Location of the IWTH25 station and its observation system. (a) Location of the IWTH25 station (red circle) plotted with other K-NET (solid triangles) and KiK-net stations (solid circles). The blue star indicates the epicenter location of the 2008 Iwate-Miyagi Nairiku earthquake. The rectangular region shows the fault area of the 2008 earthquake. The red open rectangle in the upper right inset stands for the area plotted. (b) Close-up view of the location of IWTH25 plotted with the GPS station ICNS. The map image data is from the Geological Survey of Japan through http:/cyberjapan.jp. Brown contours indicate the heights at an interval of 20 m. (c) Schematic illustration of the IWTH25 station. Beneath the observation hut, a three-component accelerometer was installed, and it was isolated from the basement of the hut. In front of the hut, a 260-m-deep borehole was dug. At the bottom of the borehole, a sensor capsule was installed that included a three-component short-period velocity seismometer, a three-component accelerometer, and a two-component tiltmeter.