In the first deep sea drilling expedition designed to gather seismic data, scientists have successfully drilled nearly a mile beneath the ocean floor into one of the world's most active earthquake zones.

Researchers aboard the drilling vessel Chikyu — meaning "planet Earth" in Japanese — used a special technology called riser drilling to penetrate the upper portion of the Nankai Trough, an earthquake zone located about 36 miles southeast of Japan. By collecting rock samples and installing long-term monitoring devices, the geologists hope to understand how stress builds up in subduction zones like Nankai, where the Philippine Sea plate is sliding beneath the island of Japan.

Riser drilling involves encasing a deep sea drill in a giant metal tube, called a riser, that extends from the ship down to the drilling site, effectively bolting the ship to the sea floor. The researchers circulate lightly pressurized mud down through the drilling tube and back up through the riser.

"One of the key benefits is the pressurized mud keeps the wall rock from collapsing on the drilling pipe, which allows you to drill deeper and with better control," geologist Timothy Byrne of the University of Connecticut wrote in an e-mail. "For example, nearly perfectly vertical holes or steeply inclined holes can be drilled," wrote Byrne, who co-led the project.

Using a riser also makes it easier to send core samples and cuttings, or small chips of rock collected during drilling, back up to the surface.

The Nankai Trough last ruptured twice in 1944 and 1946, generating earthquakes greater than magnitude 8 that shook the region and caused deadly tsunamis. Since then, the two plates have continued to move, but the boundary between them has been locked, causing pressure to build.

"We know that a locked fault is not a quiet thing, but we don’t quite understand why," said geologist Kelin Wang of the Geological Survey of Canada, who was not involved in the research. "When we understand what is meant by locking, we can understand how energy is building up for the next event."

The Nankai project is part of an international effort called the Integrated Ocean Drilling Program, designed to investigate a variety of scientific questions through drilling. The IODP chose to drill for seismic data in Nankai because of the region's history of recent earthquakes and the accessible location of the rupture zone. The drilling is not powerful enough to trigger an earthquake.

What is learned in Japan will help scientists understand other earthquake-prone plate boundaries, such as the Cascadia subduction zone, which extends along the Pacific coast from British Columbia to Northern California

"For us in North America, the good news is that the Nankai subduction zone is strikingly similar to ours," Wang said. Both zones are hotter and accumulate more sediment than average. "By studying Nankai, we North Americans can actually benefit pretty directly from the project. It's almost as if we are drilling our own subduction zone, because we'll see a lot of the same things."

The first drilling and sampling operations in Nankai began on May 12 and are expected to conclude on August 1. After the initial drilling stage, scientists lowered various gauges and logging instruments into the hole to measure temperature, stress, water pressure and rock permeability. Once they gather enough data, the scientists will prepare the hole for future installation of long-term monitoring equipment.

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Image 1: Riser aboard Chikyu vessel, JAMSTEC/IODP. Image 2: D. Sawyer, JOI/USSAC/IODP. Video: JAMSTEC/IODP.