We won't be able to say we didn't know

May 25, 2013

Panel: Giant quake at least 90% likely within 50 years

http://ajw.asahi.com/article/behind_news/social_affairs/AJ201305250047

The Asahi Shimbun

By ROKU GODA/ Staff Writer

A devastating earthquake of magnitude 8-9 has at least a 90-percent chance of striking the Nankai Trough along Japan's Pacific coastline in the next 50 years, and triggering towering tsunami in densely populated areas.

The finding is in a report released May 24 by the Earthquake Research Committee of the government's Headquarters for Earthquake Research Promotion.

The Nankai Trough is a roughly 4-kilometer depression on the seabed that extends about 700 km from Suruga Bay off Shizuoka Prefecture to areas off eastern Kyushu.

"We think the degree of urgency is very high," said panel chairman Yoshimori Honkura, professor emeritus of solid earth physics at the Tokyo Institute of Technology. "We hope the authorities will steadily promote earthquake and tsunami preparedness measures to prevent and mitigate potential disaster."

The Earthquake Research Committee began releasing earthquake probability forecasts in 2001. Since then, the panel has provided separate probabilistic forecasts for three component seismic source areas along the Nankai Trough, which are called, from east, Tokai, Tonankai and Nankai. But that approach has been amended for the first time in 12 years following the failure of scientists to foresee the magnitude-9.0 Great East Japan Earthquake of 2011, which simultaneously ruptured several different seismic source areas along the Japan Trench east of Japan.

In a major departure from the earlier approach, the latest report provides forecasts in a combined manner across the whole stretch of the Nankai Trough.

The earliest documented seismic event along the Nankai Trough is the Hakuho Earthquake of 684. Devastating earthquakes of magnitude 8 or so have since recurred along the tectonic boundary, where the oceanic plate begins to dive beneath the continental plate.

But some of those historical events broke the Nankai segment alone, whereas others only affected the Tonankai segment. Still others ruptured both, possibly also extending into the Tokai segment to the east.

Given this diversity in seismic rupture patterns, the panel decided the old approach of segment-by-segment forecasts did not fully account for the reality of the situation.

The new calculations produced occurrence probabilities of "about 20 percent" within 10 years, "60-70 percent" over the coming three decades, "about 80 percent" over the next four decades and "about 90 percent or more" within 50 years, counting from Jan. 1 of this year.

Other panels, set up under the Cabinet Office, have hypothesized a worst-case scenario of a magnitude-9.1 event along the Nankai Trough to produce their damage estimates. But the Earthquake Research Committee stopped short of publishing probabilities for such a catastrophic event.

"No evidence is currently available that earthquakes of the maximum size occurred during the past several millenniums," the May 24 report read. "It is, therefore, difficult to evaluate them quantitatively, but the magnitude-frequency distribution of earthquakes implies that their occurrence frequency is at least one decimal place smaller than that of (other) major earthquakes, which have recurred at intervals of 100-200 years."

An earlier report by the Earthquake Research Committee said a magnitude-8.1 or so Tonankai earthquake could occur at a probability of "70-80 percent," and a magnitude-8.4 or so Nankai earthquake with a likelihood of "about 60 percent," both within 30 years of Jan. 1, 2013. It also said, with a proviso of poorer forecast accuracy, that a magnitude-8 or so Tokai earthquake had an 88-percent chance of hitting during the coming three decades.