Clustering of earthquakes remains issue of debate

by Xinhua writer Mao Lei

SAN FRANCISCO, March 9, 2010 (Xinhua) --

A string of earthquakes hitting Haiti, Chile and as recently as Turkey this year have fueled the speculation that the "clustering" of temblors may signal the Earth is entering a new period of earthquake cycle, but whether the theory stands is still an issue of debate within the science community.

SIGNS OF INCREASED EARTH ACTIVITY

Stephen S. Gao, a geophysics professor at Missouri University of Science and Technology in the United States, is one of the scientists who argued that the Earth has been witnessing increased activity.

"It is clear that the Earth is significantly more active over the past 15 years than the 20 years before," he told Xinhua in a recent interview.

Gao's statement is based on the study of the so-called "moment release," a measure of the product of the area ruptured by an earthquake and the displacement between the two sides of the fault.

His calculation shows that the moment release per year between 1995 and 2010 is about four times as large as that between 1975 and 1994. Even when the 2004 Sumatra earthquake in Indonesia and its large aftershocks and this year's 8.8-magnitude earthquake in Chile are not counted, the moment release over the past 15 years is still twice as large as that of the previous two decades.

Gao believed that the increased activity could simply be natural fluctuations of the stress field in the Earth's lithosphere, or the outer solid part.

"We do not have a long-enough record of instrumentally determined earthquakes to determine if this is true or not for a longer period," noted the geophysicist.

"Although we are still trying to come up with some explanations for the higher activity over the past 15 years, I do not think global warming or human activities have much to do with it," he added.

MORE RESEARCH NEEDED

According to a recent report by the newspaper USA Today, Marcia McNutt, director of the U.S. Geological Survey (USGS), said that global earthquakes in recent years, starting with the 9.1- magnitude one in Indonesia in 2004, follow a 50-year cycle of earthquake activity.

The last cycle, in the 1960s, produced two mega-quakes with a magnitude-9.5 earthquake in Chile and a magnitude-9.2 one in Alaska. The one in Chile in 1960 is the largest earthquake ever instrumentally recorded.

Other scientists in the United States, however, are not as convinced that a new spike of major earthquakes is emerging.

Statistics by the USGS indicated that seven out of the 15 largest earthquakes since 1900 occurred in the period between 1950 and 1964, with four quakes on the top 15 list happening after the end of 2004 including the 8.8-magnitude one in Chile this year.

However, when a larger pool of earthquakes with magnitude 8.0 or above are considered, the pattern of the "clustering" or " grouping" of large quakes becomes much less apparent, said Jian Lin, a geologist at the Woods Hole Oceanographic Institution.

Whether recent earthquakes are part of a new phase of long-term cycle is subject to ongoing research, he told Xinhua.

"The answer to the question will also depend on how large the earthquakes one would like to consider in a statistical study," Lin said.

Dr. Chris Goldfinger, a marine geologist at Oregon State University, also cautioned that scientists should not rush to conclusions.

A global "clustering" of big earthquakes is very difficult to test, as "our history for most faults is not long enough," said Goldfinger, director of the university's Active Tectonics and Seafloor Mapping Laboratory.

It's not impossible that there could be such "clustering," as earthquakes may trigger other earthquakes through transfer of stress in the crust and could result in a peak in earthquake activity, the scientists said.

"Overall though, a general increase in earthquake frequency isn 't something that would be very likely without some tectonic explanation, I know of no such mechanism," he told Xinhua.

Editor: Lin Zhi

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