To investigate microseismicity around the Nagamachi-Rifu fault, the B class active reverse fault with a northeast-southwest strike and a northwest dip, Miyagi Prefecture, northeastern Japan, a temporal seismic observation composed of sixteen high-sensitivity stations was carried out for the total period of one year between 1996 and 1998. In spite of high artificial noise in the urban area of Sendai City, we succeeded in obtaining high-quality seismic data by using a DAT continuous recording system. The total number of the earthquakes recorded is 119, of which the smallest one is magnitude -0.5. The depths of the events range between 3 and 13km. Most of hypocenters are concentrated in the northwestern region of the Nagamachi-Rifu fault, while few earthquakes in the southeastern region regardless of our high sensitivity observation. This indicates that these earthquakes occur only in the hanging wall side of this active fault. Focal mechanisms in this region are not necessarily coincident with that of the Nagamachi-Rifu fault. In particular, focal-plane solutions of strike-slip are predominant along the Kagitori-Okubushi line, which is a flexure with a conjugate strike to the Nagamachi-Rifu fault. The spatial pattern of the horizontal crustal strain also changes its characteristics across this line. These results strongly suggest that not only the Nagamachi-Rifu fault but also the Kagitori-Okubushi line may control the seismogenesis of this area.
