“Earthquakes have occurred even in the shallow areas under the ocean floor,” explains Dr. Sakaguchi. “In other words, slips can occur across a far wider area than we previously believed. This means that there is the possibility of a tsunami bigger than anything we have so far envisioned. As long as there are no major changes in the environment of the area, what has happened in the past is sure to happen again. I hope the findings will be put to good use in future disaster preparation measures.”
Cross section of the drilling area in the Nankai Trough
Pinpointing Past Earthquakes from the Strata
Dr. Sakaguchi is also working to geologically date major earthquakes that occurred in the past.
In a reverse fault earthquake, plates pushing against each other result in a compressive force. Generally, the hanging wall (the plate pushed upward in a quake) is greatly damaged, while the footwall (the plate that is sinking) remains mostly undamaged. The deposits in the hanging wall are crushed by the earthquake, as a result becoming a soft mud known as mud breccia.
New deposits subsequently form in the area damaged by the earthquake, so that if the age of the mud breccia within the subsurface strata can be determined, that will indicate the year in which the earthquake occurred.
The collected samples did not reveal any special structure when observed by the naked eye, so Dr. Sakaguchi examined them in 3-D using X-ray computed tomography. He was able to confirm the presence of mud breccia in at least five strata from site C0004. Radiometric data using lead 210 (half-life 22.3 years) and carbon 14 (half-life 5,730 years) showed that the most recent mud breccia had formed in 1950, ±20 years.
“There’s very little doubt that this is from the Tonankai earthquake that struck in 1944,” says Dr. Sakaguchi. “There was mud breccia at site C0004, but none at site C0008. So it’s likely that the splay fault between these two points shifted.”
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The area of the Nankai Trough where CHIKYU is drilling |
Geological sturucture of the drilling area in the megasplay fault in the seismic zone (Site C0004) of Nankai Trough and in the shallow tip of the plate boundary fault (Site C0007). The results show that earthquakes occur even in shallow parts of the ocean floor. |
Dr. Sakaguchi is hoping to explain the size of the energy next. Rocks have a mixture of hard and soft minerals. When force is applied and then released the hard minerals return to the way they were, while soft minerals retain traces of the changes. By putting this theory to use, he says he will be able to analyze the force, and thus the energy, that was applied to the rock.
CHIKYU will continue to bring back new samples from the ocean depths, and these samples are sure to disclose new information.
Measurement of the reflectance of vitrinite from the fault zone revealed traces of historical quakes (the red areas).
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