Active tectonics and seismic hazard in Central Asia

The active tectonics of Central Asia are dominated by the on-going collision of the Indian and Eurasian plates, which has produced a long and broad belt of compressional deformation extending into the Asian continental interior. Driven by these collisional stresses, the large faults of Central Asia have the capacity to produce high magnitude earthquakes with devastating effects on human life and infrastructure. However, the earthquakes and active faults of such intraplate settings are poorly understood compared to plate boundaries.This project seeks to understand how intraplate faults typically behave in two contrasting tectonic settings, and to inform estimates of regional seismic hazard. I will focus on the active deformation around two major cities: Almaty in Kazakhstan and Dushanbe in Tajikistan. Almaty (Kazakhstan's largest city) is situated on the northern edge of the Tien Shan mountain range, a slowly deforming part of the Asian continental interior which accommodates shortening through thick-skinned (involving the crystalline basement) deformation (fig 1). The city lies on and close to a number of faults, although no big earthquake has affected Almaty in over 100 years and deformation rates are very low (1-2 mm/yr [9]). Several of these faults are known to have ruptured in a sequence of 3 destructive earthquakes: the 1887 Verny (MW7.3), 1889 Chilik (MW8.3) and 1911 Chon Kemin (MW8.0) earthquakes [1, 3]. The 1887-1911 sequence hints at a mode of behaviour which has previously been proposed to govern major slow-moving faults in other intraplate settings - networks of multiple faults simultaneously become critically stressed then rupture in quick succession, followed by a long period of quiescence [18]. Only detailed paleoseismic studies of the faults around Almaty can confirm whether they follow this mode of behaviour. With few examples of recent high magnitude events on these faults, unravelling their earthquake histories is also key to determining the present hazard they pose. The 1889 and 1911 events are of particular interest as some of the largest recorded continental earthquakes. Previous studies reveal that both events ruptured complex sets of smaller faults. Detailed fault geometries, slip magnitudes and directions are yet to be determined for many of these ruptures, and there are uncertainties regarding the attribution of particular faults to the 1889 event. Some thrusts have unexpected positions and orientations, appearing on the \wrong" side of valleys, with fault-parallel rivers owing in the hanging wall block (also seen in Suusamyr to the southwest. Unravelling the nature of these structures would give insights into to the fate of basin bounding faults in the late stages of basin closure. There is also geomorphic evidence for Holocene rupture along the Zailisky range front fault (ZRF in fig 1),which bounds the northern edge of the mountains, though earthquake magnitudes and timings are unknown [9]. Grutzner et al. (2017) [9] suggest that part of this active fault scarp is covered by the Big Almaty Canal - a detailed investigation of this fault's past behaviour is therefore particularly pertinent. In contrast to the northern Tien Shan, the Tajik basin is a fast moving, thin-skinned (largely involving the sedimentary cover) fold and thrust belt [5], bounded to the east by the Pamir via the left-lateral Darvaz fault and 1 to the south and southeast by the Afghan Pamir and the Hindu Kush [11] (fig 2). A set of north-south ridges (anticlines and inferred blind thrusts) sweep up the basin interior, curving to the east at the depression's northern margin where the transpressive right-lateral Illiac fault zone forms a boundary with the southern side of the Tien Shan mountain range, along with the Gissar fault system slightly further north [11, 5, 17, 13]. The potential for earthquakes on these faults poses a significant threat to Tajikistan's capital city, Dushanbe.