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[SCG40-P02] A future scenario earthquake for seismic hazard analysis in Kathmandu, Nepal
Keywords:scenario earthquake, seismic hazards, Kathmandu, characterized source models
We propose a distribution of the source regions of great historical earthquakes in the Main Himalayan Thrust (Figure) using Bilham (2019) and the results of trench surveys (Bt and Bg in Figure). In this distribution, the red rectangle can be the source region of a future scenario earthquake for seismic hazard analysis in Kathmandu, Nepal. The rectangle is neighboring Kathmandu (K in Figure), and the previous events in the rectangle occurred in 1255 and 1344. They are the oldest among the great historical earthquakes in the Main Himalayan Thrust (MHT). The displacement rates observed by the GNSS are similar along the southern boundary of the MHT (Tabei et al., 2020), and it can be assumed that the great historical earthquakes mostly released accumulated strain. Therefore, it is the most probable for the rectangle, where the previous events are the oldest, to generate a great future earthquake the soonest. The area of the rectangle is measured to be 17,600 km2 with a length of 220 km and a width of 80 km.
The 2015 Gorkha earthquake recently occurred in the MHT. Kobayashi et al. (2016) performed a source inversion for this earthquake, obtaining a seismic moment M0 of 7.4 x 1020 Nm (Mw 7.8) and the slip distribution. We trim insignificant slips off the slip distribution following the scheme of Somerville et al. (1999). This trimming results in a rupture area of 9,600 km2. This rupture area corresponds to M0 = 8.9 x 1020 Nm (Mw 7.9) using the formula of Somerville et al. (1999), while the area corresponds to M0 = 5.2 x 1020 Nm (Mw 7.7) using the formula of Murotani et al. (2008). The actual M0 of 7.4 x 1020 Nm (Mw 7.8) falls between these, so that we use the both formulas to determine two characterized source models for the future scenario earthquake with the rupture area of 17,600 km2. The formula of Somerville et al. (1999) gives M0 = 2.2 x 1021 Nm (Mw 8.2), an average stress drop Δσc of 2.3 MPa, an asperity area Sa of 4,000 km2, a stress drop on asperity σa of 10.1 MPa, and an effective stress in the background σb of 2.0 MPa. Meanwhile, the formula of Murotani et al. (2008) gives M0 = 1.3 x 1021 Nm (Mw 8.0), Δσc = 1.4 MPa, Sa = 4,400 km2, σa = 5.6 MPa, and σb = 1.1 MPa.
This study partly includes the results of the SATREPS project with the Department of Mines and Geology, Nepal.
The 2015 Gorkha earthquake recently occurred in the MHT. Kobayashi et al. (2016) performed a source inversion for this earthquake, obtaining a seismic moment M0 of 7.4 x 1020 Nm (Mw 7.8) and the slip distribution. We trim insignificant slips off the slip distribution following the scheme of Somerville et al. (1999). This trimming results in a rupture area of 9,600 km2. This rupture area corresponds to M0 = 8.9 x 1020 Nm (Mw 7.9) using the formula of Somerville et al. (1999), while the area corresponds to M0 = 5.2 x 1020 Nm (Mw 7.7) using the formula of Murotani et al. (2008). The actual M0 of 7.4 x 1020 Nm (Mw 7.8) falls between these, so that we use the both formulas to determine two characterized source models for the future scenario earthquake with the rupture area of 17,600 km2. The formula of Somerville et al. (1999) gives M0 = 2.2 x 1021 Nm (Mw 8.2), an average stress drop Δσc of 2.3 MPa, an asperity area Sa of 4,000 km2, a stress drop on asperity σa of 10.1 MPa, and an effective stress in the background σb of 2.0 MPa. Meanwhile, the formula of Murotani et al. (2008) gives M0 = 1.3 x 1021 Nm (Mw 8.0), Δσc = 1.4 MPa, Sa = 4,400 km2, σa = 5.6 MPa, and σb = 1.1 MPa.
This study partly includes the results of the SATREPS project with the Department of Mines and Geology, Nepal.