About the Cover

Vol. 96 No. 7 (2020)

  In subduction zones, such as the Japanese Islands, the shallow portion of the plate interface is locked by friction and stress is accumulated. When the accumulated stress reaches the strength limit of the plate interface, the locked portion of the plate interface slides rapidly to release stress. This causes interplate earthquakes such as the 2011 M9.0 Tohoku-Oki earthquake. Recently, it has become clear that not only such regular earthquakes that cause rapid sliding but also very slow sliding earthquakes, called slow earthquakes, frequently occur at the subduction plate interface to release stress. Such slow earthquakes were first discovered almost simultaneously by a Canadian research group in the Cascadia subduction zone and by Obara in the Nankai Trough subduction zone.
  Obara (this issue, pp. 297-315) reviews the characteristics of slow earthquakes occurring in Japan. The Nankai Trough subduction zone is well known for the recurrence of great earthquakes every 100 to 200 years. Great earthquakes occur as a rapid sliding of areas that are usually locked (shown by black lines in the figure). Recent studies have revealed the occurrence of a few types of slow earthquakes (shown in yellow, red, and orange in the figure) surrounding those locked areas. Because of their very low stress drops, slow earthquakes are considered to be caused by the high pore fluid pressure. Deep slow earthquakes were found at the deeper part of the locked area in the Nankai Trough subduction zone; however, they have never been detected in the northeast Japan subduction zone. This difference in deep slow earthquake activity may be caused by the difference in fluid amounts obtained from the subducting Philippine Sea plate and the Pacific plate having different temperatures and ages. A recent seismic tomography study revealed that the occurrence of these deep slow earthquakes requires not only fluids but also an impermeable structure right above the plate interface to enhance the pore fluid pressure. Shallow slow earthquakes have been detected near the Japan Trench and the Nankai Trough and are distributed close to the locked areas on the plate interface. Thus, slow earthquakes may play an important role in the loading and transfer of stress to the locked areas. The interaction between slow and great earthquakes must be clarified to improve the evaluation of the future seismic behavior along the subduction plate interface.

Akira Hasegawa
Professor Emeritus, Tohoku University