From December, 1983 to September, 1989 twelve small earthquakes were recorded for the El Dorado, Arkansas area. Although the hypocenters for these events are poorly defined, the following observations in concert support the conclusion that the quakes were triggered. Prior to 1983 no seismicity was reported in the area, suggesting that the earthquakes were not naturally occurring and may have been the result of human activity. El Dorado is located at the margin of a region of underground waste brine disposal and along a major fault zone. Elevated pore pressures resulting from brine disposal may have reduced the normal stresses across fault surfaces and triggered fault movement.
The two injection wells (Great Lakes Chemical Corporation SWD# 7 and 13) in the El Dorado South field in closest proximity to fault surfaces at the depth of injection also lie at the center of the macroseismic area of a magnitude 2.5 earthquake of December 12, 1988 and show increases in injection rates prior to periods of seismicity. These relationships suggest that pressured fluid injection triggers earthquakes in the area. Seismic energy greatly exceeds injection energy, suggesting that induced earthquakes release tectonic strain.