[S-SS09] Strong Ground Motion and Earthquake Disaster

[SSS09-P19] Development of Site Amplification Map in Bhutan

*Takumi Hayashida¹, Nityam Nepal², Dowchu Drukpa², Hiroshi Inoue³ (1.International Institute of Seismology and Earthquake Engineering, Building Research Institute, 2.Department of Geology and Mines, Ministry of Economic Affairs, Bhutan, 3.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Bhutan, Vs30, site amplification, microtremor

Bhutan is located in the eastern part of the Himalayas in the collision zone between the Eurasian and Indian plates, and is included in the world's most active seismic zones. The country has not experienced M-7 class earthquakes since the 20th century, but an Mw6.1 earthquake in September 2009 caused a disaster with casualties, and the 2011 Sikkim earthquake (Mw6.9), India, caused structural damage, mainly in Haa and Paro districts in the western part of Bhutan. Based on the risk of future earthquake occurrence in the country, detailed active fault investigations (e.g., Nakata et al., 2022JSAF), probabilistic seismic hazard assessments (Pagani et al., 2018; Stevens et al., 2020), and scenario-based seismic motion assessments (Hao et al., 2022) have recently been conducted. Nepal (2021) and Hayashida et al. (2022SSJ) compiled the results of microtremor array surveys conducted in Thimphu, Essuna, and Ura and estimated shallow S-wave velocity structures and average shear-wave velocity in the upper 30 m (Vs30) at all target sites. The estimated Vs30 values are lower than those by the US Geological Survey (USGS), derived using topographic slopes (Heath et al., 2020), suggesting that earthquake ground motions will be larger than the conventionally estimated ones. The discrepancy may be because the conventional Vs30 values were calculated based on 30-second mesh topography data and do not account for local geomorphological/geological features. In this study, we first obtained Shuttle Radar Topography Mission (SRTM) 1 arc-second topography data provided by the National Aeronautics and Space Administration (NASA) and estimated higher-resolution Vs30 distributions for the three regions using an approach shown by Allen and Wald (2009). The estimated Vs30 distribution strongly reflects the topographic features, but the correlations between the Vs30 values estimated by the microtremor array survey and those calculated with the topographic slope still need to be improved. Next, we combined the Vs30 distributions estimated from topography data and microtremor array survey to obtain the subsurface ground amplification maps in the entire areas based on existing empirical equations (e.g., Abrahamson et al., 2014; Stewart et al., 2017). Amplification factors ranged from 1 to 2 times in residential areas. The areas with larger amplification reflect the results of microtremor surveys, indicating the importance of continuing microtremor surveys to improve the accuracy of the amplification maps. We also updated the ground motion (i.e., peak ground acceleration) distribution for a scenario earthquake (Hao et al., 2022) and the probabilistic seismic hazard maps (Stevens et al., 2020) based on the updated ground amplification information.