4:00 PM - 4:15 PM
[SSS08-03] Energy-based scenarios for the Nankai trough subduction earthquakes: Stress accumulation model taking historical earthquakes into account
Keywords:Earthquake scenarios, Interplate earthquake, Nankai trough, The mechanics of fault rupture
The occurrence of earthquakes is controlled by the balance between the shear stress that drives faulting and the frictional stress that resists it. This perspective of the mechanics of faulting is necessary to create scenarios of anticipated megathrust earthquakes for preparing for unprecedented events in the Nankai trough subduction zone. Based on such an idea, Noda et al. (2020) proposed a new energy-based method to construct rupture scenarios, which is composed of two steps: (1) Estimating a coseismic slip model from the distribution of shear stress accumulation based on an interseismic slip-deficit rate distribution. (2) Examining whether the model follows the mechanics of faulting by comparing the strain energy released by the fault slip ΔW and the energy dissipated on the fault ED. They introduced a new parameter “residual energy” defined as ER≡ΔW-ED. Positive residual energy is a necessary condition for earthquake generation in terms of the law of conservation of energy. The scenario with negative residual energy is supposed to be unreasonable.
Noda et al. (2020) assumed a constant stress accumulation rate and calculated the shear stress accumulation from a reference time when the shear stress was zero over the entire seismogenic region of plate interface along the Nankai trough, for simplicity. However, the slip history of earthquake cycles in the Nankai trough is more complex. For example, the latest great earthquakes, the 1944 Tonankai and 1946 Nankai earthquakes, did not rupture the entire plate interface from the Suruga bay to off Shikoku at once. In addition, the segmentation of historical earthquakes before the 20th century remains controversial because there are insufficient observation records. It has been considered that the source region of the 1854 Ansei-Tokai earthquake comprises segments in the Kumano-nada, Enshu-nada, and Suruga bay (Ishibashi & Satake, 1998); however, Seno (2012) suggested that the source region did not include the segment in the Kumano-nada. Segments that have not ruptured for some time accumulate substantial shear stress and will release a large amount of strain energy during the next event.
In this study, for more realistic earthquake rupture scenarios, we improved the scenarios of Noda et al. (2020) by integrating the earthquake rupture history into the stress accumulation model. First, we assumed that the stress accumulation rate is constant and that the shear stress accumulated since the last event is fully released during the next event. We estimated slip models of the 1854 Ansei Tokai and Nankai earthquakes, the 1944 Tonankai earthquake, and the 1946 Nankai earthquake following the method of Noda et al. (2020) and evaluated the residual energy for each model. We found that the earthquake rupture history of Seno (2012) is more reasonable in terms of energy budget than that of Ishibasi & Satake (1998).
Next, we calculated the distribution of stress accumulation in 2021 based on Seno (2012) and generated various earthquake scenarios including single-segment ruptures and multi-segment ruptures. We found that the strain energy stored in the elastic medium is not enough for generating great thrust earthquakes in 2021 because all scenarios showed negative residual energy. However, as the shear stress continues to accumulate at the same rate, the residual energy becomes positive by 2025 in a scenario which ruptures the segment in the Suruga bay, satisfying a necessary condition of earthquake generation. For the scenario rupturing all segments along the Nankai trough, the residual energy becomes positive by 2070. It should be noted that these estimations strongly depend on the assumptions of uncertain parameters such as the dissipated energy on the fault. We will discuss the effects of the assumed parameters on the earthquake scenarios.
Noda et al. (2020) assumed a constant stress accumulation rate and calculated the shear stress accumulation from a reference time when the shear stress was zero over the entire seismogenic region of plate interface along the Nankai trough, for simplicity. However, the slip history of earthquake cycles in the Nankai trough is more complex. For example, the latest great earthquakes, the 1944 Tonankai and 1946 Nankai earthquakes, did not rupture the entire plate interface from the Suruga bay to off Shikoku at once. In addition, the segmentation of historical earthquakes before the 20th century remains controversial because there are insufficient observation records. It has been considered that the source region of the 1854 Ansei-Tokai earthquake comprises segments in the Kumano-nada, Enshu-nada, and Suruga bay (Ishibashi & Satake, 1998); however, Seno (2012) suggested that the source region did not include the segment in the Kumano-nada. Segments that have not ruptured for some time accumulate substantial shear stress and will release a large amount of strain energy during the next event.
In this study, for more realistic earthquake rupture scenarios, we improved the scenarios of Noda et al. (2020) by integrating the earthquake rupture history into the stress accumulation model. First, we assumed that the stress accumulation rate is constant and that the shear stress accumulated since the last event is fully released during the next event. We estimated slip models of the 1854 Ansei Tokai and Nankai earthquakes, the 1944 Tonankai earthquake, and the 1946 Nankai earthquake following the method of Noda et al. (2020) and evaluated the residual energy for each model. We found that the earthquake rupture history of Seno (2012) is more reasonable in terms of energy budget than that of Ishibasi & Satake (1998).
Next, we calculated the distribution of stress accumulation in 2021 based on Seno (2012) and generated various earthquake scenarios including single-segment ruptures and multi-segment ruptures. We found that the strain energy stored in the elastic medium is not enough for generating great thrust earthquakes in 2021 because all scenarios showed negative residual energy. However, as the shear stress continues to accumulate at the same rate, the residual energy becomes positive by 2025 in a scenario which ruptures the segment in the Suruga bay, satisfying a necessary condition of earthquake generation. For the scenario rupturing all segments along the Nankai trough, the residual energy becomes positive by 2070. It should be noted that these estimations strongly depend on the assumptions of uncertain parameters such as the dissipated energy on the fault. We will discuss the effects of the assumed parameters on the earthquake scenarios.