برآورد میزان شدت زمین لرزه افقی در امتداد هیمالیا Overestimation of the earthquake hazard along the Himalaya: constraints in bracketing of medieval earthquakes from paleoseismic studies

Introduction Te Himalaya is a 2500  km long belt of mountains, which are the result of the progressive under thrusting of the Indian Plate beneath Tibetan Plate along Main Himalayan Trust (MHT) (Zhao et  al. 1993; Molnar and Tapponnier 1977). Global Positioning System (GPS) measurements indicate 4–5  cm/year of convergence rate between these two plates (Banerjee and Bürgmann 2002) of which 18 mm/year is accommodated by thrust systems along the Himalayan arc. Tectonic deformation has resulted in the formation of four major south-verging thrust systems viz. South Tibetan Detachment (STD), which separates Tibetan sedimentary and Higher Himalayan meta-sedimentary sequences, Main Central Trust (MCT), which bounds the Higher Himalayan Crystalline rocks, and Upper Precambrian to Paleozoic rocks of Lesser Himalaya. MCT has not been observed to rupture the Quaternary deposits and is considered to be inactive (Nataka 1989; Valdiya 1992). Main Boundary Trust (MBT) marks the southern edge of Lesser Himalaya. Te southernmost thrust is Himalayan Frontal Trust (HFT), which has displaced Tertiary and Quaternary alluvial and molasse sediments and is expressed as discontinuous range front scarps (Nakata 1972; Valdiya 1980, 1992). Tese fault systems are the result of the southward propagation of the Himalayan Front through time and each fault merges into the basal detachment fault, i.e., Main Himalayan Trust (Zhao et al. 1993; Pandey et al. 1995). Based on the active fault topography, the MBT and HFT are considered as active (Nakata 1989; Valdiya 1980; Malik and Mohanty 2007). Historical archives indicate that Himalaya has experienced several damaging earthquakes of intensity >XI in the last millennium (Iyengar et al. 1999; Chitrakar and Pandey 1986). Instrumental records and geodetic data indicate that several earthquakes have occurred along the basal detachment and propagated southward to produce surface rupture at Himalayan Front such as 1934 Nepal–Bihar (Sapkota et al. 2013) (Fig. 1c), but few nucleate below the Higher Himalaya and produced ruptures in the higher reaches only, 1905 Kangra Earthquake is one such example which produced surface rupture along Kangra Valley Fault (KVF) (Fig. 2a) (Malik et al. 2015). Te variable nature of the rupture pattern raises several questions: (a) does all stored strain developed due to India–Tibet convergence is released during a single event? (b) Is the strain accumulation similar along the entire Himalaya, or (c) how does the variation in the Himalayan strike and segmentation efects the variable rupture pattern?