Ōmachi earthquake

USGS ShakeMap for the Omachi earthquake

On Saturday night a strong earthquake hit Nagano Prefecture in central Japan. The United States Geological Survey (USGS) reports the magnitude as 6.2, with Japan Meteorological Agency (JMA) suggesting 6.8. The epicenter was located 16 km NNE of Ōmachi, a city of approximately 30,000, and 24 km west of Nagano, a city of 387,000 famous for hosting the 1998 Winter Olympics. Nagano Prefecture is landlocked and highly mountainous, with 9 of the 12 highest mountains in Japan.

The Ōmachi earthquake reached a seismic intensity of 6 on the JMA scale. Seismic intensity is a measure of the degree of shaking at the earth’s surface (unlike moment magnitude and the outdated but often mentioned Richter Scale, which measure the total energy released), with 6 on the JMA scale sufficient to make it difficult to stand. The earthquake produced several other effects expected for an earthquake of this intensity - severe damage to buildings, surficial ground cracks and landslides. The earthquake was felt across a large portion of central Japan, from Osaka to Tokyo and Niigata. The earthquake was felt in Hamamatsu, the base for the QuakeRecNankai team until earlier this month. Shaking here reached 2 on the JMA scale, sufficient for most people to feel, but not enough to do any damage. Our field season in Japan finished without any of us feeling a single earthquake. There was no risk of a tsunami from the Ōmachi earthquake - these are generally associated with larger earthquakes along the subduction zones to the south and east of Japan. 

The Japan Times is reporting that the government's Earthquake Research Committee is attributing the earthquake to slip on the Kamishiro Fault, part of the Itoigawa-Shizuoka Tectonic Line (ISTL). The ISTL is a major fault system running approximately North-South across central Japan. Slip rates on the ISTL are high, averaging 10 m per thousand years, but historical seismicity has been low, with no large earthquakes recorded since AD 841 (Okamura, 2001). Trenches dug across the Kamishiro Fault in 1995 and 1996 suggest four earthquakes over the last ~7000 years, with the last probably reflecting the historically documented AD 841 earthquake (Okamura et al., 1998).

References:

• Okumura,K., R. Imura, T. Imaizumi, M. Togo, H. Sawa, K. Mizuno, Y. Kariya, and E. Saito, 1998, Recent surface faulting events along the northern part of the Itoigawa-Shizuoka tectonic line -Trenching survey of the Kamishiro fault and east Matsumoto basin faults, central Japan. Zisin, 50, 35-51.
• Okamura, K. 2001. Paleoseismology of the Itoigawa-Shizuoka tectonic line in central Japan. Journal of Seismology, 5, 411-431.