Tohoku earthquake unleashes multiple ways to explore earthquake risk changes: RMS

The 2011 Tohoku earthquake and tsunami has caused the seismic hazard research community to explore different ways to predict whether other related damaging earthquakes can be expected around Japan and whether Tohoku may have affected the timing of other earthquakes in the region, a recent RMS research paper notes.

The paper explores changes to risk in the area due to both static stress changes and elevated seismicity. Both of these approaches must be considered, RMS notes in its paper, to assess whether there is an elevated or decreased earthquake risk in areas of Japan.

Taken together, these two approaches reveal a broad variability of potential earthquake risk changes in Japan.

“Estimated occurrence rate changes, based only on the calculated static stress changes, indicate that short-term earthquake risk to the Tokyo region, where approximately 10% of Japan‘s population resides, has remained relatively unchanged following the 2011 Tohoku event,” the RMS report says. “Considering increased patterns of post-event seismic activity, however, average annual loss estimates (AALs) can potentially increase up to 70%.”

Static stress changes reflect the redistribution of stress in a region due to the stress release by the main shock fault rupture, the RMS report says. This stress release is permanent and is offset slowly over time.

This static stress change analysis should be supplemented by an analysis of dynamic stress changes, which occur as seismic waves from the main shock pass through other seismic sources, triggering additional earthquakes within seconds to hours after the main event.

“Given the elevated seismicity rates following the 2011 Tohoku earthquake … and the presence of many unknown seismic sources in this complex tectonic region, occurrence rate changes cannot be resolved exclusively by analyzing static stress changes on known seismic sources,” RMS notes. “Therefore, RMS complemented the static stress change analysis with a detailed examination of the seismicity in the Tokyo region, as well as other prefectures along the Northern Honshu coastline, before and after the 2011 Tohoku event, to understand the impact of post-event seismicity on occurrence rates.”