From the Izu-Bonin to the north of Hokkaido : how did the M9.0 Tohoku earthquake affect the Pacific plate seismicity ?

The last twenty years have seen a number of large, devastating earthquakes on subduction zones. In many ways, the M9.0 Tohoku-oki earthquake was bewildering for the seismological community. It occurred on a previously identified coupled area but ruptured a larger zone than expected and, above all, the large amount of near-trench coseismic slip was a surprise.

Because Japan is one of the best area instrumented in the world, the 2011 Mw 9.0 Tohoku-oki earthquake is one of the world's best-recorded ruptures. Many studies have analyzed with great details the pre-seismic, co-seismic and post-seismic phases of the Tohoku earthquake. Researchers also focused on the triggering of on-land seismicity following the mega-thrust earthquake. However, no study zoom out and considered the consequences of this earthquake on the Pacific plate in this area.

 

In this study, we analyzed the Japanese Meteorological Agency seismic catalog over ten years of data to assess the consequences of such large mega-thrust earthquake over the Pacific plate from the Izu-Bonin area to the north of Hokkaido island. We studied the seismicity from 0 to 700km depth, taking advantage of one of the most complete subduction zone catalogue.

Our results show that the seismic rate south of Japan experienced a decrease at the time of Tohoku about 30% and an increase of 20% underneath the Hokkaido island. The subduction zone that is downdip Tohoku doesn’t seem affected by the megathrust earthquake. While it is difficult to understand and to model such large scale effects of the Tohoku earthquake on the Pacific plate, we think it is primordial to observe and detail them with precision.