The Gulf of Mexico is nearly aseismic; no earthquake of Richter magnitude larger than 5.0 has been reported there in historic time. An unusual earthquake with a magnitude of about 5.0 did occur on July 24, 1978, and for this event it has been possible to obtain a focal mechanism and a reliable location, including an accurate depth of focus. The event occurred near the edge of the Mississippi Fan at a depth of 15 km, which is about the depth of the Moho. Its location and reverse-faulting focal mechanism suggest that it may be related to stresses associated with the downwarping of the lithosphere caused by the accumulation of sediments from the Mississippi River. A crude calculation confirms that the rate of accumulation of stress caused by downwarping is large enough to cause the observed seismicity. Other earthquakes that have occurred in the Gulf of Mexico are situated near the boundaries of distinct geologic regions, suggesting that these may represent areas of weakness in the crust.