Seismological Research Letters, Volume 62, No. 2, April - June, 1991 POSSBILE TRIGGERINC OF EARTHQUAKES BY UNDERGROUND WASTE DISPOSAL IN THE EL DORADO, ARKANSAS AREA Randel Tom Cox Department of Geology University of Missouri Columbia, Missouri 65211 ABSTRACT 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 sup- port the conclusion that the quakes were triggered. Prior to 1983 no seismi- city 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 magni- tude 2.5 earthquake of December 12, 1988 and show increases in injection rates prior to periods of seismicity. These relationships suggest that pres- sured fluid injection triggers earthquakes in the area. Seismic energy greatly exceeds injection energy, suggesting that induced earthquakes release tec- tonic strain. INTRODUCTION El Dorado is located on the seismically quiet northern Gulf Coastal Plain, and prior to 1983 no earthquakes had been reported within 75 km of the city. Between Dec. 10, 1983 and Feb. 5, 1989 five earthquakes between magnitude 2.0 and 3.0 in the El Dorado, AR area were instrumentally recorded by the Center for Earthquake Research and Information (CERI) in Memphis, TN. By Sept. 1, 1989 seven more events (magnitude 1.0 to 1.7) probably originating in the same area were recorded. (Following compilation of this report another earthquake (magnitude 2.2) occurred in the El Dorado area on February 4, 1991). The absence of macroseismicity in the El Dorado area prior to the sequence of tremors that began in 1983 suggests the possibility that the seismicity is not natur- ally occurring and was induced by human activity. Various activities have been recognized to trigger earthquakes, including reservoir impoundment, strip mining, subsurface mining, all recovery, underground explosions, and underground fluid waste injection (Simpson, 1986). Oil production in the El Dorado area began in the early 1920's and peaked before mid-century, unaccom- panied by detected seismicity. Only minor amounts of oil have been produced in recent decades. However, underground disposal of waste brine generated by the bromine industry began in the early 1970's, and in 1983 large volumes began being injected under pres- sure into wells in the El Dorado South field near the epicentral area. Prior to this time, the only injection under pressure in the field involved a single low volume hazardous waste disposal well (Cox and VanArsdale, 1991). Underground fluid disposal by pressured injection can induce seismicity on preexisting fractures at pres- sures lower than those necessary to fracture rock. The normal stress on faults is reduced by increasing pore pressure, thereby triggering fault movement under existing shear stress (Simpson,1986). This region of the northern Gulf Coast is characterized by an east- west oriented maximum compressional stress (Haim- son, 1977; Zoback and Zoback, 1981; Dart, 1987) that may be capable of generating earthquakes if fault strength is reduced. Three kilometers of Mesozoic and Cenozoic sedi- ments overlie a transition from folded Paleozoic base- ment to extended basinal crust (Keller et al., 1989). The South Arkansas fault zone, a segment of a major structure known as the Mexia-Talco fault zone in Texas and the Pickens-Gilbertown fault zone in Mississippi and Alabama, transects the region (Fig. 1). This structure was initiated as down-to-the-basin basement faulting during the opening of the Gulf of Mexico. The fault zone experienced recurrent activity through Mesozoic and early Cenozoic time, warping the overly- ing sediments and giving rise to a zone of horst and graben blocks (Murray, 1961). Typical development of this horst-graben system can be seen in Figure 2. The fault zone is nearest to the field of waste brine disposal at the point of seismicity (Fig. 1), suggesting that this structure or a subsidiary structure could be the source of the earthquakes. 113