Science of the Sichuan earthquake

The first detailed portrait of the ground motions generated by the earthquake that devastated China's Sichuan province on May 12 will be presented today in a press conference and a scientific session 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 will discuss the Sichuan earthquake at a press conference today (May 30, 2008) at 11:30 am Eastern Daylight Time at the meeting. Reporters are invited to attend this event in the Press Conference Room (Room 301, Level 3, Conference Center), or to follow the instructions below to call in to the press conference and view the speakers' slides on the Web.

Chavez will also give a presentation today on the Sichuan earthquake findings in a scientific session on “Observation, Modeling, and Economics of Extreme Events” (Session U53A), which begins at 1:30 pm.

Call-in instructions:

* From USA and Canada, call (toll free): +1 888 481 3032
* From other locations, call: +1 617 801 9600

When prompted, please enter this access code: 115139
Viewing speaker slides on the Web::

To view slides shown at Chavez's press conference as they are presented, please go to this website:
http://www.visualwebcaster.com/event.asp?id=48707

Then, type in your name, employer, and email address, and click “Register”.
At the new web page that appears, click on the “Slides Only” button. When another web page then opens (the 'player' window), please click on its “Ask A Question” button, type in your name again, and click “Submit”. Slides shall be displayed one at a time in a portion of the player window.