Seismic Shifts Could Increase Quake Chances in Japan

NU Online News Service, Feb. 17, 9:20 a.m. EST

The 30-year rupture probability of magnitude 6.7 earthquakes in the Kanto Plain in Japan—which includes Tokyo—may have increased from 72 percent, to between 81 percent and 93 percent since the massive Tohoku earthquake and tsunami, which killed 20,000 people, according to a study by AIR Worldwide.

AIR says its scientists have conducted a detailed analysis of whether and where the stresses relieved by the Tohoku earthquake have been transferred to neighboring faults. The information is released a new report titled, “Understanding Earthquake Risk in Japan Following the Tohoku-Oki Earthquake of March 11, 2011.”

Because seismologists so sharply underestimated the chances of such a quake, the report says, there is a widespread sense of urgency in reexamining the seismicity of regions neighboring the Tohoku rupture—and in assessing the event’s impact on the probability of another destructive quake occurring in nearby regions.

AIR says the results of its study indicate that the 30-year probability for another Kanto-type event has increased from 0.76 percent to 1.1–1.6 percent, depending on the assumptions.

Because ofJapan’s highly comprehensive seismic monitoring network, the Tohoku event is the most thoroughly recorded mega-earthquake in history and will be of primary interest at many research institutions for years to come, AIR notes.

While quantifying the stress changes that resulted from the Tohoku earthquake is critically important, AIR says, any overall reassessment of seismic risk in Japan must be performed in a larger context—which includes explicit consideration of previously un-modeled risk factors, namely, tsunami and liquefaction.

Rather than focus on one event, AIR says, it is important to identify all of the factors that contribute toJapan’s seismic risk. For example, the most significant factor may not be the consequences of the Tohoku earthquake, but rather the choice of modeled maximum magnitudes for the Nankai and Sagami Troughs. Any large event in those regions would have devastating consequences forTokyo.

A study published last fall by researchers atTokyoUniversity’s Earthquake Research Institute (ERI)—which has since received media attention—estimated an occurrence probability of 70 percent for a magnitude 7.0 earthquake striking in the vicinity of theBosoPeninsulawithin the next four years.

AIR, however, says its analysis produces a corresponding four-year probability of between 23 percent and 28 percent, significantly lower than that of ERI.

Although damage from the Tohoku earthquake is most closely associated with the massive tsunami—which in places reached a height of more than 30 meters and demolished nearly all structures within its footprint—by AIR’s estimate, the tsunami was responsible for only about 30 percent of overall insured losses from this event.

Shake damage was far more widespread—and shake damage would have been significant within the area subsequently impacted by the tsunami, according to AIR.

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