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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 June 2000


News Notes
 Hazards Engineering

Taiwan quake is data boon

Seismologists and engineers say that data from last September’s multiple earthquakes in Taiwan have given them a better understanding of how buildings are affected by ground motion. The enormous amounts of data from the eight Chi-Chi earthquakes will lead to advances in building design, they say. The earthquakes, which killed more than 2,400 people, occurred nearly in the center of a high-density network of recording stations and created the most thorough picture of what happens to the ground during a fault rupture.
 
“Engineers have eight times more data to study how buildings, bridges, freeways, dams and harbors can be designed more earthquake resistant,” says Leon Teng, a professor in the Department of Earth Sciences at the University of Southern California (USC).

Teng and researchers from the Institute of Earth Sciences, Academia Sinica and the Taiwan Central Weather Bureau, both in Taipei, created a movie of the wavefield showing the acceleration, velocity and displacement of wave motions. The movie — presented at the April Seismological Society of America meeting in San Diego, Calif. — showed that the rupture evolved differently than was previously thought.


    The Chi-Chi Earthquake caused by the Chelungpu 
    fault surface rupture cut through the Ta-Chia River, 
    forming a fault scarp and an instant six-meter-high
    waterfall. LeonTeng, USC.

The current theory of fault rupturing is analogous to tearing a sheet of paper, Teng says, with the rupture velocity a measure of how fast the tearing point travels. This tearing point was thought to be regular in space and time, but this description may no longer be entirely accurate. Analysis of the movie indicated that the Chi-Chi earthquakes did not follow the accepted model.

“The spots of tearing change with time and do not seem to follow any obvious order. This is new to seismology and is important to earthquake shaking hazard evaluation,” Teng says. Seismologists will no longer be able to calculate the rupture velocity of earthquakes because the velocity calculation assumed that fault rupture points move regularly through time and space.

Although not knowing the rupture velocity makes evaluating hazards from earthquakes more difficult, advances in building designs are possible because having so complete a data set of earthquake wave motions allows engineers to determine the predominant wave frequencies that shake a building to its foundation.
 

    The Chelungpu fault runs through the Kuang-Fu
    School running track, forming a three-meter scarp.
    Leon Teng, USC.
“Those are the numbers that engineers have been craving,” says James Dolan, a colleague of Teng’s at USC, referring to the Chi-Chi data set. “I think that we’ll see enormous benefits from these data,” he says.

Digital strong-motion recorders over the 100-kilometer-long Chelungpu fault surface rupture are only three to five kilometers apart, creating a network that records wave motion from the epicenter outward. “It just happened that the earthquakes occurred in the middle of the network and everything was recorded,” Dolan says.

The data gave seismologists and engineers a unique view of the complete response of the ground to seismic waves, including the initiation of the quakes and the propagation of waves. Teng says that this network and others like it act as observation tools for understanding the physics of earthquakes. And the more than 70,000 aftershocks that occurred generated data that will help to clarify the geometry of the fault plane.

Some engineering changes could be immediate, Teng says, but every country faced with potential earthquake disasters will respond differently. The Taiwan Central Weather Bureau released the data set to the public within a month after the earthquakes, allowing anyone to access it from the Web.

Julia Cole
Geotimes contributing writer

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