文中收集了新疆测震台网2009-2018年记录的492个 MS2.5以上的地震事件, 以 MS=3.5为界, 分别用CAP方法和FOCEMEC程序计算其震源机制解, 并收集了GCMT记录的该区域历史地震事件的震源机制解结果。 按照全球应力图分类标准对计算得到的震源机制解数据进行分类, 发现区域内中强地震主要以逆冲型为主, 兼有一定的走滑分量。 采用阻尼区域应力场反演方法获取新疆天山中段的构造应力场空间分布特征, 结果表明研究区内最大主压应力轴由西向东呈扇形旋转, 自西段的NW向逐渐转为NE向, 仰角近水平, 最大主张应力轴近EW向, 仰角近直立。 受喀什河断裂、 那拉提断裂、 博阿断裂、 准噶尔南缘断裂和北轮台断裂等大型断裂带的影响, 局部区域应力场呈现出复杂的多样性。 在帕米尔和塔里木块体持续向N挤压的影响下, 天山整体被挤压缩短, 但由南向北、 由西向东缩短速率逐渐降低, 应力形因子自西向东逐渐增大, 且中间主压应力轴由偏压缩成分转为偏拉张成分。 研究区南侧最大主压应力轴方向为N15°E, 而北侧则为近SN向, 这与塔里木块体的顺时针旋转有直接关联。 区内近期发生的2次 MS6.6地震均造成区域应力场的逆时针旋转, 震后应力场与主震震源机制解趋于一致。
The middle part of the Tianshan Mountains in Xinjiang is located in the north-central part of the Tianshan orogenic belt, between the rigid Tarim Basin and Junggar Basin. It is one of the regions with frequent deformation and strong earthquake activities. In this paper, 492 MS>2.5 earthquake events recorded by Xinjiang seismograph network from 2009 to 2018 were collected. The MS3.5 earthquake was taken as the boundary, the focal mechanism solutions of the earthquake events in this region were calculated by CAP method and FOCEMEC method respectively. At the same time the focal mechanism solutions of GCMT recorded historical earthquake events in this region were also collected. According to the global stress map classification standard, the moderate-strong earthquakes in the region are mainly dominated by thrust with a certain slip component, which are distributed near the combined belts of the Tarim Basin, Junggar Basin, Turpan Basin and Yili Basin with Tianshan Mountains. The thrust component decreases from south to north, while the strike-slip component increases. The spatial distribution characteristics of the tectonic stress field in the middle section of the Tianshan Mountains in Xinjiang are obtained by using the damped regional-scale stress field inversion method. The maximum principal compressive stress in axis the study area rotated in a fan shape from west to east, the NW direction in the western section gradually shifted to NE direction, its elevation angle is nearly horizontal, in the state of near horizontal compression. The minimum principal compressive stress axis is nearly EW, and the elevation angle is nearly vertical. Influenced by large fault zones such as Kashi River, Bolhinur, Nalati, Fukang, the southern margin of the Junggar and the north Beiluntai, the local regional stress field presents complex diversity. Under the influence of the northward extrusion of Pamir and Tarim blocks, the whole Tianshan is shortened by compression, but its shortening rate decreases from south to north and from west to east, the stress shape factor increases gradually from west to east, the intermediate principal compressive stress axis exhibits a change in compression to extension. There are some differences in the characteristics of tectonic stress field between the north and south of Tianshan Mountains. The regional maximum principal compressive stress axis is 15° north by east on the south side, while it is nearly NS on the north side. The deformation of the Tianshan Mountains and the two basins on both sides is obviously larger than that in the inside of the mountain. Changes in the crustal shortening rate caused by the rotation of the rigid Tarim block and Junggar block to the relatively soft Tianshan block, as well as the uplifts of Borokonu and Bogda Mountains, the comprehensive influence of the material westward expansion constitute the stress field distribution characteristics of the north and south sides of the middle section of Tianshan Mountains. The recent two MS6.6 earthquakes in the region caused the regional stress field to rotate counterclockwise. The post-earthquake stress field and the main source focal mechanism solution tend to be consistent. The seismic activity in the study area is week in the south and strong in the north. The focal depth is about 20km. Most strike-slip earthquakes occur near the junction belt of the Tianshan and Junggar Basin.
新疆天山中段位于天山造山带的中北部, 展布于刚性的塔里木盆地和准噶尔盆地之间。 受印度大陆和欧亚大陆碰撞远程效应的影响, 区内分布着数条近EW向的逆冲-褶皱活动构造带, 是大陆内部变形和强震活动频繁的地区之一。 该区历史上曾发生11次6.5级以上强震, 其中7级以上地震4次, 分别为1716年特克斯7.5级地震、 1786年霍城7.5级地震、 1906年玛纳斯7.7级地震和1994年乌苏7.2级地震。 该区较强的地震活动性为研究盆山地区的结构及活动大陆构造学提供了丰富的数字地震资料, 一直以来都是国内外地震研究学者重点关注的地区之一。
本研究选取新疆天山中段2009-2019年记录的MS2.5以上地震事件492个, 其中2.5~3.5级地震353个, 3.5~4.5级地震116个, 4.5级以上地震23个(图 1)。 根据新疆天山中段的测震台站布局和地震事件信噪比的约束, 由P波初动和振幅比解算MS2.5~3.4地震的震源机制解, MS> 3.5地震事件的震源机制解由CAP方法计算。 此外, 本文搜集了全球矩心矩张量目录(GCMT)研究区范围内1977-2008年的震源机制解数据12个, 共同参与研究区构造应力场的解算。
为研究新疆天山中段构造应力场的分布特征, 图 4 给出了区域内504个地震震源机制解的主压应力P轴和张应力T轴的水平投影分布。 P轴方位整体近SN向, 倾角较小, 由于地震事件的震级跨度范围较大, 区域构造应力场对部分中小地震事件的发震模式控制较弱, 导致P轴的方位分布较乱, 不能准确分析其应力轴的具体方位。 T轴的方位总体呈近EW向, 倾角较大, 个别小地震受区域断层的控制, 方向发生了一定的偏转, 但整体以SN向挤压应力为主。
从应力形因子分析, 研究区R值均约0.5, 自西向东由小到大变化, 无明显的极大值和极小值。 反演构造应力场时断层面的选取对应力方位影响较小, 但会对应力形因子产生显著影响。 表2给出本研究应力形因子的统计分析结果, 研究区内R值的不确定度集中在[-0.25, 0.25]的置信区间内, 个别区域的置信区间较大, 不确定度并无明显随空间变化的规律。 研究区西段(T2)较小的应力形因子表明, 这里主压应力值与中间应力值较为接近, 使得EW向的扩展量较小, 主要表现为物质的隆升; 天山中东段(T1)R> 0.5, 表明这里主张应力值和中间应力值较为接近, 中间应力轴有一定拉张成分, 以EW向扩展为主, 走滑型地震事件的数量增多。
总体而言, 研究区内近2次MS6.6地震震中附近的最大主压应力方位变化显著, 强震的发生均使原有的应力场发生了逆时针偏转, 但未能改变区域内主应力方位整体呈近SN向的趋势。 强震震源类型对区域应力场的应力结构有显著影响。 2次强震既反映了其震源区及其附近的受力状态, 也显示了大区域范围内不同断层构造受控于同一动力源的力学作用。
震源机制解是研究区域构造应力场特征的主要基础资料。 本研究收集并计算了新疆天山中段504个地震的震源机制解数据, 采用区域阻尼构造应力场反演程序分析了新疆天山中段现今地壳应力状态的总体特征与分区域特征, 主要得到以下结论:
(1)新疆天山中段的震源机制解类型主要以逆冲型为主, 震源深度约20km; 走滑型地震主要分布在天山与准噶尔盆地交会的区域, 反映了在帕米尔和塔里木块体向N推挤的过程中, 作用力不断减弱, 且由于准噶尔盆地的阻挡作用, 研究区北侧走滑型地震事件增多, 区内近期发生的中强地震的震源深度普遍约20km。
(2)构造应力场反演结果显示, 新疆天山中段最大主压应力场方向整体近SN向, 大致由西向东呈扇形旋转, 由西段的NW向逐渐顺时针转为NE向, 最大主压应力轴的倾角均近水平。 受喀什河断裂、 那拉提断裂、 博阿断裂、 准噶尔南缘断裂和北轮台断裂等大型活动断裂带影响, 局部区域最大主压应力轴方向呈现一定的多样性。 此外, 哈萨克地台对天山俯冲的影响也不可忽视。 受塔里木块体顺时针旋转的影响, 地壳缩短速率自西向东递减, 应力形因子自西向东由小到大, 中间主压应力由压缩转为扩张分量, 并在乌鲁木齐坳陷区受博格达隆起的影响形成突变。 区域内近期发生的2次强震均造成了区域应力场的逆时针旋转, 震后区域应力场状态与主震震源机制解趋于一致。
(3)新疆天山中段南北两侧构造应力场的分布特征具有一定的差别, 南侧最大主压应力轴为NNE向, 北侧则为近SN向。 区域变形特征表明天山内部的构造应变较均匀, 而天山与南北两侧盆山结合带的变形明显大于山体内部。 刚性的塔里木块体和准噶尔块体相对软弱的天山发生旋转导致地壳缩短速率发生变化, 以及博罗科努山和博格达山的隆起使物质向W扩展的综合影响构成了新疆天山中段南北侧的应力场分布特征。