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Deep-sea sampling explains large slip-up off Japan

The Japan Trench Fast Drilling Project project (Integrated Ocean Drilling Program exp. 343/343T) used the remotely-operated vehicle KaikoII7000 to retrieve an 820-meter-long string of temperature sensors from a borehole crossing the plate boundary fault in the Japan Trench, at a water depth of ~7 km.
[Image courtesy of JAMSTEC]

It was the fault zone's fault that the 2011 Tohoku-Oki earthquake in Japan was so big; the fault zone was too thin and too weak, reports a series of extraordinary ocean drilling studies.

On March 11, 2011, when the seafloor east of Sendai, Japan, cracked open, a portion of the earth moved upwards—in places as much as 50 meters. This was the largest recorded movement along a fault in a single earthquake, and it came as a surprise to seismologists (the scientists that study earthquakes). This is because such massive slips typically don't happen in shallow fault zones, like the Japan Trench, where this earthquake occurred.

Now, in the 6 December issue of the journal Science, three teams of scientists from the Japan Trench Fast Drilling Project (JFAST) weigh in on whether the large slip in the shallow part of the fault is characteristic of subduction zone faults like this one, or whether it was a fluke -- caused by conditions unique to the seafloor in this region.

They were able to make this assessment using insights gained from work aboard the drilling ship Chikyu, which can drill at deeper depths than any existing vessel. It can reach through 7 kilometers of water to get to the rupture zone.

The three teams that conducted studies onboard Chikyu were led by Kohtaro Ujiie of Japan's University of Tsukuba, Patrick Fulton of the University of California, Santa Cruz, and Frederick Chester of Texas A&M University, respectively.

These scientists and their colleagues describe rock samples retrieved from the rupture zone where the earthquake happened. They report detailed seafloor measurements, too.

Their collective observations suggest that an important reason for the quake's large size was that that the fault was thin (just millimeters in some places) and weak (made largely of clay sediments).

The work of these JFAST teams brings seismologists closer to understanding why a large slip can happen in a shallow fault zone, which has been debated. Results from other drilling projects such as those off Costa Rica will allow for comparisons between the behavior of this and other subduction zones.