About the Cover

Vol. 90 No. 8 (2014)

  Earthquakes like the 1995 Kobe and 2011 Tohoku-Oki have significantly affected society and economics in Japan. One of the current social issues is the next big earthquake near the Nankai trough, which is supposed to occur probably within several decades.
  While large earthquakes are rare, we experience a lot of moderate to small earthquakes in daily life. Moreover, as has been just recently discovered, a large part of the underground beneath Japan moves slowly with little shaking. These phenomena are called slow earthquakes, in contrast to ordinary, or fast, earthquakes. The geographical proximity of these slow earthquakes to historical giant earthquakes in western Japan, such as those in 1707, 1854, 1944 and 1946, and the presence of several types of slow earthquakes as a family, LFE (low frequency earthquake), VLF (very low frequency earthquake) and SSE (slow slip event), suggest that fast and slow earthquakes are in distinctly different categories but are strongly coupled to each other, and should be understood unitedly.
  What physical processes are working behind giant earthquakes, moderate to small earthquakes, and slow earthquakes? Ide (this issue, pp. 259-277) summarizes the current answers to these questions from his unique viewpoint as a researcher studying a wide variety of earthquake-like phenomena. Earthquakes are usually modeled as fracture and frictional slip in elastic media. Physics of fracture and laws of friction have been fairly established, but scaling problems remain unclear and complicate modeling earthquakes. He proposes that some hierarchal structures have to be introduced to model fast and slow earthquakes as integral parts of earthquake-related processes. Hierarchal structures in an earthquake source region make deterministic prediction quite difficult, or even impossible.