11:45 〜 12:00
[SCG44-22] Energy-based scenarios for interplate great earthquakes taking aseismic slips outside seismogenic zone into account
キーワード：プレート境界地震、南海トラフ、非地震性すべり
A new framework for generating earthquake rupture scenarios considering the energy balance, named the energy-based method, was proposed by Noda et al. (2021 JGR). They generated energy-based scenarios for great earthquakes in the Nankai trough subduction zone based on a stress accumulation model estimated from interplate slip-deficit rate distribution. In this study, we expand this framework to a longer time scale beyond seismic cycles. This expansion will enable us to construct more realistic earthquake scenarios by incorporating information on past earthquake history. To generate scenarios consistent with the concept of seismic cycles on long time scales, we develop a method to incorporate the effect of aseismic slip outside seismogenic zone to the stress accumulation model.
Noda et al. (2021) defined the region where stress accumulates during interseismic periods as a seismogenic zone (asperity) and assumed that seismic slips occur within the asperities. However, to apply the method to seismic cycles, it is necessary to consider the slip budget not only inside the asperity but also outside it so that it is consistent with long-term plate subduction. In this study, we consider that the slip deficits accumulating during an interseismic period are almost canceled by forward slips (slip excess) during the seismic cycle which is defined as from just after the last earthquake to just after the next earthquake. In other words, the cumulative slip at the plate interface for one seismic cycle is approximately equal to the plate convergence rate multiplied by the period of one seismic cycle regardless of whether it is inside or outside the asperity.
We use slip-deficit rate distribution estimated by Noda et al. (2018 JGR), referred to hereafter as the N2018 model, to generate the energy-based scenarios. The N2018 model was inverted from GNSS displacement rate data from Mar. 2005 to Feb. 2011 as the long-term average of the slip-deficit rates. In the entire seismic cycle, however, there should be some periods in which the slip-deficit rates are different from the average rate because of postseismic aseismic slips, slow slip events, and preseismic shrinkage of locked zones. Such aseismic slip events will release slip deficits accumulating outside the asperities. To construct the stress accumulation model just before the next earthquake, we take the total amount of aseismic slips outside the asperities occurring during one seismic cycle into account without identifying the cause and timing of the aseismic slips. The total amount of aseismic slips is calculated by multiplying the slip-deficit rates outside the asperities by the period of seismic cycles. The aseismic slips basically do not occur inside the asperities, but we set a transition zone where the aseismic slip smoothly decreases to zero from the outside to the inside of the asperity to avoid an unrealistically extreme stress concentration.
The stress changes inside the asperities calculated from the aseismic slips outside the asperities are comparable to those calculated from the steady slip-deficit rates (the N2018 model). We recalculate earthquake scenarios considering stress changes due both to steady slip-deficit rates and aseismic slips by the same procedure as Noda et al. (2021). It results in earthquake scenarios with a slip amount and seismic moment that are approximately twice as large as those of the scenarios without considering the aseismic slips. The maximum slip of the scenarios is consistent with the accumulated slip deficit during the interseismic period, which means that the interseismic slip deficits within the asperity due to interplate locking are almost canceled by the seismic slip. These results suggest the importance of aseismic slips outside the asperities for estimating stress accumulation inside the asperities.
We generate source models of historical earthquakes by applying the energy-based method with the improved stress accumulation model to the earthquake history in the Nankai trough subduction zone. The estimated moment magnitudes of the 1854 Tokai, 1854 Nankai, 1944 Tonankai, and 1946 Nankai earthquakes are compatible with those of the actual earthquakes based on historical and seismic records.
Noda et al. (2021) defined the region where stress accumulates during interseismic periods as a seismogenic zone (asperity) and assumed that seismic slips occur within the asperities. However, to apply the method to seismic cycles, it is necessary to consider the slip budget not only inside the asperity but also outside it so that it is consistent with long-term plate subduction. In this study, we consider that the slip deficits accumulating during an interseismic period are almost canceled by forward slips (slip excess) during the seismic cycle which is defined as from just after the last earthquake to just after the next earthquake. In other words, the cumulative slip at the plate interface for one seismic cycle is approximately equal to the plate convergence rate multiplied by the period of one seismic cycle regardless of whether it is inside or outside the asperity.
We use slip-deficit rate distribution estimated by Noda et al. (2018 JGR), referred to hereafter as the N2018 model, to generate the energy-based scenarios. The N2018 model was inverted from GNSS displacement rate data from Mar. 2005 to Feb. 2011 as the long-term average of the slip-deficit rates. In the entire seismic cycle, however, there should be some periods in which the slip-deficit rates are different from the average rate because of postseismic aseismic slips, slow slip events, and preseismic shrinkage of locked zones. Such aseismic slip events will release slip deficits accumulating outside the asperities. To construct the stress accumulation model just before the next earthquake, we take the total amount of aseismic slips outside the asperities occurring during one seismic cycle into account without identifying the cause and timing of the aseismic slips. The total amount of aseismic slips is calculated by multiplying the slip-deficit rates outside the asperities by the period of seismic cycles. The aseismic slips basically do not occur inside the asperities, but we set a transition zone where the aseismic slip smoothly decreases to zero from the outside to the inside of the asperity to avoid an unrealistically extreme stress concentration.
The stress changes inside the asperities calculated from the aseismic slips outside the asperities are comparable to those calculated from the steady slip-deficit rates (the N2018 model). We recalculate earthquake scenarios considering stress changes due both to steady slip-deficit rates and aseismic slips by the same procedure as Noda et al. (2021). It results in earthquake scenarios with a slip amount and seismic moment that are approximately twice as large as those of the scenarios without considering the aseismic slips. The maximum slip of the scenarios is consistent with the accumulated slip deficit during the interseismic period, which means that the interseismic slip deficits within the asperity due to interplate locking are almost canceled by the seismic slip. These results suggest the importance of aseismic slips outside the asperities for estimating stress accumulation inside the asperities.
We generate source models of historical earthquakes by applying the energy-based method with the improved stress accumulation model to the earthquake history in the Nankai trough subduction zone. The estimated moment magnitudes of the 1854 Tokai, 1854 Nankai, 1944 Tonankai, and 1946 Nankai earthquakes are compatible with those of the actual earthquakes based on historical and seismic records.