While Mexico City digs out from under the destruction of Thursday's earthquake and Friday night's major aftershock, seismologists are beginning to dig into the data the events have generated. As was the case with a major Chilean quake March 3, the Mexico quake occurred in a heavily instrumented area. Seismologist James Burne at Scripps Institution of Oceanography says that, in terms of data on seismic waves, ``it will be the best-recorded quake in history.''
Thus scientists are beginning to build a data base from intensely monitored earthquakes that they hope will offer new insight into quake mechanisms. Building engineers and land-use planners also should learn more about how to minimize potential earthquake damage.
During the March Chilean quake, instruments recorded in detail strong ground motion and building damage, among other measurements. Many modern reinforced concrete high-rise buildings came through the severe shaking fairly well, according to Bruce Bolt of the University of California at Berkeley, who inspected the damage at the time. He also reported that he had never seen such ``energetic ground motion records from any California earthquake.''
Now the Mexican quake has added more detailed seismic records for study. It was not yet known, at this writing, how various types of buildings fared in Mexico City. The silt of the ancient lake bed on which central Mexico City rests is probably too fine to have liquified and damaged buildings by sudden slumping in most cases. Early reports suggest that damage mainly occurred because of severe shaking. However, slow subsidence of that silt has weakened many structures and may have made them more suscepta ble to damage by shaking.
Dr. Brune cautions that it will take months to sketch what happened geophysically Thursday, when the quake erupted at 6:18 a.m. Pacific daylight time, a moderately severe magnitude of 7.8 on the Richter scale. This scale ranks quake intensity according to the amplitude of certain seismic waves. An increase by one magnitude means a tenfold increase in this wave amplitude and roughly a 30-fold increase in seismic energy. Friday's aftershock registered 7.3.
Analysis may eventually help explain the so-called seismic gap, which enabled seismologists to anticipate the quake. The epicenter was some 250 miles southwest of Mexico City, just inland from the Pacific Coast in a region that had been unusually inactive. This suggested that a major shock was perhaps 10 to 20 years overdue. So, Brune says, ``We anticipated the quake'' and instrumented the area.
Seismologists have identified a number of these quiet gaps in seismically active areas around the rim of the Pacific Ocean basin where they suspect major earthquakes are overdue. A similar suspicion led Karen McNally of the University of California at Santa Cruz, then with the California Institute of Technology, and Lautaro Ponce of the University of Mexico to string seven seismometers between the mountains and the coast just in time to catch the 7.8 magnitude quake that hit Mexico's Oaxaca region in 19 78.
Most earthquakes are generated along the edges of the 10 or so major plates into which Earth's crust is broken. Sometimes two plates slide past each other. Quakes there represent the sudden slippage that releases strain where sections of the two moving plates have locked together.
In other places, the ocean plate dives under a continental plate -- a process called subduction. Again, where the two plates stick, strain accumulates until rock breaks and the plates slip. They typically may move several meters. This is the process that generated Thursday's quake.
There may be more to Mexico's earthquake risks than the action believed responsible for Thursday's quake. Three geologists -- two from Mexico and one from the United States -- point out that the Richter magnitude 8.0 Oaxaca quake of 1931 probably resulted from a vertical break in the subducted Cocos plate, well inland from the coast. This so-called normal faulting is unusual. In the Sept. 5 issue of Nature, the three geologists warn, ``Such earthquakes, for which the periods of recurrence are unknown, c onstitute an extremely high seismic risk to population centers of Mexico.'' ILLUSTRATION: PACIFIC OCEAN Undersea trench Rupture in Coscos plate caused 1931 Oaxaca earthquake. Pressure builds Pressure released MEXICO COCOS PLATE Acapulco Mexico City Oaxaca