From December, 1983 to September, 1989 twelve small earthquakes were recorded for the El Dorado, Arkansas area. Magnitudes of these earthquakes were well below damaging levels. Prior to this time 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 (locking) stresses across fault surfaces and triggered fault movement. Two injection wells (Great Lakes Chemical Corporation SWD# 7 and 13) in the El Dorado South field are in closest proximity to fault surfaces at the depth of injection. The two wells also lie at the center of the macroseismic area and show increases in injection rates prior to periods of seismicity. These relationships suggest that pressured fluid injection triggers earthquakes in the area. Future research to corroborate these results should include detailed seismological studies of the El Dorado South field and detailed studies of formation pressures, in situ stresses and geologic structure for all sites of pressured fluid injection and secondary oil recovery operations in the region.
Cox, Randel and VanArsdale, Roy B.. 1991. Investigation of the Relationship of Earthquakes and Underground Waste Disposal in The El Dorado Area, Arkansas. Arkansas Water Resources Center, Fayetteville, AR. MSC076. 39