Rock Samples May Explain Causes Of High-Magnitude Earthquakes

What causes large-scale destructive earthquakes, like that of the 2011 Tohoku Earthquake in Japan? Rock samples from the Pacific Ocean may reveal the answer.

AsianScientist (Jun. 30, 2011) – What are the causes of large-scale destructive earthquakes, like that of the Tohoku Earthquake that struck Japan in mid-March?

New samples of rock and sediment from the depths of the eastern Pacific Ocean, uncovered during a recent month-long Integrated Ocean Drilling Program (IODP) Costa Rica Seismogenesis Project (CRISP) Expedition, may reveal the answer.

The sediment core samples, retrieved by the drilling vessel JOIDES Resolution, contain detailed records of about two million years of tectonic activity along a seismic plate boundary. Scientists aim to use the samples to better understand the processes that trigger large earthquakes at subduction zones, where one tectonic plate slides beneath another.

“Knowing how the plates interact at the fault marking their boundary is critical to interpreting the behavior and frequency of earthquakes in the region,” says co-chief scientist Kohtaro Ujiie of the University of Tsukuba in Japan, who led the expedition with co-chief scientist Paola Vannucchi of the University of Florence in Italy.

The recent Tohoku Earthquake in Japan was generated in an erosive portion of the plate interface.

More than 80 percent of global earthquakes above 8.0 magnitude occur at subduction zones, which are formed at convergent margins along the coasts of the East Pacific from Alaska to Patagonia, New Zealong, Tonga and Marianas – all the way to Japan and the Aleutians. For this reason, the Pacific Ocean is a primary target for research into the triggering mechanisms of large quakes.

Though the only one of its kind that is accessible to research drilling, the seismically active CRISP research subduction zone is representative of 50 percent of global subduction zones, making scientific insights gleaned here relevant to earthquake-prone regions all around the Pacific Ocean.

This unique expedition focuses on the properties of erosional convergent margins, where the overriding plate gets “consumed” by subduction processes. These plate boundaries are characterized by trenches with thin sediment covering less than 400 meters (1,312 feet), fast convergence between the plates at rates greater than eight centimeters per year, and increased seismicity.

In the future, the CRISP team hopes to return to the same drill site to directly sample the plate boundary and fault zone before and after seismic activity in the region. The changes observed may provide new insights into how earthquakes are generated.

Other IODP geoscience research drilling programs include the Nankai Trough Seismogenic Zone Experiment (NantroSEIZE) near the southeast coast of Japan. Japan’s Nankai Trough was the center of two magnitude 8 earthquakes in 1944 and 1946.

Data from the NanotroSEIZE project will similarly inform on earthquakes that were formed along accretionary margins, such as the 1964 Alaska and the 2004 Sumatra quakes.

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Source: National Science Foundation
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Wendy Yang is a public health sciences major at the University of California, Irvine. She enjoys covering science and research news from Asia.

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