A quantitative interpretation on possible correlations between intraplate seismic activity and interplate great earthquakes along the Nankai trough

Abstract

The Tonankai earthquake occurred in 1944 along the Nankai trough between the continental Eurasian plate and the subducting Philippine Sea plate. We investigated spatial and temporal correlations between the occurrence of this event and intraplate seismic activities in the inland region of southwest Japan. In order to elucidate the possible relationship and dynamic coupling between the 1944 Tonankai and the 1946 Nankaido earthquakes, we calculated stress changes due to the Tonankai earthquake by applying a three-dimensional finite element method and compared them with post-seismic activity. Agreement between them appears quite well at a depth of ∼ 10 km, indicating that stress release in an E-W direction associated with the Tonankai earthquake may have reduced the seismic activity in the inland region. Particularly, the observed drastic decrease of seismicity in the Wakayama region just after this event can be explained by significant decrease of compressive stresses in the E-W direction. Extensive seismicity quiescence preceding the 1946 Nankaido earthquake, which occurred adjacent to the source region of the Tonankai event, may be related to the enhancement of N-S oriented compressive stress due to accelerating northward movement of the Philippine Sea plate. Therefore, in addition to the decrease of stresses in the E-W direction, the increase of the N-S oriented compressive stresses might play an important role to yield low seismic activity just after the Tonankai event. The calculated results show that the post-seismic shear stress increased and the normal stress decreased near the hypocentral area of the 1946 Nankaido earthquakes, while the shear stress decreased and normal stress increased in its surrounding area on the fault. This indicates that the Tonankai earthquake made it easier for the rupture of the Nankaido earthquake to originate at the observed hypocenter. This triggering mechanism may explain the synchronous pattern of historical great interplate earthquakes along the Nankai trough; a great earthquake occurs in the western region within a few years after the first breakage in the eastern block.