Interplate coupling in the Kanto district, central Japan, deduced from geodetic data inversion and its tectonic implications

Abstract

Interplate coupling between the Philippine Sea (PHS), the North American (NAM), and the Pacific (PAC) plates in the Kanto district, central Japan, has been investigated through the inversion analysis of geodetic data using Akaike's Bayesian Information Criterion (ABIC). The data used for the analysis are annual rates of level changes (1972–1985) and horizontal length changes (1973–1990), which presumably represent average crustal movements during the interseismic period. Disregarding effects of steady plate subduction, we may ascribe the observed crustal movements to the effects of locking or faulting on some parts of the plate boundaries. In our model, the effect of locking is represented by back slip on the plate boundary, and that of faulting by forward slip. The results of the inversion analysis show the existence of a strongly coupled area on the southwestern, shallower part of the NAM-PHS plate boundary, which almost coincides with the faulting area of the 1923 Kanto earthquake (M 7.9), and a notable faulting area on the northeastern, deeper part of the PHS-PAC plate boundary. In the southwestern strongly coupled area, the rate of back slip reaches 3.5 cm/yr, and its direction is oriented N33°W, which is almost opposite to the direction of fault slip (S29°E) at the time of the 1923 event. This suggests that tectonic stress accumulation for the next large event is effectively proceeding there, and its recurrence time is roughly estimated as 245 years. The strongly coupled region is also recognized as corresponding to a recent seismic gap. The inverted forward-slip distribution on the PHS-PAC plate boundary, which has the average rate of 1.9 cm/yr and the average direction of N56°E, may be explained by aseismic northeastward slip in relation to the process to form a peculiar configuration of downward bend of the PHS plate beneath the Kanto district.