Tectonic implications of the 1944 Tonankai and the 1946 Nankaido earthquakes

The tectonic process taking place in the south-western part of Japan to the north of the Philippine Sea is discussed on the basis of the focal mechanism of the 1944 Tonankai earthquake (M ≈ 8.0 to 8.3) and the 1946 Nankaido earthquake (M ≈ 8.1 to 8.4), as well as on the basis of the spatial distribution of small earthquakes in the neighbouring region. It is found that these two earthquakes represent a low-angle thrust faulting whose slip vector is almost parallel to that of other great earthquakes in the north-western Pacific; the oceanic side is underthrusting beneath the continent. The spatial distribution of small earthquakes in the Kii peninsula region shows a remarkable two-dimensional wedge-like distribution to a depth of 70 km. The strike of this two-dimensional distribution is almost perpendicular to the slip vector of the Tonankai and the Nankaido earthquakes suggesting that the Philippine Sea plate started underthrusting about 2 × 10^6 y ago and has now reached a depth of 70 km or so. This picture is consistent with the local travel time anomaly, crustal structure, electrical conductivity anomaly, etc., observed in this region. The source parameters of the 1944 Tonankai earthquake are: latitude 33.70° N, longitude 136.05° E, depth 30 km, dip angles δ = 10° (fault plane) and δ = 80° (auxiliary plane), dip directions φ = 306° (fault plane) and φ = 126° (auxiliary plane), dislocation 3.1 m, stress drop 33 b, released strain energy 5 × 10^(23) erg. The source parameters of the 1946 Nankaido earthquake are: latitude 33.13° N, longitude 135.84° E, depth 30 km, dip angles δ = 10° (fault plane), and δ = 80° (auxiliary plane), dip direction φ = 310° (fault plane) and φ = 130° (auxiliary plane), dislocation 3.1 m, stress drop 33 b, released strain energy 5 × 10^(23) erg