as of December 1983, has produced over 30,000 events. Three of these events had duration magnitudes greater than 4.0. Fifty-three locatable events, in the magnitude range 3.8 and below, were recorded during the 2 1/2 weeks our 10 digital seismographs were in place. The hypocentral locations are confined to a tight cluster, with a radius of approximately 1 km. Depths range between 4 and 7 km. Focal mechanisms determined for 10 of the events show a combination of strike slip and normal faulting.
Source parameters—seismic moment, source radius, and stress drop—were calculated for 48 of the events. The seismic moments range from 1018 to 1021 dyne-cm while the stress drops range from 0.2 to 60 bars. Acceleration spectra are band-limited at the high end at 30 Hz. The stress drops increase with seismic moment up to 1020 dyne-cm but appear to be constant at greater moment values. The Arkansas source parameters are similar to other body-wave source parameters determined from near-source investigations of earthquakes in Eastern and Central North America (e.g., New Brunswick and South Carolina) and in California (e.g., Mammoth Lakes, Oroville, and Anza), but differ substantially from Lg-wave source parameters estimated for earthquakes in the Eastern and Central United States.