Stress change in southwest Japan due to the 1944–1946 Nankai megathrust rupture sequence based on a 3-D heterogeneous rheological model

Abstract The Philippine Sea plate subducts under the southwest (SW) Japan arc at the Nankai trough, repeatedly causing large megathrust earthquakes at intervals of 100 to 200 years. According to the studies of historical earthquakes, the inland region of SW Japan has a seismically active period from 50 years before to 10 years after megathrust earthquakes. To assess the activities of inland earthquakes, we need to quantitatively evaluate the stress accumulation on the inland source faults. For the periods before megathrust earthquakes, we can calculate inland stress accumulation rates from a nearly steady locking pattern. For the periods after megathrust earthquakes, however, we need to consider the various rupture patterns of past events and postseismic viscoelastic relaxation. In this study, we focused on the most recent ruptures, the 1944 Tonankai and the 1946 Nankai earthquakes, estimating the 4-year stress change on the source faults in SW Japan. This rupture sequence was followed by severe ~ M7 inland earthquakes, such as the 1945 Mikawa and 1948 Fukui earthquakes. For realistic stress calculation, we used a highly detailed finite element model (FEM) incorporating the actual topography and the plausible viscoelastic underground structure from past studies. The computation cost of the FEM was reduced by the analysis method optimized for crustal deformation. The calculated inland stress field shows the dominance of the coseismic change during the 1944 and 1946 earthquakes and little contribution from viscoelastic relaxation. In contrast, viscoelastic relaxation has a significant effect on stress in the slab, indicating the importance of understanding the viscosity structure. Based on the calculated stress with an effective friction coefficient of 0.4, we evaluated the change in the Coulomb failure stress (ΔCFS) on each source fault. The ΔCFS is generally positive on the strike-slip faults east of 135°E due to the 1944 rupture. In contrast, the ΔCFS on the faults west of 135°E, including the Median Tectonic Line segments, became positive due to the 1946 rupture. For faults in Kyushu, the westernmost part of SW Japan, the ΔCFS remained negative. The occurrence of damaging earthquakes such as the 1945 Mikawa and 1948 Fukui earthquakes can be explained by this basic trend. This trend is less dependent on the viscosity structure and the variation of the slip distribution of the megathrust earthquakes. However, if the viscosity in the thin layer along the lithosphere-asthenosphere boundary is extremely low, the ΔCFS on the eastern source faults, including that of the 1891 M8.0 Nobi earthquake, was positive immediately after the 1944–1946 rupture sequence but decreased gradually due to viscoelastic relaxation, which is not consistent with earthquake occurrence. Also, some faults, including that of the 1995 M7.3 Kobe earthquake, exhibit behavior more sensitive to the slip distribution on the megathrust than to viscoelastic relaxation. These results imply the usefulness of the stress calculation combined with seismicity for determining the constraints of the viscosity structure and the slip distribution.