Active tectonics and seismic hazard in Central Asia

中亚的活动构造和地震灾害

• 批准号: 2102553
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2102553
• 关键词: Active; tectonics; seismic; hazard; Central

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.

中亚的活动构造主要是印度板块和欧亚板块的持续碰撞，产生了一条延伸到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对人类生活和基础设施造成破坏性影响的高震级地震。然而，与板块边界相比，对这种板内构造的地震和活动断层的了解甚少。本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活动变形：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）位于天山山脉的北方边缘，天山山脉是亚洲大陆内部缓慢变形的一部分，通过厚皮（包括结晶基底）变形来适应缩短（图1）。城市位于并靠近许多断层，尽管在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[9]）。已知这些断层中有几个在3次破坏性地震中破裂：1887年Verny（MW7.3），1889年Chilik（MW8.3）和1911年Chon Kemin（MW8.0）地震[1，3]。1887-1911年的序列暗示了一种行为模式，这种行为模式以前曾被提出来控制其他板内环境中的主要缓慢移动断层-多个断层的网络同时受到临界应力，然后快速连续破裂，随后是长时间的平静[18]。只有对阿拉木图周围的断层进行详细的古地震研究才能确认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，解开它们的地震历史也是确定它们构成的当前危险的关键。1889年和1911年的地震是有记录以来最大的大陆地震之一。先前的研究表明，这两次事件都破坏了复杂的小断层。详细的断层几何形状，滑动幅度和方向尚未确定的许多这些破裂，并有不确定性的属性特定的断层1889年的事件。一些冲断层的位置和方向出乎意料，出现在山谷的“错误”一侧，在上盘块体中有断层平行的河流（也见于西南方向的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层的命运。还有地貌证据表明，全新世断裂沿着Zailisky山脉前缘断层（图1中的ZRF），该断层界定了山脉的北方边缘，但地震的震级和时间尚不清楚[9]。Grutzner等人（2017年）[9]认为，这一活动断层陡坎的一部分被大阿拉木图运河覆盖-因此，对该断层过去行为的详细调查特别相关。与北方天山相反，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[5]，东界帕米尔，通过左侧达尔瓦兹断层，南界和东南界阿富汗帕米尔和兴都库什[11]（图2）。一组南北向的山脊（背斜和推测的盲冲断层）扫过盆地内部，在凹陷的北方边缘向东弯曲，在那里，逆冲右行的伊利亚克断层带形成了与天山山脉南侧的边界，沿着与Gissar断层系统稍微向北延伸[11，5，17，13]。这些断层上发生地震的可能性对塔吉克斯坦首都杜尚别构成了重大威胁。