Abstract

An earthquake that occurred 9 April 1993, about 80-km south of San Antonio, is the largest earthquake known (mbLg = 4.3) in south-central Texas in historic times. The earthquake was felt over a 4000-km2 area, and modified Mercalli intensities were as high as VI at a gas production facility near Fashing, Texas. Similar, smaller events have occurred since 1973 at three sites near Fashing, Pleasanton, and Falls City, Texas. We propose a list of nine yes-no questions to assess whether a sequence of earthquakes may have been induced by fluid withdrawal. The criteria suggest that at least two sites (Fashing and Pleasanton) had earthquakes that were induced by fluid withdrawal related to ongoing oil and gas production. An alternative possibility is that these events are the southwesternmost examples in a band of weak, natural seismic activity between 135 and 240 km from the coast of the Gulf of Mexico and extending from this region east to the Texas-Louisiana border. In addition, some injection occurs at all three sites, although the volumes of fluid injected at Fashing and Pleasanton are low enough as to make injection an unlikely cause.

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