【地球科学】2011年東北沖地震の地質学的基盤

A detailed study of the geological structure of the region where the March 2011 Tohoku-oki earthquake took place, in northeast Japan, helps to elucidate earthquake behaviour in the area and possibly elsewhere. The findings are reported in this week’s Nature.

Only the second giant earthquake (with a moment magnitude of at least 9) to have occurred in the last 50 years, the 2011 Tohoku-oki earthquake is one of the most widely known disasters of the last decade. Although high-resolution data describing the kinematics of the earthquake rupture - the sudden release of energy associated with fault movement - are available, the physical or structural characteristics that contributed to the rupture were not previously clear.

Dan Bassett and colleagues used topographical and gravity data to constrain the geological structure of the area where the earthquake occurred. They focus regionally on the wedge of material above the subduction zone, where the Pacific plate is diving beneath Honshu. Their data reveal the existence of an abrupt boundary in this overthrusting plate, which the authors interpret to be an offshore continuation of the so-called Median Tectonic Line that can be observed onshore, juxtaposing rocks of different origins and densities. They propose that the geological structure of the overthrusting plate is important in controlling earthquake behaviour and that these results may be used to understand seismic hazards in other areas of the globe with a similar geological make-up.

In a Comment piece published in the same issue of Nature, Masahiro Sugiyama and colleagues reflect on the five years since the events of March 2011, distilling the lessons learned in energy, policy and beyond. Japan needs to take to make its scholarship more international and more interdisciplinary, they conclude: “Going global is the key, and will pay dividends”.