Peru Quake Occurred Outside Seismic 'Gap,' Bolstering Scholar's Theory

December 06, 1996

CHAMPAIGN, Ill. -- The occurrence of a large earthquake near the town of Nazca in southern Peru on Nov. 12 lent further support to a University of Illinois researcher's theory that large earthquakes will not happen in another region near the quake site.

In a paper to be published in the Dec. 15 Journal of Geophysical Research, Wang-Ping Chen, professor of geology and theoretical and applied mechanics at the U. of I., and former graduate student Honn Kao (now at the Institute of Earth Sciences, Academia Sinica, in Taipei, Taiwan) predicted that large earthquakes would not occur in a certain region along the coast of Peru.

"Most of the world's earthquakes occur along subduction zones, where one tectonic plate is plunging beneath another," Chen said. "We found a correlation between the shape of the descending plate, the occurrence of large 'normal faulting' earthquakes offshore, and nearby regions of low seismicity, called seismic gaps."

Based on their study of major subduction zones, Chen and Kao identified nine areas where such a correlation holds and conspicuous gaps exist in the seismic distributions. These seismic gaps are permanent features, Chen said, and will not be filled in the future by major earthquakes. A model developed by the researchers accounts for the absence of seismic activity within the gaps by relating the spatial patterns of seismicity around the intersecting plates to the mechanical properties of the subducting plate.

"If the downward moving plate is bent sharply enough, it yields and deforms easily under any additional stress," Chen said. "Because the plate interface does not accumulate any stress, no significant seismicity occurs in the area, even when nearby faults rupture."

The recent earthquake in Peru, which claimed 22 lives and left more than 9,000 people homeless, occurred just south of an area tentatively identified by the researchers as another permanent seismic gap.

"All the evidence fits our model," Chen said, "but we are missing a critical piece of information from a large earthquake that occurred offshore in 1908. We can determine the size and location of that earthquake, but we don't know anything about the mechanism behind it."

As a result, the researchers treated the region in Peru as a test case, and made two predictions:

-- If another large earthquake occurs off the coast, it will be of the normal faulting variety.

-- When future inland earthquakes occur, they will not fall within the seismic gap.

"In the past, some researchers have identified this region as a seismic gap that would be filled by a large earthquake, and they tended to concentrate their predictions in this region," Chen said. "We contend, based on our model, that the gap is a permanent feature and will not see any major earthquake activity. The fact that this latest earthquake occurred just outside the gap is consistent with our theory."

University of Illinois at Urbana-Champaign

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