Efforts to map earthquake trigger-points across Japan have created a patchy picture at best, 17 years after the government set up an agency to draft a definitive list of alerts.
It is thought that about 2,000 active fault lines lie beneath Japan's land mass, but all 14 earthquakes of at least magnitude-6 which struck in that period sprang from underground faults that the agency had not identified as a major threat. The figures do not include tremors triggered by under-sea subduction zones, such as the March 2011 quake and tsunami.
The findings emerged in an investigation by The Asahi Shimbun, which conducted extensive interviews with officials from agencies, including the Headquarters for Earthquake Research Promotion. The government set up HERP in July 1995 after officials failed to predict the devastating Kobe earthquake earlier that year.
Since 2001, it has been affiliated with Japan's science ministry.
Active fault lines move less frequently than ocean-floor subduction trenches, but can cause far more damage to urban areas because the tremor is usually just beneath the surface. One example was the Kobe quake, which killed more than 6,000 people even though the energy released was just 0.1 percent of the magnitude-9.0 quake last year that spawned towering tsunami.
HERP has listed about 100 major active fault lines, most of them 20 kilometers or more in length and visually identifiable by geological features such as surface uplift. But a separate survey by research bodies, including the National Institute of Advanced Industrial Science and Technology, found many active fault lines may be much longer—and, therefore, more powerful—than previously thought.
The survey examined 98 active fault lines, the majority of which were not on the HERP list, and found on average they extended twice further underground than their surface features suggest. Some fault lines likely exceed 20 km in length.
In June 2008 a magnitude-7.2 quake struck northeastern Japan. Like the other major tremors, the Iwate-Miyagi Nairiku earthquake fault line had not been flagged by HERP as a major risk. But this time the error was compounded by the discovery that the line stretched a massive 30 km underground.
In the wake of that, HERP decided to produce a more exhaustive list of hazardous active faults, reviewing its alert list over the course of a decade and re-assessing even short fault lines. But after two years of work it still has not completed work in Kyushu, the first region to be re-examined.
The Japanese government has published probable magnitudes and likelihoods for 100 or so faults on the HERP high-alert list, but little data is available to predict which will strike next.
Active fault lines can be compared to time bombs. There is almost no telling whether a fissure will shift tomorrow or next century. If the fissure lies beneath an urban area, the existing early-warning system, designed to pre-empt shocks, may not be very useful.
(This article was compiled from reports by Yu Kotsubo and Tairiku Kurosawa, a senior staff writer.)
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