The Active Fault Research Center, the Institute for Marine Resources and Environment and the Institute of Geoscience of the National Institute of Industrial Science and Technology (AIST), one of independent administrative institutions, have revealed, in collaboration with the U.S. Geological Survey (USGS), that outsize tsunamis were generated in the past by multi-segment rupture of inter-plate earthquakes along the Kuril Trench.

Since the 19th century, the subduction of the Pacific plate have generated recurrent earthquakes of magnitude (M) 8 or so along the Pacific Coast of Eastern Hokkaido, producing serious damages from ground shaking and tsunamis. Traces of tsunami (tsunami deposits) discovered by the AIST-USGS collaboration were distributed over 3 km or longer inland from the coast, surpassing the scale of tsunamis reported since the 19th century. The analysis of volcanic ash, also found in strata together with tsunami deposits, revealed that such unusually large tsunamis occurred about every 500 years on average in the past, most recently in the 17th century.

The computer simulation of tsunami to identify the cause of outsize tsunami demonstrated that the prehistoric traces of tsunami deposits could not be accounted for with a tsunami generated by a single segment earthquake, but that by multi-segment rupture at the plate boundary off Tokachi and off Nemuro [Fig. 1]. The present study disclosed for the first time that multi-segment inter-plate earthquakes along the Kuril Trench off the South-East Area of Hokkaido had occurred to generate an earthquake of scale greater than that having been hitherto reported.

Fig. 1. Inter-plate earthquakes recorded at the Kuril Trench since the 19th century and the earthquake source areas