11:00 AM - 1:00 PM
[STT41-P04] Stress loading on the intra-plate faults in western Japan due to the 1944 Tonankai and 1946 Nankai earthquakes
Keywords:Stress loading, Nankai trough, Tonankai and Nankai earthquakes, Philippine Sea plate, Intra-plate fault, Finite element method
The Philippine Sea Plate is descending under the Nankai Trough off the coast of western Japan, causing M~8 earthquakes at intervals of 100-200 years. The most recent earthquakes were the 1944 Tonankai and the 1946 Nankai earthquakes. Since then, more than 70 years later have passed, and there are concerns about the occurrence of another large earthquake and tsunami. Studies of historical earthquakes have shown that inland earthquakes were activated in western Japan before and after the occurrence of a large earthquake. Therefore, it is important to mechanically estimate the impact of Nankai Trough earthquakes on inland faults, i.e., stress changes, in order to evaluate the risk of inland earthquakes. Since the Nankai Trough earthquakes have shown various occurrence patterns, it is necessary to investigate the stress changes caused by each occurrence pattern. The 1944 and 1946 earthquakes were of the “half-to-total-rupture” type. These earthquakes were followed by damaging earthquakes such as the 1945 Mikawa earthquake and the 1948 Fukui earthquake. In particular, the 1945 Mikawa earthquake occurred just after the half rupture in 1944 and before the total rupture in 1946, indicating the importance of estimating the impact of the half rupture as well. Therefore, in this study, we calculate the stress changes induced on each inland fault in western Japan through the “half-to-total” events of 1944 and 1946.
First, we estimated the stress changes preliminarily using the semi-infinite elastic model by Okada (1992). We used the plate boundary geometry by Koketsu et al. (2009, 2012), and took the slip direction along the relative velocity model of Miyazaki & Heki (2001). The location, strike, dip, and rake of inland faults are based on the National Seismic Hazard Map (Headquarters for Earthquake Research Promotion, 2005). We calculated Coulomb stress change on each inland fault by giving uniform slip of 2 m and 4 m for the Tonankai and Nankai source areas, respectively, with an apparent friction coefficient of 0.4. The Coulomb stress change due to the 1944 half rupture is positive for faults in the northern Chubu and the northern Kinki and negative for faults in the southern Kinki and Shikoku. After the subsequent Nankai earthquake, the Coulomb stress on the faults in the southern Kinki and Shikoku switched from negative to positive. The maximum stress change due to half-to-total-rupture in 1944 and 1946 was 0.2 MPa. On the contrary, assuming that the Nankai earthquake occurred first, the faults from Shikoku to the southern Chubu region were subjected to positive Coulomb stress, while the faults from Kinki to the northern part of the Chubu region were negative. In the subsequent total rupture, the negative and positive stresses are switched on most of the faults in the Chubu region.
In this presentation, we will examine effects on fault stress from topography, heterogeneous crustal structure and elastic-viscoelastic properties of the southwest Japan arc to the Philippine Sea slab, slip distributions from previous literature, and so on. Such calculations will enable us to estimate the risk of inland earthquakes associated with the occurrence of the Nankai Trough earthquake.
First, we estimated the stress changes preliminarily using the semi-infinite elastic model by Okada (1992). We used the plate boundary geometry by Koketsu et al. (2009, 2012), and took the slip direction along the relative velocity model of Miyazaki & Heki (2001). The location, strike, dip, and rake of inland faults are based on the National Seismic Hazard Map (Headquarters for Earthquake Research Promotion, 2005). We calculated Coulomb stress change on each inland fault by giving uniform slip of 2 m and 4 m for the Tonankai and Nankai source areas, respectively, with an apparent friction coefficient of 0.4. The Coulomb stress change due to the 1944 half rupture is positive for faults in the northern Chubu and the northern Kinki and negative for faults in the southern Kinki and Shikoku. After the subsequent Nankai earthquake, the Coulomb stress on the faults in the southern Kinki and Shikoku switched from negative to positive. The maximum stress change due to half-to-total-rupture in 1944 and 1946 was 0.2 MPa. On the contrary, assuming that the Nankai earthquake occurred first, the faults from Shikoku to the southern Chubu region were subjected to positive Coulomb stress, while the faults from Kinki to the northern part of the Chubu region were negative. In the subsequent total rupture, the negative and positive stresses are switched on most of the faults in the Chubu region.
In this presentation, we will examine effects on fault stress from topography, heterogeneous crustal structure and elastic-viscoelastic properties of the southwest Japan arc to the Philippine Sea slab, slip distributions from previous literature, and so on. Such calculations will enable us to estimate the risk of inland earthquakes associated with the occurrence of the Nankai Trough earthquake.