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[SSS10-14] An earthquake scenario assessment due to Himalayan megathrust segmentation in southwest Bhutan
Keywords:Bhutan, Seismic Hazard Assessment, Earthquake Scenario Simulation, Main Himalayan Thrust
Introduction : . Located in a seismic gap of the eastern part of Main Himalayan Thrust (MFT), Bhutan is most likely under an earthquake threat in a long-term prediction. Some low exceedance probabilities of Probabilistic Seismic Hazard Assessment (PSHA) (Pagani et al. 2018, Stevens et al. 2020) could be affected by incomplete historical records such as omission misconceptions. In the southwestern part of Bhutan, the investigation of active faults has hardly progressed despite the plate boundary thrust, such as Main Front Thrust (MFT) and Main Boundary Thrust (MBT), which went through from east to west. Aerial photographs were used to create anaglyphs and contour maps to understand the detailed location and shape of active faults (Nakata et al, 2020). Moreover, field investigation and trench have been carried out from Samtse to Sipsu in the southwestern border of Bhutan (Kumahara et al, 2020, Drukpa et al, 2020). Based on these information, we try to develop a huge earthquake scenario for the damage estimation and aseismic preparation in southwestern Bhutan.
The southwest Bhutan MHT Fault Model : . In almost the same MHT geological environment of Nepal, the 2015 Gorkha earthquake (Mw=7.9) ruptured a base of the MHT at a depth of about 15 km but did not break the area ranging from MFT to Kathmandu (Grandin et al, 2015). Considering the possibility of MHT seismogenic potential rupture from shallow MFT to the main MHT base, a huge Mw = 7.8 earthquake scenario was assumed that the MFT/MBT Bhutan southwest portion ruptured from a 2km depth dipping to the northward, consequently the main MHT rupture simultaneously.
By the scaling rule of fault length with seismic moment (Takemura 1998), a 130 km-length of MHT is modeled along the 26.94 Latitude from Samtse to Gelephu on the southern boundary of Bhutan. The fault width is calculated up to 20km, 40km, and 60km.Considering the uncertainty of fault rupture, and capital Thimphu, the result of width 60km was emphasized. In profile, the fault model ruptures from the subsurface of 2 km, northward dipping steeply at 30 Degree in 18km-width, then dipping gently at 7 Degree in 42km-width approaching to a depth of 17km around Thimphu.
Evaluation method and parameters: . The scenario of the MHT fault model with parameters as below was simulated by using the OpenQuake engine (GEM, 2013). The model with a length of 130km and a width of 60km, an inclination angle changed from 30 degrees to 7 degrees, with an epicenter Mw = 7.8, an epicenter depth H = 10km, and the site surface layer with Vs = 800m/s. We found Ground Motion prediction Equations (GMPE) play an important role in the scenario simulation, and discussed the variations of GMPEs.
Summary : . We simulated earthquake generated by the Himalayan thrust fault segmentation in southwestern Bhutan. The scenario results show very significant shaking over a wide area from MFT to Thimphu as shown in Figure. The ground shaking intensity PGA = 0.6G (MMI seismic intensity VIII class) could cause severe earthquake damage. It raises a crucial issue under the reality that about 66% of households in Bhutan live in buildings constructed with traditional materials and techniques.
Acknowledgment : . This work was supported by the research of Protection of traditional Bhutanese architecture made with rammed earth and stone masonry techniques from earthquake damage, led by Aoki Takayoshi and Jigme Thinlye Namgyal.
References: . Grandin, R. et al. (2015), Geophys. Res.Lett., doi:10.1002/2015GL066044 ; . Kumahara, Y. et al. (2020),The General Meeting of the AJG Spring 2020 ; . Nakata, T. et al. (2020), Japanese Society for Active Fault Studies, p8-9 ; . Drukpa, D. et al. (2020), International Symposium on Active Faulting, p23-24 ; . GEM (2013), http://www.globalquakemodel.org ; . Pagani, M. et al. The 2018 version of the Global Earthquake Model ; . Stevens V. L. et al. 2020, Seismic hazard and risk in Bhutan, Natural Hazards.
The southwest Bhutan MHT Fault Model : . In almost the same MHT geological environment of Nepal, the 2015 Gorkha earthquake (Mw=7.9) ruptured a base of the MHT at a depth of about 15 km but did not break the area ranging from MFT to Kathmandu (Grandin et al, 2015). Considering the possibility of MHT seismogenic potential rupture from shallow MFT to the main MHT base, a huge Mw = 7.8 earthquake scenario was assumed that the MFT/MBT Bhutan southwest portion ruptured from a 2km depth dipping to the northward, consequently the main MHT rupture simultaneously.
By the scaling rule of fault length with seismic moment (Takemura 1998), a 130 km-length of MHT is modeled along the 26.94 Latitude from Samtse to Gelephu on the southern boundary of Bhutan. The fault width is calculated up to 20km, 40km, and 60km.Considering the uncertainty of fault rupture, and capital Thimphu, the result of width 60km was emphasized. In profile, the fault model ruptures from the subsurface of 2 km, northward dipping steeply at 30 Degree in 18km-width, then dipping gently at 7 Degree in 42km-width approaching to a depth of 17km around Thimphu.
Evaluation method and parameters: . The scenario of the MHT fault model with parameters as below was simulated by using the OpenQuake engine (GEM, 2013). The model with a length of 130km and a width of 60km, an inclination angle changed from 30 degrees to 7 degrees, with an epicenter Mw = 7.8, an epicenter depth H = 10km, and the site surface layer with Vs = 800m/s. We found Ground Motion prediction Equations (GMPE) play an important role in the scenario simulation, and discussed the variations of GMPEs.
Summary : . We simulated earthquake generated by the Himalayan thrust fault segmentation in southwestern Bhutan. The scenario results show very significant shaking over a wide area from MFT to Thimphu as shown in Figure. The ground shaking intensity PGA = 0.6G (MMI seismic intensity VIII class) could cause severe earthquake damage. It raises a crucial issue under the reality that about 66% of households in Bhutan live in buildings constructed with traditional materials and techniques.
Acknowledgment : . This work was supported by the research of Protection of traditional Bhutanese architecture made with rammed earth and stone masonry techniques from earthquake damage, led by Aoki Takayoshi and Jigme Thinlye Namgyal.
References: . Grandin, R. et al. (2015), Geophys. Res.Lett., doi:10.1002/2015GL066044 ; . Kumahara, Y. et al. (2020),The General Meeting of the AJG Spring 2020 ; . Nakata, T. et al. (2020), Japanese Society for Active Fault Studies, p8-9 ; . Drukpa, D. et al. (2020), International Symposium on Active Faulting, p23-24 ; . GEM (2013), http://www.globalquakemodel.org ; . Pagani, M. et al. The 2018 version of the Global Earthquake Model ; . Stevens V. L. et al. 2020, Seismic hazard and risk in Bhutan, Natural Hazards.