I found this at:
http://wwwneic.cr.usgs.gov/neis/bulletin/990930163113_setting.html

Canie

Seismotectonic Summary for the MW 7.4 Oaxaca, Mexico Earthquake of September 30,1999

This normal-faulting earthquake occurred at a depth of 53 kilometers, within the subducting Cocos plate. It occurred at 11:31 local time and its epicenter is located at 16.1°N and 96.7°W. Earthquakes this deep are unable to make direct hits on population centers by surface faulting, such as recently occurred with the MW 7.4 Turkey earthquake of August 17, 1999 and the MW 7.6 Taiwan earthquake of September 20, 19999. Only 3½ months ago, on June 15, 1999, central Mexico was jarred by a MW 6.9 earthquake. With a focal depth of 80 kilometers, that earthquake also occurred within the Cocos plate. Although earthquakes in the depth range of 50-80 kilometers are capable of causing significant losses, these losses typically are much less than caused by the same sized earthquakes striking near the Earth's surface.

The Cocos plate is moving northeastward at about 7.3 centimeters/year and collides with the Pacific coast of Mexico (part of the North American plate). The Cocos plate moves beneath (subducts) coastal Mexico and leads to "shallow" earthquakes such as the catastrophic 1985 Michoacan event and the MS 7.5 Oaxaca earthquakes of 1965 and 1968.

Deeper earthquakes, with characteristics like those of the 9-30-99 event, probably are caused by the sinking of the subducting plate.

The preceding summary was prepared by seismologists at the U. S. Geological Survey, National Earthquake Information Center, Golden, Colorado.