19 Janvier 2013
January 19, 2013
The number of active faults in Kyushu that could cause earthquakes with a magnitude of 7 or higher has doubled from eight to 16, according to a draft of the government's Earthquake Research Committee reevaluation of national active faults.
After releasing the Kyushu-related portion of the reevaluation, the committee plans to release reevaluation results for each region--starting with the Kanto region.
A government project to evaluate earthquake magnitudes and their probability of occurring by designating active faults capable of causing large earthquakes began after the 1995 Great Hanshin Earthquake.
As there is believed to be a link between the length of an active fault and the magnitude of an earthquake, the committee chose 110 active faults nationwide with surface lengths of 20 kilometers or more as research targets. These active faults are suspected to be capable of causing magnitude-7 or stronger earthquakes.
However, the committee decided to review the targeted faults based on a new 2010 standard as destructive earthquakes occurred at nontargeted faults, including the magnitude-6.8 Niigata Prefecture Chuetsu Earthquake in 2004.
The new standard has expanded the target to include active faults capable of causing a magnitude-6.8 earthquake or larger with a length of about 15 kilometers. Additionally, elements such as the length of fractures underground and in coastal areas were also included. Active faults with long intervals between activity were also included.
As a result, 16 active faults in Kyushu were included among the targeted faults and judged capable of causing magnitude-7 or stronger earthquakes .
Meanwhile, according to a 2009 survey, there are nearly 100 active faults nationwide with a surface length of 15 kilometers or longer.
Close examinations of these faults are currently being carried out, and it is expected that many of them will be covered under the new standard. As a result, it is highly likely the number of targeted faults nationwide will be doubled.
The committee also calculated for the first time the possibility of a magnitude-6.8 earthquake or greater within the next 30 years by sections within the Kyushu region. The probability of such an earthquake occurring in Kyushu as a whole was calculated to be about 40 percent.
Active faults at 27 locations were covered in the committee's calculations for this purpose, including those at 11 additional locations that stretched from 10 to 15 kilometers but were deemed capable of causing a magnitude-6.8 earthquake.
The probability differs when taking into consideration differences in stratal architecture. By this measure, the probability of a major earthquake in northern and southern Kyushu is about 10 percent, and about 20 percent in central Kyushu. A detailed evaluation of the Kyushu region will be released as early as by the end of this month.