Spatial distribution of earthquakes in the Kii peninsula, Japan, south of the Median Tectonic Line

Precise data obtained by a high-sensitivity micro-earthquake observatory network are used to determine simultaneously the crustal structure and the spatial distribution of small earthquakes in the Kii peninsula region, Japan. The spatial distribution of hypocenters thus determined clearly shows two distinct groups of earthquakes: (1) a group of shallow (H ⩽ 10 km) earthquakes on the western coast of the Kii peninsula near Wakayama; (2) a group of mantle earthquakes, having a depth ranging from 30 to 70 km and trending NE-SW, in the central part of the Kii peninsula. Along the trend of the second group, a marked structural anomaly is found which suggests the presence of a high-velocity zone at depths below 20 km. A projection of the hypocenters of the earthquakes belonging to the second group onto a vertical plane strikingNW-SE shows a wedge-like distribution to a depth of 70 km. The spatial relation between this wedge-like distribution and the 1944 Tonankai earthquake (M = 8.0) suggests a common tectonic process which is now taking place in the Kii peninsula region. The activity of the earthquakes of the first group terminates abruptly to the north at the Median Tectonic Line. This activity is represented by numerous but relatively small events (M < 5) without any conspicuous major earthquakes in history. It is suggested that the strength of the crust in this region of shallow activity is too weak to sustain stresses large enough to be released in a major event; rather, the stresses which probably originate from the tectonic activity represented by the earthquakes of the second group are released by numerous minor fracturings of the low-strength crust. A possibility of using the weak crust for detecting a remote stress accumulation is suggested