新疆及周缘构造破裂特征及地震序列类型

横亘新疆境内的天山及其周边的西昆仑、阿尔金和阿尔泰是中国大陆著名的强构造运动区和地震活动带。在对新疆构造区应力环境、动力过程、断层运动变形特征和地震序列分析讨论的基础上,对新疆及其周缘主要构造区地震破裂方式和序列类型进行研究,得出如下结论:(1)西昆仑构造区受来自青藏块体和塔里木块体NS和NW向水平压应力和垂向力的作用,构造运动呈现出走滑与逆冲特征,震源破裂以走滑型为主,数量较少的逆断型地震主要分布在西昆仑帕米尔一侧的深震挤压区,正断型地震主要出现在西昆仑与阿尔金交汇的拉张盆地及附近。该区主余型地震占63%,6级以上地震序列也存在多震类型。(2)阿尔金断裂带位于西昆仑北缘断裂和北祁连断裂过渡带,受青藏块体向北和向西的推挤,断裂本身的左旋位移量通过两端逆冲挤压而转化,使得青藏高原北边界不断向外扩展。在此力源下,阿尔金断裂带震源破裂以走滑为主,也有少量的逆冲型地震。地震序列中主余型和孤立型地震占比相同(占44%)。(3)在印度板块和亚欧大陆碰撞效应影响下,天山地区产生近NNE向水平压应力,构造运动显现出带旋性特征的逆冲和走滑,震源破裂方式与之相吻合。而天山构造大跨度的空间展布、扩展形式的多样性和地震破裂的两重性,又影响到地震序列类型的多样性,使得主余型、孤立型和多震型地震在不同构造部位呈现优势分布。(4)阿尔泰的构造运动可能受到了来自印度板块与亚欧板块碰撞的远程效应和西伯利亚块体南向运动的双向影响,形成NNE和SW向水平挤压力,主要大型发震断裂做右旋剪扭错动,而一些深断裂则以逆冲运动为主。震源破裂呈现出走滑(占64%)和部分的逆冲(占27%),6级以上地震序列主要为主余型,5级左右地震则多为孤立型。

Tectonic Rupture Characteristics and Earthquake Sequence Types in Xinjiang and Its Adjacent Areas

In this study, we analyzed the characteristics of different types of earthquakes occurring in the north rim of the Tibetan Plateau, Tianshan, and Altai in view of regional geodynamic background, focal mechanisms, and research results of seismic sequences of the Xinjiang area considering the effect of earthquake rupture and sequences.West Kunlun is located in the eastern wing of Pamir arc, with a tectonic style of extrusion conversion system consisting of both strike-slip and thrust structures, thus indicating upliftment of West Kunlun in the Cenozoic.In the area, earthquakes have a combination of characteristics of strike-slip and major-after earthquakes, with the dominant strike-slip characteristics accounting for 60% of the sequences, and major-after characteristics accounting for 63% of sequences. Earthquakes with magnitudes greater than 6 are primarily major-after and individual multiple-shock type earthquakes. Seismic activity characteristics of major-after earthquakes indicate that the stress state of West Kunlun is strong and the rupture strength is medium.The Arkin fault has been pushed to the north and west by the Qingzang Block. The escape wedge formed by the Arkin and Karakoram faults causes the thrust of northern West Kunlun and North Qilian faults to gradually transit through the Arkin fault zone. On the other hand, the sinistral displacement of the Arkin fault transforms through both-end thrust extrusion, resulting in an arc bending at the eastern and western end of the West Kunlun and North Qilian. Therefore, the northern border of the whole plateau extends outward. Similarly, earthquake sequence types in the Arkin fault are characterized by strike-slip, major-after, and isolated earthquakes. Strike-slip ruptures are dominant in the area, and major-after and isolated earthquakes each account for 44% of the seismic sequence. Isolated earthquakes typically have a magnitude below 6.5. The 7.3 magnitude Yutian earthquake that occurred in 2014 conformed to the characteristics of this type of earthquake.Owing to the effect of India-Asia continental collision and its effect on the later continental convergence distance, Tianshan shows complete vertical crust shortening under the horizontal compressive, oblique shear transform deformation, and lateral deformation expanding on both sides of the basin. Active faults include reverse and strike-slip faults with rotating characteristics. Seismic rupture mode of Tianshan is complex, with strike-slip and thrust earthquakes being dominant (46% and 38%, respectively). Contact ways of tectonics in different directions and movement characteristics add to the complexity and diversity of seismic sequence types in the Tianshan region. Major-after earthquake sequences account for 50% of seismic sequences, whereas isolated earthquake sequences account for approximately 40% of sequences. In individual regions such as the western part of south Tianshan, tectonics crisscross with strong movement, and the earthquake sequence shows multiple aftershocks.The Altai fault exhibits dextral shear dislocation under NNE and SW horizontal extrusion, and some of the deep fractures experienced significant thrust movement with obvious fracture topography. Strike-slip, thrust and major-after, and isolated combined earthquake seismic types are formed under different conditions such as regional tectonic stress and fault movement. Isolated earthquakes are primarily medium earthquakes with a magnitude of approximately 5.