17.08.: Seafloor under pressure

The origin and evolution of seismogenic faults –
New evidence from an international ocean drilling expedition

To gain a better understanding of what triggers earthquakes, the long term research project, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), has been initiated under the auspices of the Integrated Ocean Drilling Program (IODP). Within NanTroSEIZE, international teams of scientists investigate the Nankai Trough located beneath the ocean off the southwest coast of Japan. Dr. Michael Strasser from MARUM at Bremen University, Germany, together with colleagues from the USA, Japan, China, France, and Germany obtained samples from the sea floor which gave a deeper insight into the geologic past of the area. The team publishes their results in the upcoming issue of Nature Geoscience (Online publication, August 16th, 2009).

In the Nankai Trough, 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 so far has been based on seismic or modelling experiments accomplished over the last twenty years. But with the core samples we took on board the CHIKYU, for the first time, it became possible to reconstruct the geological history of such a fault in great detail.“ says Dr. Michael Strasser. Together with his co-workers, Strasser found out 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. After an initial period of high activity, the movement along the fault slowed down. But since about 1,55 million years ago this fault has been reactivated, favoring ongoing megasplay slip along it.

“I am absolutely fascinated about being part of NanTroSEIZE, which aims at sampling and monitoring at the point where earthquakes actually originate, something that is really completely new and an innovative field in scientific drilling“ Dr. Michael Strasser explains. “Ultimately, we all hope to detect signals occurring right 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 well into the seventh century. Moreover, 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.

During the upcoming expeditions of CHIKYU, the Nankai Trough drill holes will be equipped with borehole instruments to establish an ocean observatory network. Currently scientists are already making preparations to install monitoring devices for continous measurements of the Nankai Trough. Prof. Gaku Kimura of the University of Tokyo, Japan, who led the Stage 1 Expedition 316 as the Co-Chief Scientist says, “not only can we have new insights about history of fault activities in Nankai Trough, but it strongly suggests the megasplay fault may be one of the key factors of occurrence of large earthquake in the future”. Also he continues, “to understand much about the processes of earthquake and tsunami in the active plate subduction zone by drilling with the Chikyu, I’m expecting to be able to contribute for society”

Contact:
Albert Gerdes
MARUM Public Relations