New observatory installed beneath seafloor at site of Tohoku earthquake

UCSC seismologist Emily Brodsky was part of an international team aboard the drill ship Chikyu for the successful installation of a temperature observatory on the fault

emily brodsky
Emily Brodsky
underwater tv image of borehole
This image from an underwater TV camera shows the deployment of the temperature observatory in the borehole 7 kilometers below the sea surface. (Photo credit: JAMSTEC/IODP)

On July 16, an international team aboard the drill ship Chikyu successfully completed installation of the temperature observatory that was the main objective of the Japan Trench Fast Drilling Project (JFAST). The project is collecting data and samples that will allow scientists to better understand the fault that caused the devastating Tohoku earthquake and tsunami in March 2011.

Emily Brodsky, professor of Earth and planetary science at UC Santa Cruz, said the project was an "epic adventure" that required extra time on the drill ship in addition to the original JFAST expedition in April and May.

"It is the deepest, fastest, most extensive instrumentation after an earthquake ever," Brodsky said in an email sent from aboard Chikyu. "The observatory installation was a tricky business that involved effectively threading a needle (actually a steel casing into a borehole) under 7 kilometers of water. This certainly had never been done before."

The successful installation of the observatory means that scientists for the first time will be able to measure the frictional heat produced by the fault slip of a great subduction zone earthquake. "The observatory is now in place and collecting temperature data that will ultimately tell us about the friction on the fault during the earthquake," said Brodsky, who serves on the project management team and will take a lead role in the data analysis. "Fault friction is one of the key unknowns in earthquake physics. This knowledge gap is a major obstacle in the way of a full, first-principle understanding of earthquake mechanics."

The Integrated Ocean Drilling Program (IODP) Expedition 343 took place in April and May of this year, and successfully drilled across the Tohoku earthquake fault and into the subducting Pacific plate. The follow-up expedition (343T) returned to the site this month to complete the work. Over the past week, the onboard scientists and engineers installed the temperature observatory into one of the previously drilled boreholes.

"During the original expedition last April and May, it seemed at almost every step of the way there was some problem or delay," said James Mori of Kyoto University, co-chief scientist who led this expedition.

There were many steps in the deployment of the observatory: preparation of the underwater camera system, running the long pipe to the ocean floor, setting the wellhead, borehole re-entries, underwater instrument releases, borehole drilling, casing installation, and temperature instrument programming.

"There were difficult technical aspects for the borehole, which reached 855 meters below the seafloor in nearly 7000 meters of water. Chikyu, or any other research ship, has never drilled in such deep water, so there were many engineering challenges," Mori said.

JFAST had two main objectives: to collect geological samples to analyze the physical properties of the fault zone and to measure the temperature near the fault to understand the frictional heat produced by fault slip. During the original expedition of over 50 days, Chikyu drilled two boreholes and achieved the first goal to collect geological samples and other geophysical data to analyze the physical properties of the fault zone. The deepest hole set a new record depth for ocean scientific drilling with a total of 7768.5 m measured from the rig floor (7740 m below sea level).

The temperature sensors, however, were not deployed during that expedition because of many delays caused by technical problems and bad weather. A remotely operated vehicle (the Kaiko 7000II ROV) will collect the sensors later this year, at which time the data analysis will begin.