17:15 〜 19:15
[SCG45-P51] Numerical modeling of recurrences of two types of Kanto earthquakes and Boso slow slip events
キーワード：関東地震、数値シミュレーション、スロースリップイベント
On the upper boundary of the subducting Philippine Sea Plate beneath the Kanto region, Japan, devastating megathrust earthquakes repeatedly occur at the interval of 200-300 years, and slow slip events (SSEs) repeat off the Boso Peninsula at the intervals of 6-7 years. Recently, Saito and Noda (2023) estimated the stress accumulation rate in this region and suggested four major regions with high-stress accumulation rate (i.e., the Region Odawara, Miura, Awa, and Boso), applying their newly developed geodetic inversion method (Saito and Noda, 2022) to this region. Several studies suggest that the major slip of the 1923 Kanto earthquake, which is also called the Taisho Kanto earthquake, occurred in the Odawara and the Miura region. On the other hand, it is suggested that the major slip of the 1703 Kanto earthquake, which is also called the Genroku Kanto earthquake, occurred in the Miura and the Awa region, at least (e.g., Satake, 2023). In this study, we tried to reproduce the recurrence of the Genroku-type and the Taisho-type Kanto earthquakes, and SSEs in the Boso region, in a single model.
To model megathrust earthquakes and SSEs, we adopt a similar approach as in our previous studies (Matsuzawa et al., 2010, 2013), in which rate- and state-dependent friction law (RS-law) with cutoff velocities is used. Based on Saito and Noda (2023), we assume negative (a-b) values in the RS-law within the high-stress accumulation region, while positive (a-b) values (i.e., stable sliding) are assumed in the other region. The negative (a-b) region is defined as the region where there exists a high stress accumulation rate region (more than 0.05MPa/year in this case) within 12 km. In the off-Boso SSE region, effective normal stress is set to a lower value than that at the same depth to reproduce SSEs (e.g., Matsuzawa et al., 2013). We examined the cases with two times (Model 1), and four times (Model 2) higher effective normal stress region around the Awa region, and without such region (Model 3). The subducting Philippine Sea plate is modeled by about 73,000 triangular elements. The temporal evolution of slip velocity is numerically simulated, introducing the elastic response of semi-infinite medium and realistic configuration of the plate interface.
In the numerical result of Model 1 and 2, the Taisho-type earthquakes repeat at the interval of 200-300 years and the Genroku-type Kanto earthquakes occur as super-cycle events of the Taisho-type earthquakes, while most megathrust earthquakes are the Genroku-type in Model 3. This result suggests that the variation of two types of earthquakes may be controlled by parameter distribution. In Model 1, the typical recurrence interval of the Genroku-type earthquakes is about five hundred years, and the Genroku-type and the Taisho-type earthquakes tend to occur alternately. In Model 2, the typical recurrence interval of the Genroku-type earthquake is one thousand years, while the sequences are slightly complex. The recurrence interval of the actual Genroku-type earthquakes is estimated to be, for example, 2000-2700 years (Shishikura et al., 2014), or 1300-2000 years (Komori et al., 2021), and still have large ambiguity, although these are longer than the case in Model 2. Further studies both in the numerical simulation and geodetic analysis would be important to evaluate the risk of the hazard in very populated metropolitan areas.
To model megathrust earthquakes and SSEs, we adopt a similar approach as in our previous studies (Matsuzawa et al., 2010, 2013), in which rate- and state-dependent friction law (RS-law) with cutoff velocities is used. Based on Saito and Noda (2023), we assume negative (a-b) values in the RS-law within the high-stress accumulation region, while positive (a-b) values (i.e., stable sliding) are assumed in the other region. The negative (a-b) region is defined as the region where there exists a high stress accumulation rate region (more than 0.05MPa/year in this case) within 12 km. In the off-Boso SSE region, effective normal stress is set to a lower value than that at the same depth to reproduce SSEs (e.g., Matsuzawa et al., 2013). We examined the cases with two times (Model 1), and four times (Model 2) higher effective normal stress region around the Awa region, and without such region (Model 3). The subducting Philippine Sea plate is modeled by about 73,000 triangular elements. The temporal evolution of slip velocity is numerically simulated, introducing the elastic response of semi-infinite medium and realistic configuration of the plate interface.
In the numerical result of Model 1 and 2, the Taisho-type earthquakes repeat at the interval of 200-300 years and the Genroku-type Kanto earthquakes occur as super-cycle events of the Taisho-type earthquakes, while most megathrust earthquakes are the Genroku-type in Model 3. This result suggests that the variation of two types of earthquakes may be controlled by parameter distribution. In Model 1, the typical recurrence interval of the Genroku-type earthquakes is about five hundred years, and the Genroku-type and the Taisho-type earthquakes tend to occur alternately. In Model 2, the typical recurrence interval of the Genroku-type earthquake is one thousand years, while the sequences are slightly complex. The recurrence interval of the actual Genroku-type earthquakes is estimated to be, for example, 2000-2700 years (Shishikura et al., 2014), or 1300-2000 years (Komori et al., 2021), and still have large ambiguity, although these are longer than the case in Model 2. Further studies both in the numerical simulation and geodetic analysis would be important to evaluate the risk of the hazard in very populated metropolitan areas.