The first detailed portrait of the ground motions generated by the earthquake that devastated China's Sichuan province on May 12 were presented last week at the AGU 2008 Joint Assembly.

Seismic waves calculated by a computer model, and verified by ground-motion velocities measured during the earthquake at Chinese seismological stations, indicate that the ground suddenly jumped, shifted, or sank by at least one and a half meters (5 feet) in a matter of seconds, finds Mario Chavez, Professor of Seismological Engineering at the National Autonomous University of Mexico in Mexico City.

By indicating how much and in which direction the ground moved locally at millions of one-kilometer-square areas throughout a vast region including the earthquake's epicenter, Chavez's preliminary results could help the Chinese target aid to the hardest hit of those areas not yet reached by emergency crews. The data may also help pinpoint which of hundreds of dams in the stricken region are the most at risk, he adds.

"Even though the findings are preliminary, they can be used. It's better to have this information than no information," Chavez says The ground-motion data offer "an explanation of why we observed so much damage in the region," Chavez adds.

Displacements of at least 1.5 meters (5 feet) would have shattered rock and could readily explain, for instance, the landslide that formed a fragile dam across the Jianhe River and forced the evacuation of approximately 160,000 people.

Chavez's study also indicates that the ground motions caused by the 7.9 magnitude quake took place at velocities of at least 65 centimeters (2.1 feet) per second.

In size and speed, the motions are similar to those that were recorded at the epicenter of an 8.1 magnitude earthquake that killed 30,000 people in Mexico City in 1985, Chavez says. When tested against the ground motions from that quake, an earlier version of the computer model proved highly accurate, he notes.

To verify the calculations this time, he required seismographic measurements from the Chinese quake. His 23-year-old son Erik, who is a student in China and speaks the language, helped him obtain those measurements from Chinese scientists just last week. Chavez says he'll make his new ground-motion findings, which he completed Tuesday, available to Chinese colleagues as soon as he can.

Chavez discussed the Sichuan earthquake at a press conference May 30, 2008.