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

The 30-year rupture probability of magnitude 6.7 earthquakes inthe Kanto Plain in Japan—which includes Tokyo—may have increasedfrom 72 percent, to between 81 percent and 93 percent since themassive 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 ofwhether and where the stresses relieved by the Tohoku earthquakehave been transferred to neighboring faults. The information isreleased a new report titled, “Understanding Earthquake Risk inJapan Following the Tohoku-Oki Earthquake of March 11, 2011.”

Because seismologists so sharply underestimated the chances ofsuch a quake, the report says, there is a widespread sense ofurgency in reexamining the seismicity of regions neighboring theTohoku rupture—and in assessing the event's impact on theprobability of another destructive quake occurring in nearbyregions.

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

Because ofJapan's highly comprehensive seismic monitoringnetwork, the Tohoku event is the most thoroughly recordedmega-earthquake in history and will be of primary interest at manyresearch institutions for years to come, AIR notes.

While quantifying the stress changes that resulted from theTohoku earthquake is critically important, AIR says, any overallreassessment of seismic risk in Japan must be performed in a largercontext—which includes explicit consideration of previouslyun-modeled risk factors, namely, tsunami and liquefaction.

Rather than focus on one event, AIR says, it is important toidentify all of the factors that contribute toJapan's seismic risk.For example, the most significant factor may not be theconsequences of the Tohoku earthquake, but rather the choice ofmodeled maximum magnitudes for the Nankai and Sagami Troughs. Anylarge event in those regions would have devastating consequencesforTokyo.

A study published last fall by researchers atTokyoUniversity'sEarthquake Research Institute (ERI)—which has since received mediaattention—estimated an occurrence probability of 70 percent for amagnitude 7.0 earthquake striking in the vicinity oftheBosoPeninsulawithin the next four years.

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

Although damage from the Tohoku earthquake is most closelyassociated with the massive tsunami—which in places reached aheight of more than 30 meters and demolished nearly all structureswithin its footprint—by AIR's estimate, the tsunami was responsiblefor only about 30 percent of overall insured losses from thisevent.

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