Abstract
Abstract. In spite of an increasing number of paleoseismic studies carried out over the last decade along the Himalayan Arc, the chronology of historical and prehistorical earthquakes is still poorly constrained. In this paper, we present geomorphologic and paleoseismic studies conducted over a large river-cut exposure along the Main Fontal Thrust in southwestern Bhutan. The Piping site reveals a 30 m high fault-propagation fold deforming late Holocene alluvial deposits. There, we carried out detailed paleoseismic investigations and built a chronological framework on the basis of 22 detrital charcoal samples submitted to radiocarbon dating. Our analysis reveals the occurrence of at least five large and great earthquakes between 485±125 BCE and 1714 CE with an average recurrence interval of 550±211 years. Coseismic slip values for most events reach at least 12 m and suggest associated magnitudes are in the range of Mw 8.5–9. The cumulative deformation yields an average slip rate of 24.9±10.4 mm yr−1 along the Main Frontal Thrust over the last 2600 years, in agreement with geodetic and geomorphological results obtained nearby.
Highlights
The Himalayas, accommodating ∼ 50 % of the India–Eurasia collision at a shortening rate of ∼ 20 mm yr−1 (e.g., Lavé and Avouac, 2000; Ader et al, 2012; Burgess et al, 2012; Marechal et al, 2016), are a region of sustained seismicity as illustrated recently by the 2015 Mw 7.8 Gorkha earthquake in Nepal (e.g., Avouac et al, 2015; Grandin et al, 2015)
All these units are bounded by major faults including the South Tibetan Detachment (STD), the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT), which is the most recent expression of the thrust sequence that accommodated the deformation over geological timescales (Gansser, 1964; Le Fort, 1975; McQuarrie et al, 2008; Long et al, 2011a)
These four major north-dipping thrust faults connect to the Main Himalayan Thrust (MHT), a mid-crustal decollement under which the Indian plate subducts beneath the Himalayas and Tibet
Summary
The Himalayas, accommodating ∼ 50 % of the India–Eurasia collision at a shortening rate of ∼ 20 mm yr−1 (e.g., Lavé and Avouac, 2000; Ader et al, 2012; Burgess et al, 2012; Marechal et al, 2016), are a region of sustained seismicity as illustrated recently by the 2015 Mw 7.8 Gorkha earthquake in Nepal (e.g., Avouac et al, 2015; Grandin et al, 2015). Even the Bollinger et al (2014) study constitutes a rather short catalog when compared to data available for smaller structures such as the ∼ 1300 km long San Andreas Fault or the ∼ 1000 km long Dead Sea Fault or the North Anatolian Fault (e.g., Meghraoui et al, 2012; Rockwell et al, 2015) This issue is mostly due to the accommodation of a high shortening rate along the frontal thrust faults leading to surface ruptures with vertical offsets of up to 10 m (e.g., Kumar et al, 2010; Le Roux-Mallouf et al, 2016) and an average recurrence interval of 500–1000 years Our results allow us to discuss the timing and the magnitude of five surfacerupturing events that occurred in Bhutan during the last 2600 years
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