Seismotectonic perspectives on the Himalayan arc and contiguous areas: Inferences from past and recent earthquakes

Spread over countries including Pakistan, India, Nepal, Bhutan, and China, the Himalayan mountain chain, the most spectacular result of the Indo-Eurasian plate collision, is a locus of destructive earthquakes. Past earthquakes from this region have impacted vast swathes of frontal Himalaya and stretches of alluvial plains south of the range front. Risk from future earthquakes has increased, considering the burgeoning population and an ever-expanding built-environment in the region. While considerable ambiguities exist on the locations, ruptures, and sizes of the earthquakes during the first half of the last millennium (1255, 1344, and 1505 CE), those during the latter half (1803 and 1833 CE) are quite well-documented, all reported from the central Himalayan segment comprising eastern Nepal, Kumaun, and Garhwal. While dormancy prevailed in the central segment in the intervening period, the Himalayan arc elsewhere witnessed three large/great earthquakes in the last century, namely, 1905 Kangra (Mw 7.8), 1934 Bihar-Nepal (Mw 8.2), and 1950 Upper Assam (Mw 8.6), the last one being the largest intra-continental earthquake in the recorded history. The April 25, Gorkha (Nepal) earthquake (Mw 7.8) located in the central seismic gap terminated the period of low-level seismic productivity that followed the 1950 event. The Himalayan arc and its contiguous regions are now being investigated using various tools including paleoseismology. Quite understandably, the interpretations and tectonic models do not always converge, given the diverse quality and type of data. The data from variously sized modem-day earthquakes sourced in the diverse structural settings of the Himalaya provide better constraints on the source properties of the Himalayan earthquakes. The source complexity and diversity is a striking point of interest for the post-1950 events, and at least some of them seem to deviate from the generally accepted model of great ruptures on a shallow dipping detachment. A prime example of such anomalies is the 2005 Kashmir earthquake (Mw 7.6) sourced on an out-of sequence thrust. The sources of the 1991 Uttarkashi (Mw 6.8), 1999 Chamoli (Mw 6.6), and the 2015 Gorkha (Nepal) (Mw 7.8) earthquakes - all in the central Himalayan segment - are attributed to the ramp-flat on the down-dip part of the Main Himalayan Thrust (MHT). In contrast, the 2011 Sikkim (Mw 6.9) and the 2016 Imphal (Mw 6.7) earthquakes are intraplate events sourced within the subducting slab. Source models of modern-day earthquakes (1991-2016) discussed in this paper bring out this plurality of mechanisms expected of a complex subduction-cum-collisional boundary. This review presents the status of current seismotectonic understanding of the Himalaya from the analyses of significant earthquakes, from the past as well as recent events with a focus on the central Himalayan segment