Active tectonics and seismic hazard in Central Asia

中亚的活动构造以印度板块和欧亚板块的持续碰撞为主导，形成了一条延伸至亚洲大陆内部的长而宽的挤压变形带。在这些碰撞应力的驱动下，中亚的大断层有能力产生高震级地震，对人类生活和基础设施造成毁灭性影响。然而，与板块边界相比，人们对这种板内构造的地震和活动断层知之甚少。该项目旨在了解板内断层在两种截然不同的构造环境中的典型表现，并为区域地震危险性的估计提供信息。我将重点关注两个主要城市周围的主动变形：哈萨克斯坦的阿拉木图和塔吉克斯坦的杜尚别。阿拉木图（哈萨克斯坦最大的城市）位于天山山脉的北部边缘，这是亚洲大陆内部缓慢变形的一部分，可以通过厚皮（包括结晶基底）变形来适应缩短（图1）。阿拉木图位于并靠近许多断层，尽管100多年来没有大地震影响阿拉木图，变形率非常低（1-2毫米/年）。已知这些断层中的一些在3次破坏性地震序列中破裂：1887年的Verny (MW7.3)， 1889年的Chilik （MW8.3）和1911年的Chon Kemin （MW8.0）地震[1,3]。1887-1911年的序列暗示了一种行为模式，这种模式先前被提出用于控制其他板内环境中的主要缓慢移动断层——多个断层网络同时受到临界应力，然后快速连续破裂，随后是长时间的静止。只有对阿拉木图周围断层进行详细的古地震研究才能证实它们是否遵循这种行为模式。由于这些断层上最近发生的高震级事件很少，了解它们的地震历史也是确定它们目前构成的危险的关键。1889年和1911年的地震是有记录以来最大的大陆地震之一，值得特别关注。先前的研究表明，这两个事件都破裂了复杂的小断层集。许多断裂的详细断层几何形状、滑动震级和方向尚未确定，1889年地震的具体断层成因也存在不确定性。一些逆冲断层具有意想不到的位置和方向，出现在山谷的“错误”一侧，断层平行的河流位于上盘块体中（也见于西南的Suusamyr）。揭示这些构造的性质将有助于深入了解盆地闭合后期盆地边界断裂的命运。此外，还有全新世沿着扎利斯基山脉前缘断层（图1中的ZRF）断裂的地貌证据，该断层位于山脉的北部边缘，尽管地震的震级和时间尚不清楚。Grutzner等人（2017）认为，这一活动断层崖的一部分被大阿拉木图运河覆盖——因此，对这一断层过去行为的详细调查特别有意义。与天山北部相比，塔吉克盆地是一个快速移动的薄皮（主要涉及沉积盖层）褶皱和冲断带bb1，其东部通过左侧达瓦兹断层与帕米尔相连接，南部和东南部由阿富汗帕米尔和兴都库什盆地相连接（图2）。一套南北走向的隆起（背斜和推断的盲冲断层）在盆地内部蔓延，在坳陷北缘向东弯曲，在那里逆压的右侧伊利亚克断裂带与天山山脉的南侧形成边界，再往北一点则是吉萨尔断裂系统[11,5,17,13]。这些断层上潜在的地震对塔吉克斯坦首都杜尚别构成了重大威胁。

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.