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New Report Highlights Stress Change During 2011 Tohoku-Oki Earthquake

February 8, 2013
Image Credit: concept w / Shutterstock

Alan McStravick for redOrbit.com – Your Universe Online

As we are quickly approaching the two-year anniversary of the Tohoku-Oki earthquake that prompted the devastating Honshu tsunami, we learned this week about a rapid-response drilling operation at the site of the earthquake.

The March 2011 earthquake, a magnitude 9.0 temblor, was responsible for producing a 164-foot-tall slip along the fault. This is the single largest slip ever recorded in an earthquake. Needless to say, such a significant fault movement in a shallow portion of the megathrust boundary could easily produce so devastating a tsunami as the world was witness to that day.

But what was especially surprising to geologists and seismologists was that this portion of the subduction zone had not been thought to be accumulating stress prior to the earthquake. For this reason, scientists from the Integrated Ocean Drilling Program (IODP) wanted to rapidly assess the stress state on the fault that controlled the very large slip.

Their study, published in this week´s issue of the journal Science, offers compelling evidence that large slips actually occur as a result of a complete stress drop during the earthquake. The IODP Japan Trench Fast Drilling Project (JFAST) team believes their findings will aid in the understanding of earthquakes and tsunamis the world over.

IODP is comprised of an international consortium of scientists who are dedicated to advancing the scientific understanding of the Earth via drilling, coring and monitoring of the subseafloor. Scientific partners hail from the US, Japan, Europe, Australia, New Zealand, China, Korea and Brazil.

“The study investigated the stress change associated with the 2011 Tohoku-Oki earthquake and tested the hypothesis by determining the in-situ stress state of the frontal prism from the drilled holes,” says a lead author Weiren Lin of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). “We have established a new framework that the large slips in this region are an indication of co-seismic fault zone and nearly the total stress accumulated was released during the earthquake.”

DRILLING FOR UNDERSTANDING

The JFAST study was conceived, designed and implemented by the international scientific community. The impetus was in service of gaining a more complete understanding of the 2011 Tohoku-Oki earthquake. Over the months of April to July 2012, the JFAST expedition was carried out aboard the scientific drilling vessel Chikyu. The team selected their drill sites some 135 miles off the coast of Honshu, Japan. Their selected sites were located almost 4.5 miles below them on the floor of the ocean.

“The project is looking at the stress and physical properties of the fault zone soon after a large earthquake,” co-author James Mori of Kyoto University, co-chief scientist who led the JFAST expedition explains.

The JFAST team was entering uncharted waters, so to speak. This expedition represents the first time a “rapid-response drilling” has ever been attempted. This study, conducted within 13 months immediately following the earthquake, was undertaken in order to measure the temperature across the subduction fault zone.

The researchers say that the rapid mobilization is necessary to observe time sensitive data, such as the temperature signal. The entire basis of the scientist´s investigation was to observe and collect data from the large displacement by drilling from the ocean floor to the plate boundary. The team achieved a maximum depth of more than 2800 feet below the seafloor.

“Understanding the stress conditions that control the very large slip of this shallow portion of the megathrust may be the most important seismological issue for this earthquake.” Mori says.

In the team´s published results, in an area they believe contains the main slip zone for the March 2011 earthquake, were breakouts located in a borehole some 2700 feet under the surface of the ocean. The nature of the breakouts helped the team to determine the stress field. To achieve this measurement, Lin and his co-authors pored over an abundance of borehole-logging data obtained as a result of drilling with Logging-While-Drilling (LWD) tools. The borehole breakouts, known as a compressive failure, are formed in the borehole wall during the drilling. After their formation, the compressive failures are imaged with the LWD tools.

OVERTURNING CONVENTIONAL WISDOM

The researchers were looking specifically for the orientation and size of the breakouts. These two indicators allowed the team to infer the present direction and magnitudes of the stress field. The results presented in their paper show that the present shear stress on the fault is nearly zero. This low stress indicates there was a nearly complete stress change during the earthquake. The paper´s presentation of this fact flies in the face of the prevailing wisdom that earthquakes will typically only release a portion of the stress on the fault.

“This was the first time for such nearly complete stress change has been recognized by direct measurement in drilling through the ruptured fault. This is the first time direct stress measurements have been reported, a little over a year after a great subduction zone earthquake.” Lin says.

In addition to the discovery of the near complete stress change, Lin and his team, with their expedition aboard the Chikyu, broke through previous research records for deepwater ocean drilling. Drilling nearly 25,500 feet below sea level allowed the team to obtain and analyze deep core. Their samples were derived from the Japan Trench boundary.

The samples and data retrieved have allowed the team to learn about residual heat, co-seismic frictional stress, fluid and rock properties, along with other factors related to megathrust earthquakes. The team also drilled parallel boreholes which they outfitted with borehole logging data observatories.

“We will be able to address very fundamental and important questions about the physics of slip of the thrust near the trench, and how to identify past events in the rock record.” says Frederick Chester, Texas A&M University, co-author of the Science report and the other expedition co-chief scientist.

Though their findings have already been published, this team’s research continues. The work requires the input from both ship-board and shore-based scientists. Their further investigation into large slip earthquakes has them focused intently on the core samples they collected and the borehole logging data retrieved. Additional data, to be obtained from the observatories they left in the boreholes, will be collected later this month using the JAMSTEC ROV Kaiko7000II. The team will combine this newly retrieved data with the current results in order to continue to increase the understanding of the processes involved in this large slip earthquake.

“We anticipate that the results from the JFAST expedition will provide us with a better understanding of the faulting mechanisms for this critical location,” says Mori. “Investigations and research findings from the expedition have obvious consequences for evaluating future tsunami hazards at other subduction zones around the world, such as the Nankai Trough in Japan and Cascadia in the Pacific of North America.”


Source: Alan McStravick for redOrbit.com - Your Universe Online



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