Introduction to the special section for the 2005 M7.2 Miyagi-oki earthquake

Abstract

An M=7.2 earthquake occurred on the plate boundary off Miyagi Prefecture, northeastern Japan, at 11:46 a.m. Japanese Standard Time on 16 August 2005, when the number of people returning to their homes after the holidays was at a peak. This earthquake resulted in the Japanese bullet train of Tohoku/Akita/Yamagata/Shinkansen falling out of service for half a day, and it took a whole day for the service to get back to normal. A suspended ceiling also fell at a public facility in Sendai city, and many people were injured. Overall, this earthquake injured 79 people, and caused partial damage to 339 buildings. Large earthquakes with a magnitude of ∼7.5 have occurred repeatedly at a recurrence interval of ‘37 years along the plate boundary off Miyagi Prefecture. When the M=7.2 event occurred in the source area of the anticipated Miyagioki earthquake, more than 27 years had passed since the last event, and the possibility of an earthquake occurrence within the next 10 years exceeded 50% according to the evaluation report by the Headquarters for Earthquake Research Promotion (2003) of the Japanese government. Among all of the anticipated earthquakes in Japan, this Miyagi-oki earthquake had the highest probability of occurrence. Although the August 2005 Miyagi-oki earthquake occurred within the source area of the anticipated Miyagioki earthquake, the magnitude was slightly smaller than M=7.5 that had been anticipated. For this reason, the Headquarters for Earthquake Research Promotion predicted that this earthquake is not the anticipated Miyagi-oki earthquake. However, if this 2005 earthquake is not the one anticipated—subsequently denoted here as the “coming” earthquake—when are we to expect the Miyagi-oki earthquake? Moreover, what is happening within the source area of the coming Miyagi-oki earthquake at this moment? It is critical for us to provide answers to those questions in order to get a clear picture of the coming Miyagi-oki earthquake. Dense nationwide seismic and GPS networks, which were recently constructed as the “Kiban” networks [in Japanese, this means the basic observation network (Okada et al., 2004; Sagiya, 2004)], were successful in recording the mainshock rupture, aftershock activity and post-