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In the Tsunami's Wake, New Knowledge About Earthquakes

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If any good has come from the recent devastating earthquakes off Sumatra, it is that they are providing scientists with unprecedented clues to understanding how these large undersea earthquakes occur and how they create tsunamis.

In the wake of the devastating earthquake and tsunami on Dec. 26, 2004, and the March 28 magnitude-8.7 quake, scientists have gathered new information to answer provocative questions: Did the Dec. 26 earthquake shift stresses in the Java Trench where the earthquakes occurred and help trigger the March 28 quake? Was the Dec. 26 quake shallow, and did this contribute to generating such a great tsunami? Why did the March 28 quake generate only a minor tsunami even though it was a great quake?

The earthquakes occurred in a subduction zone where two of Earth’s great crustal plates are colliding. As one plate is thrust beneath another to form the deep Java Trench, it causes the buildup of stress in rock formations, which slip suddenly to generate earthquakes.

Shifting stress

Jian Lin, a marine geophysicist at WHOI, and his colleague Ross Stein of the U.S. Geological Survey in Menlo Park, Calif., have investigated thrust earthquakes both on land and under the oceans for several years. In a study published in early 2004, the two scientists theorized that as thrust earthquakes alleviate stress in one region, they can shift that stress to adjacent areas.

In a domino-like effect, stress can transfer through the crust, interact with neighboring faults, and trigger another earthquake elsewhere. Lin and Stein noted that such domino-like reaction appears to have occurred in a series of moderate-sized thrust quakes in central California in the 1980s.

Is the domino-like reaction also occurring in the Java Trench? Recently a number of research groups had calculated that the Dec. 26 quake shifted additional stress further south along the Java Trench. That could have helped trigger the March 28 earthquake, which occurred about 120 miles southeast of the December quake.

“If this is true, this could be good news for the people living close to a large part of the Java Trench where the stress was released by the recent great quakes,” Lin said. But unfortunately both quakes have now added stress to some other fault lines in this region, making them more susceptible to future quakes.

Comparing the greatest quakes

With an estimated magnitude as large as 9.3, the Dec. 26 earthquake could be the second largest ever recorded. The largest earthquake ever recorded was a magnitude-9.5 event in a geologically similar ocean trench off Chile in South America in 1960, which killed an estimated 2,000 people.

Lin found a striking comparison between the1960 and 2004 earthquakes: Both ruptured the seafloor for distances as long as 1,000 to 1,300 kilometers (about 625 to 810 miles), and both caused ocean-spanning large tsunamis. However, the rupture plane of the 2004 quake appears to be significantly shallower and narrower in its down-dip direction, resembling a long, “skinny” stripe.

“What we learn from the Dec. 26 event will probably define what we know about tsunami-generating earthquakes for a long time,” said Lin, who plans to survey the rupture zone on the Java Trench this fall on a research cruise with a team of international colleagues. “The new observations should help us to improve earthquake models and bring us closer to understanding the behavior of these devastating events.”

But there are still many unsolved mysteries of the Dec. 26 and March 28 events, including why the March 28 quake created only a minor tsunami. “It is clear that a tsunami early warning system in the Indian Ocean is badly needed,” Lin said.

Images

A DOMINO-LIKE EFFECT?—Scientists theorize that the magnitude-9.3 earthquake off Sumatra on Dec. 26 may have shifted additional stress further south along the Java Trench, which may have helped trigger the March 28 magnitude-8.7 earthquake. (Map provided by K. Sieh, California Institute of Technology)

STRESS TRIGGERING—The March 28 earthquake occurred to the south of the December 2004 earthquake rupture zone, supporting the theory that the 2005 earthquake may have been triggered by the 2004 quake. Such stress triggering behavior is very similar to a sequence of thrust earthquakes in central California in the 1980s, reported by Lin and Stein in 2004. The small yellow circles in the pink rupture zone, which is many times larger than the 2005 rupture zone shown in blue, represent aftershocks within the first three weeks after the Dec. 26 earthquake. The data were provided by the U.S. Geological Survey. (Illustration by Jack Cook, WHOI)

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