Ocean-drilling Expedition Cites New Evidence Related To Origin And Evolution Of Seismogenic Faults

Since September 2007, rotating teams of scientists have spent months aboard Japan's drilling vessel, CHIKYU, investigating the Nankai Trough, a seismogenic zone located beneath the ocean off the southwest coast of Japan. Drilling operations, managed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its Center for Deep Earth Exploration, have resulted in a collection of cored samples from the sea floor, which have provided scientists with deeper insights into the geologic past of the area.

Discussion in the above-noted article focuses on the Nankai Trough, in which the Philippine Sea plate slips below the Eurasian Plate with a velocity of 4 cm per year. This area is one of the most active earthquake zones on the planet. While being subducted, sediments are scraped off the oceanic plate and added to the overriding continental plate. Due to the movement of the plates these so-called accretionary wedges are exposed to enormous stress that form large faults. The landward wedge in the Nankai Trough is completely intersected by such a prominent fault which extends laterally over more than 120 km. Scientists refer to this structure as "the megasplay." Movements along such megasplay faults during large magnitude earthquakes generated at depth may rupture the ocean floor and generate tsunamis.

"Our knowledge of megasplay faults up till now has been based on seismic or modelling experiments accomplished over the last twenty years," says Michael Strasser of Post-Doc Fellow of the Center for Marine Environmental Sciences (MARUM) at University of Bremen. "For the first time, with cored samples brought onto the CHIKYU, it has become possible to reconstruct the geological history of a fault in great detail." With his associates, Dr. Strasser found that the fault in the Nankai Trough originated about two million years ago. From the information recorded in the cores, the research team can draw conclusions on the mechanics of the accretionary wedge. They also can infer in which geological time periods the fault was most active.

"Our most significant conclusion is that splay fault activity varies through time," Dr. Strasser states. According to Strasser, after an initial period of high activity, the movement along the fault slowed down. Since about 1.55 million years ago, this fault has been reactivated, favoring ongoing megasplay slip along it.

"It is absolutely fascinating to be part of NanTroSEIZE," says Strasser, noting that the expedition series aims to sample and monitor activity at the point where earthquakes originate. "NanTroSEIZE is something completely new and innovative in scientific drilling," Strasser explains. "Ultimately, we hope to detect signals occurring just before an earthquake to get a better understanding of the processes leading to earthquakes and tsunamis."

The Nankai Trough is particularly suited for this experiment because historical records of earthquakes and tsunamis in this area date back into the seventh century. Additionally, the area where earthquakes are generated, the so-called seismogenic zone, is located at a relatively shallow depth of about six kilometers below the seafloor.

In 2007 and 2008, during the first stage of NanTroSEIZE, the deep sea drilling vessel CHIKYU carried out three expeditions. This drilling project consists of four stages in all, and ultimately focuses on "ultra-deep" drilling that can reach the seismogenic zone, where great earthquakes have occurred repeatedly.

During upcoming expeditions, the Nankai Trough boreholes will be equipped with instruments to establish an ocean observatory network. Currently, scientists are making preparations to install monitoring devices for continous measurements of the Nankai Trough. Prof. Gaku Kimura of University of Tokyo, who led an earlier NanTroSEIZE expedition 316 as Co-Chief Scientist says, "Not only do we have new insights about historic fault activities in Nankai Trough, but the data strongly suggests that the megasplay fault may be a key factor in the occurrence of large earthquakes in the future." He adds, "Greater understanding about the processes of earthquake and tsunami generation in the active subduction zone will be a great contribution to society."