Mexican quake update.

The earthquake that killed more than 4,600 people when it rattled southwestern Mexico two weeks ago (SN: 9/28/85, p. 196) was larger than seismologists originally reported. Last week, the National Earthquake Information Center in Golden, Colo. upgraded the main shock from a magnitude 7.8 to a magnitude 8.1 quake -- meaning that three times the amount of energy was released than thought prevously. A large aftershock which followed the main jolt the next day was also upgraded from magnitude 7.3 to magnitude 7.5 on the Richter scale.

The reason for the promotion is that data from more than 10 seismic stations, located in many directions from the quake epicenter, such as in Europe and Asia have become availabe; the original estimates were based on a few stations only and these were only situated to the north of Mexico. Such upgrades are not uncommon, says Russell Meedham of the National Earthquake Information Center. The magnitudes of at least 50 percent of the 1,000 or so earthquakes the center reports every month are routinely changed as more information comes in, he says. The center is still wating for Mexican stations to provide data on the main shock.

The center also reports that there have been at least 12 aftershocks -- the three largest have measured 7.5, 5.3 and 5.0 on the Richter scale.