Very Dense Seismic Array Observations in Furukawa District, Japan

Abstract

On 11 March 2011, the Tohoku earthquake ( M w 9.0) hit the eastern part of Japan’s mainland off the Pacific coast, and killed more than 10,000 people, due mainly to a great tsunami in the immediate onshore region. Strong ground motions during the earthquake were observed in almost the whole region of Japan by K‐NET, KiK‐net organized by National Research Institute for Earth Science and Disaster Prevention (NIED), and the other seismometer networks. At least 17 of the K‐NET and KiK‐net stations observed over 980 cm/s2 of peak ground acceleration (PGA) in horizontal components, and two stations observed over 6.5 of high seismic intensity on the Japan Meteorological Agency (JMA) scale. However, damage in areas due to ground motion do not correspond to either the large PGA or the high seismic intensity (e.g., Motosaka, 2012; Goto and Morikawa, 2012, forthcoming). We focus on the Furukawa district of Osaki City, where severe residential damage occurred downtown. Ground‐motion records in the downtown area are available from two stations, MYG006 (K‐NET) and JMA Furukawa (JMA). The damage level was different between the areas within several hundred meters from the MYG006 and JMA Furukawa stations, which are separated by about 1 km. The severe damages were concentrated within an area approximately 1×1 km2 including the JMA station. This implies that the ground‐motion characteristics were not uniform in subkilometer scale, and the existing two stations are not enough to clarify the damage distribution. In the aftermath of the earthquake, we distributed dozens of low‐cost seismometers, namely the ITK sensor, around an area about 3×2 km2 in the Furukawa district. The observed data were sent to the remote server through an Internet connection in real time. The seismometers were installed beside the volunteers’ houses. The volunteers can access the interactive information service, namely the online viewer …