Seismic hazard and risk in Bhutan

Abstract

We present the first modern seismic hazard and risk assessment in the Bhutan Himalaya. We used a fault-based probabilistic seismic hazard analysis based on fault locations, slip-rates and paleoseismic earthquake data. We worked with two seismic intensity measures: the peak-ground acceleration (PGA) and Modified Mercalli Intensity (MMI). We extend the hazard analysis to risk by using local building distribution data and making various assumptions about building distribution and fragility. We find, unsurprisingly, that the Main Himalayan Thrust (MHT) is the primary source of hazard, with oblique strike-slip faults cutting across and beneath the Himalaya, and extensional grabens on the northern edge of Bhutan a secondary hazard. The hazard is highest in the southern part of Bhutan where the MHT is shallow, and site conditions lead to amplification of shaking. The risk does not reflect the hazard solely, but also the distribution of exposure, which is concentrated in the cities. We also simulated the 1714 MW8 earthquake, producing 10,000 possible shakemaps in terms of PGA and MMI; we find that many locations could experience PGA values of over 1 g, and on average, up to 18% of the Bhutanese population could be affected. Refining the probable frequency of larger events on the MHT in this region, developing local Ground Motion Prediction Equations, creating tailored vulnerability models for typical Bhutanese buildings, and improving the exposure mapping would most improve the hazard and risk results shown here. The existing building code of Bhutan, adopted from the Indian Seismic Zonation of 2002 (BIS-1893, 2002), uses a PGA of 0.36 g uniformly applied across the entire country. Our study, however, presents a non-uniform hazard level across the country and thus questions the relevancy of the current code of construction practices in the country.

Building the Himalaya from tectonic to earthquake scales

“…Such integrated efforts should improve our ability to understand deformation and earthquake potential in one of the most densely populated seismic regions of the world. We hope that integrated earthquake science will eventually provide actionable assessments of spatially variable hazard 200,201 to guide infrastructure development and enable the construction of earthquake early warning systems on large scales [202][203][204] . The societal impact of this effort would be multiplied by combining it with more science education initiatives to increase earthquake awareness in the local population 205,206 .…”

Comment on nhess-2021-279

“…Most of the land area of Bhutan is underlain by the Main Himalayan Thrust (MHT), which covers the entire length of the Himalayan Arc. An indication of fault creeping in some locations is the main reason for the release of interseismic loading for major earthquakes, and such activities have shown significant microseismicity for Bhutan (Stevens et al, 2020). Some studies also pointed out seismicity in the eastern fringe of the Himalayan arc can cause strong to moderate shaking across Bhutan, for example, Berthet et al (2013).…”

“…Both the simulated and the recorded ground motions can, however, be used for site-specific response studies (Ansal & Tönük, 2007). In absence of a national seismic code, Bhutan is assumed to fall under Indian seismic zone IV and V with PGA of 0.24 g and 0.36 g respectively for Maximum Considered Earthquake (MCE) (IS:1893(IS: , 2002, and in many instances, PGA of 0.36 g is applied uniformly across the entire country (Stevens et al, 2020). For the two zones mentioned, the PGA for earthquakes with a return period of 475 years is expected to be half of MCE, i.e., 0.12 g and 0.18 g. From the global seismic map (GSHAP), the PGA depicted are in the range 0.20 g to 0.28g, with an increasing pattern to the east of the country (Tempa, Chettri, Gurung, et al, 2021).…”

Sensitivity analysis of input ground motion on surface motion parameters in high seismic regions: a case of Bhutan Himalaya

“…So far, few studies on local seismic response have been in Bhutan, using a single strong motion record, but the reported studies mainly focus on the role of bedrock depth in ground response parameters (Tempa et al, 2020;Tempa et al, 2021a, b). The ground motion response analysis may not adequately address the accuracy in predicting the response when the information is limited regarding site characteristics and their variations within the same soil column (Stevens et al, 2020). In the case of data-scarce regions such as Bhutan, the variation in terms of material characteristics can be possibly accounted for using sensitivity analysis.…”