Recently the telemetering seismic network of Shiraki Micro-Earthquake Observatory, Earthquake Research Institute, University of Tokyo, has been constructed in Chugoku District and its vicinity, Western Japan by the fourth stage of National Earthquake Prediction Project. There are seven seismic observation sites in the network, three of which are linked by the telephone lines and four of which, newly installed, are linked by radio to Shiraki Micro-Earthquake Observatory. The latter four are located in the southern part of the network. Thus, the accuracy of hypocenter determination has been improved, especially in the southern part. According to hypocenter determination based on the revised Ichikawa-Mochizuki travel-time table, the following features of the seismic activity in the period from July, 1983 to October, 1984 were obtained:
1. In the land area of Chugoku District, earthquakes occurred in the upper part of the crust.
2. In the southern region of 34°N latitude line, most of earthquakes took place in the upper mantle. An east-west cross section of hypocenters clearly shows a simple Wadati-Benioff seismic zone with a thickness of about 20km, but a north-south cross section of hypocenters shows complicated seismic zones, possibly more than two. The dip of the seismic zone west of 132°E is a little larger than that east of 132°E in the east-west cross section.
Furthermore, detailed description is given to main events during the period from July, 1983 to October, 1984, such as earthquakes M6.8 and M5.2 on August 26, 1983 and June 25, 1984, respectively.