12-km Tachikawa earthquake fault open for viewing

Huge earthquake fault trench in western Tokyo temporarily opened for public viewing

By Cherrie Lou Billones / The Japan Daily Press, February 9, 2013

The University of Tokyo‘s Earthquake Research Institute has opened to the public a trench it dug to survey Tokyo’s only active fault, on February 8 and 9, from 10 a.m. to 3 p.m. The Tachikawa fault is a 21-kilometer fault that runs from Ome to Tachikawa and Fuchu in Tokyo; it also includes a 12-kilometer fault that stretches from the Saitama Prefecture city of Hanno to Ome.

According to the central government’s Earthquake Research Committee, the March 2011 Great East Japan Earthquake has caused a tectonic deformation to the fault, which increased chances of it triggering an earthquake. It had been difficult to study the fault because it runs through very populated areas; and thus, its activities have been practically unknown to experts. Fortunately, automobile giant Nissan Motor Co., whose Murayama factory used to sit atop the fault, has agreed to cooperate with the survey. Hence, the trench that measured 250 meters long, about 30 meters wide and some 10 meters deep—abnormally large in comparison to a regular trench dug for the same purpose.

Tatsuya Ishiyama, leader of the survey team, hopes that this endeavor will shed light to the possibilities of the fault triggering an earthquake, so that they could see the level of risks that it could pose. There is no specific end in sight for the study, but the team is likely to continue through fiscal 2014. The results will then be submitted to the Earthquake Research Committee, which will use the same to formulate a disaster prevention plan.

For contact information, see University of Tokyo’s Earthquake Research Institute page.

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Earthquake Research Institute,
The University of Tokyo
1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-0032, JAPAN
TEL: +81-3-5841-5666 FAX: +81-3-3816-1159 http://www.eri.u-tokyo.ac.jp/
If you have any questions about this pamphlet,
please feel free to contact the Outreach Office.
outreach@eri.u-tokyo.ac.jp

The following information was excerpted from “Disaster Prevention Measures by the Tokyo Metropolitan Government” pamphlet:

Earthquake occurring in an active fault (Tachikawa Fault Zone
(M7.4), etc.)
The Tachikawa fault zone is an active fault zone that runs from Naguri Village, Iruma County in Saitama Prefecture through Ome City and Tachikawa City to Fuchu City in Metropolitan Tokyo. This fault causes large earthquakes in Sagami when it slips, but the government evaluation shows the average activity interval is 10,000 to 15,000 years. The national government has announced that the crustal movement caused by the Great East Japan Earthquake “may possibly have increased the probability of an earthquake occurring” in the Tachikawa fault zone, but the current occurrence probability is 0.5 to 2.0%, so the occurrence frequency is thought to be very much less compared to an earthquake directly beneath Tokyo.