[S-TT43] Creating future of solid Earth science with high performance computing (HPC)

[STT43-P02] Intra-plate stress changes due to the 1944 Tonankai and the 1946 Nankai earthquakes, southwestern Japan, based on a high-fidelity 3-D rheological structure model

*Akinori Hashima¹, Takane Hori¹, Takeshi Iinuma¹, Sota Murakami², Kohei Fujita², Tsuyoshi Ichimura² (1.Japan Agency for Marine-Earth Science and Technology, 2.Earthquake Research Institute, University of Tokyo)

Keywords:Stress loading, Nankai trough, Tonankai and Nankai earthquakes, Philippine Sea plate, Intra-plate fault, Finite element method

In the southwest Japan arc, the Philippine Sea Plate is subducting beneath the continental plate from the Nankai Trough, and megathrust earthquakes repeatedly occur at intervals of 100-200 years. According to studies of historical earthquakes, there is a period of active inland damaging earthquakes in southwestern Japan from 50 years before to 10 years after a megathrust earthquake. To predict the activity of these inland earthquakes, it is necessary to quantify the stress accumulation on the inland source faults. For the period before a megathrust earthquake, it is possible to calculate the inland stress accumulation rate from the nearly steady-state locking pattern. However, for the period after the megathrust earthquake, it is necessary to calculate the stress accumulation considering various past coseismic rupture patterns and subsequent viscoelastic relaxation beforehand. In this study, we focus on the most recent ruptures, the 1944 Tonankai and 1946 Nankai earthquakes, and estimate the four-year stress changes on inland faults in southwest Japan due to these ruptures and viscoelastic relaxation. These ruptures were followed by a series of inland ~M7 damaging earthquakes, including the 1945 Mikawa earthquake and the 1948 Fukui earthquake. A highly detailed finite element model based on a crustal deformation analysis method, which can reduce computational cost, was used to calculate stresses considering realistic viscous structures. The calculated stress field in the inland area of southwestern Japan is essentially dominated by elastic changes during the 1944 and 1946 earthquakes, with negligible viscoelastic changes. In the slab, on the other hand, the viscoelastic changes are significant, indicating the importance of knowing the viscosity structure of the subsurface. Using these calculated stress fields, change in the Coulomb failure stress (ΔCFS) on each fault was calculated with an effective friction coefficient of 0.4. ΔCFS is basically positive for strike-slip faults east of 135°E after the 1944 rupture. On the other hand, after the 1946 Nankai earthquake, ΔCFS is positive on fault segments of the Median Tectonic Line on the Shikoku Island and nearby faults. Faults in the Kyushu region consistently have negative ΔCFS. The occurrence of inland damaging earthquakes such as the 1945 Mikawa earthquake and the 1948 Fukui earthquake could be explained by this basic trend. These trends did not depend much on the viscosity structure or the slip distribution of the megathrust earthquakes. However, in the case of assuming a very low viscosity in the layer along the slab base, ΔCFS become positive immediately after the 1944 and 1946 ruptures, but it decreases gradually on the source fault of the 1891 Nobi earthquake, which is inconsistent with earthquake occurrence. Some faults, such as the source fault of the 1995 Kobe earthquake, are more sensitive to the slip distributions of the 1944 and 1946 ruptures than to the viscosity structure. The results of these stress calculations might be used to constrain the rheological structure and slip distribution by comparing the inland earthquake activities.