We cannot prevent undersea earthquakes and the tsunamis that follow them, but for many years scientists have been struggling to understand these natural phenomena. Rock samples obtained by the Deep Sea Scientific Drilling Vessel CHIKYU are expected to help explain the mechanism of earthquakes and tsunamis and to contribute new knowledge to the field of seismic research. We talk to Dr. Arito Sakaguchi, a researcher who has joined the expedition and discovered traces of past earthquakes at the tip of the plate subduction zone for the first time ever by studying an organic matter called vitrinite.
(Published in March 2012)
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Interviewee Dr. Arito Sakaguchi Research Scientist, Institute for Research on Earth Evolution (IFREE) |
New Evidence Defies Conventional Perception of Earthquakes
A major earthquake measuring in the range of magnitude 8 occurs in the Nankai Trough, south of Japan’s main island of Honshu, once every 100 to 150 years. As the Philippine Sea Plate to the seaward side of the trough sinks under the continental plate on the landward side, energy is stored up in a massive fault known as a megasplay fault. When the fault reaches its limit, it rapidly slips, causing the earth to shake. This is currently believed to be the scenario that results in the Tonankai earthquakes afflicting the central part of the Nankai Trough. However, there are actually a number of faults, and we still do not know exactly which of these will become active, or when.
One of the rock samples collected by CHIKYU is a sample drilled at site C0004 on the surface of the megasplay fault. Dr. Sakaguchi believed that if traces of frictional heat were found in this sample, it would demonstrate that this fault had caused an earthquake.
“When the fault rapidly slips, the surface momentarily reaches between 300 and 400 degrees centigrade, altering some of the organic matter in the rock,” he explains. “This means that if we investigate the organic matter in the samples we have collected and find traces of alteration due to heat, this would be physical evidence that a slip actually occurred.”
The organic matter that Dr. Sakaguchi focused on is a type of coal called vitrinite. A high-speed fault slip should result in the vitrinite at the site of the movement being more carbonized according to the frictional heat. Vitrinite carbonization results in changes to its reflectance, so measuring the reflectance of the sample will show how intensely it has been exposed to frictional heat.
Researchers use vitrinite reflectance when investigating the composition of coal, but the vitrinite in Dr. Sakaguchi’s samples was several orders of magnitude smaller than the samples normally used. Dr. Sakaguchi had to devise his own instruments for measuring the reflectance. It was a mind-numbing task, taking a full eight-hour working day to carry out measurements on a 2 cm segment of the core sample. However, it has steadily produced results—clear traces of frictional heat have been found.
“Faults may or may not be seismic,” says Dr. Sakaguchi. “However, this research has shown that the megasplay fault of the Nankai Trough was undoubtedly the result of earthquakes.”
Samples from sites other than C0004 have also been measured in the same way, and a sample from site C0007 has also displayed alterations due to heat. This is located at the tip of the continental plate and the toe of the accretionary wedge. This sample was collected from soft rock at a shallow depth below the ocean floor. Such areas were traditionally believed to be aseismic, but this discovery has revealed a completely new picture.
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