2017年M_W6.5九寨沟地震激发的同震库仑应力变化及其对周边断层的影响

Coseismic Coulomb stress changes associated with the 2017 M_W6.5 Jiuzhaigou earthquake(China) and its impacts on surrounding major faults

2017年8月8日21时19分在我国四川省北部阿坝州九寨沟县发生了M_W6.5左旋走滑型地震.该地震发生在青藏高原巴颜喀拉块体东北缘,东昆仑断裂南东段的塔藏断裂、岷江断裂和虎牙断裂的交汇地带.包括此次地震,近年来在巴颜喀拉块体周缘已发生了九次6级以上强震,表明巴颜喀拉块体周缘主要活动断裂上的应力水平仍处于不断调整之中.本文采用库仑应力模型研究2017年M_W6.5九寨沟地震激发的库仑应力变化、该地震与周边地震的应力触发关系以及强震对周边主要活动断裂的应力扰动.强震序列包括周边区域1536—1975年M≥6历史强震和1976—2017年的M_W≥6 gCMT地震目录中的强震,共计32个.研究结果表明:(1)2017年M_W6.5九寨沟地震激发的同震库仑应力变化仅在局部范围内超过0.1×10~5Pa,且75%的余震(～12.7天)受到该地震明显的同震应力触发作用,而其余25%的余震落在应力影区,采用最优破裂面可以进一步提高同震库仑应力变化与余震分布的空间相关性;(2)2008年M_W7.9汶川地震对2017年M_W6.5九寨沟地震的发生有一定的促进作用,在后者震源处激发的同震库仑应力变化为(0.026～0.263)×10~5Pa,震后黏弹性库仑应力变化为(0.010～0.032)×10~5Pa.该库仑应力的变化范围取决于汶川地震源断层参数和九寨沟地震接收断层参数.2013年M_W6.6芦山地震对九寨沟地震的发生几乎没有影响(<0.001×10~5Pa);(3)1654年M8.0甘肃天水南地震对九寨沟地震的发生有明显的促进作用,在九寨沟地震震源处激发的同震库仑应力变化为(0.410～1.266)×10~5Pa,震后库仑应力变化为(0.147～0.490)×10~5Pa.1879年M8.0甘肃武都地震可能有比1654年M8.0甘肃天水南地震更强的应力触发作用,但也有可能对九寨沟地震的发生起到抑制作用.在选取的8个九寨沟地震接收断层面上,其中6个接收断层面上该地震所激发的同震库仑应力变化为(0.913～2.364)×10~5Pa,2个接收断层面上该地震所激发的同震库仑应力变化为(-1.326～-0.454)×10~5Pa;在4个接收断层面上震后库仑应力变化为(0.094～1.072)×10~5Pa,在另外4个接收断层面上震后库仑应力变化为(-1.593～-0.106)×10~5Pa.1933年四川叠溪地震对九寨沟地震的发生影响较弱,其所激发的同震库仑应力变化为(0.015～0.080)×10~5Pa,震后库仑应力变化为(-0.029～0.025)×10~5Pa;(4)九寨沟地震仅在其附近的岷江断裂北段、塔藏断裂和虎牙断裂南段造成较明显的同震库仑应力变化,其分别为0.09×10~5Pa、(0.14～2.03)×10~5Pa和0.25×10~5 Pa.而进一步顾及其余31个强震的库仑应力作用则发现,同震库仑应力增加非常显著的主要活动断裂分段为:岷江断裂北段南侧和岷江断裂南段的库仑应力变化分别升高5.6×10~5Pa和9.8×10~5Pa.鲜水河断裂北段南侧库仑应力升高23.0×10~5Pa,鲜水河断裂南段道孚—康定段的北侧库仑应力升高9.0×10~5Pa,而最南端库仑应力升高3.0×10~5Pa;龙门山断裂带中段的北侧库仑应力变化为(6.1～7.4)×10~5 Pa,中段库仑应力增加(2.1～11.5)×10~5Pa;西秦岭北缘断裂东段库仑应力变化为4.4×10~5 Pa;龙日坝断裂北段最北侧的库仑应力变化为2.0×10~5Pa;小金河断裂北段库仑应力变化为1.7×10~5Pa;安宁河断裂北段库仑应力变化为1.6×10~5Pa;(5)由于下地壳和上地幔的黏弹性松弛作用,所有强震在九寨沟地震震后20年造成的黏弹性库仑应力变化在鲜水河断裂、龙门山断裂中段、塔藏断裂以及秦岭南缘断裂西段比较显著,其分别为:(1.0～3.0)×10~5Pa、2.8×10~5Pa、(2.3～2.7)×10~5Pa和0.9×10~5Pa.但总体上黏弹性库仑应力变化没有改变各断裂上的同震库仑应力变化空间分布.总的库仑应力变化在鲜水河断裂北段南侧和南段的道孚至康定段北侧、龙门山断裂中段北侧、岷江断裂南段和北段南侧、虎牙断裂、塔藏断裂以及西秦岭北缘东段很显著(均超过4×10~5Pa).由于库仑应力明显升高可能预示着地震潜在危险性增强,因此这些断裂分段可能将来需要重点加以关注.

At 21：19 PM, 8 August 2017 （local time） a MW6.5 sinistral strike-slip earthquake （hereinafter termed the Jiuzhaigou earthquake） struck at the Jiuzhaigou county, Sichuan province, China. This strong earthquake occurred in the proximity of the northeast rim of the Bayan Har block on the Tibetan Plateau. Locally, it took place at the confluence zone of the Tazang fault to the north that is the southeastern segment of the NW Eastern Kunlun fault, the NS Minjiang fault to the west and the NW Huya fault to the south. Apart from the Jiuzhaigou earthquake, over the past twenty years there have occurred eight MW≥6 strong earthquakes along the periphery of the Bayan Har block, suggesting that stress levels on major active faults bounding the Bayan Har block have been being adjusted profoundly. In this paper, we use the Coulomb failure model to explore the Coulomb stress triggering effects in terms of mainshock-aftershock triggering, mainshock-mainshock triggering, and mainshock-fault interplay, so as to understand the relations between the Jiuzhaigou earthquake and its aftershocks, whether the Jiuzhaigou earthquake was triggered or promoted by the 2008 MW7.9 Wenchuan earthquake or 2013 MW6.6 Lushan earthquake and to what extent the surrounding major faults were loaded or unloaded. We consider additional 31 surrounding MW≥6 earthquakes comprising 19 historic earthquakes from 1536 to 1975 and 12 other MW≥6 earthquakes from the gCMT catalog from 1976 to 2017. Our results indicate that： （1） The Jiuzhaigou earthquake aroused significant Coulomb stress changes （larger than 0.1×105Pa） merely in a local region. In addition, 75% of the total 1867 relocated aftershocks with relocation precision less than 0.7 km during the approximately 12.7 days lay in stress triggering zones on a profile where the source fault is included, whereas the remaining 25% still took place in stress shadows. The inconsistency between the spatial patterns of these aftershocks and coseismic Coulomb stress changes can be reconciled using the 3D optimally oriented failure planes （OOPs） on which Coulomb stress changes are resolved. Because of more freedom being allowed for the OOPs, most aftershocks can lie in stress triggering zones; （2） The 2008 MW7.9 Wenchuan earthquake somewhat promoted the Jiuzhaigou earthquake by imposing positive coseismic Coulomb stress changes ranging from 0.026×105Pa to 0.263×105Pa and postseismic Coulomb stress changes being in the range of （0.010~0.032）×105Pa at its hypocenter, depending on the source fault of the Wenchuan earthquake and the receiver fault of the Jiuzhaigou earthquake. On the other hand, the 2013 MW6.6 Lushan earthquake has negligible triggering effect on the Jiuzhaigou earthquake （〈0.001×105Pa）; （3） The 1654 M8.0 South Tianshui （Gansu） earthquake significantly enhanced the occurrence of the Jiuzhaigou earthquake by increasing coseismic Coulomb stress changes ranging from （0.410~1.266）×105Pa and postseismic Coulomb stress changes ranging from （0.147~0.490）×105Pa. The triggering effect of the 1879 M8.0 Wudu （Gansu） earthquake is unclear, because on two of the eight receiver faults of the Jiuzhaigou earthquake it imposed negative coseismic Coulomb stress changes ranging from （-1.326^-0.454）×105Pa but on the other six receiver faults it imposed coseismic Coulomb stress changes ranging from （0.913~2.364）×105Pa, quite larger than those for the 1654 M8.0 earthquake. Likewise, when considering four of the eight receiver faults the postseismic Coulomb stress changes accrue by 0.094×105Pa to 1.072×105Pa, whereas the postseismic Coulomb stress changes decrease by-1.593×105Pa to-0.106×105Pa when considering other four receiver faults. The 1933 Diexi earthquake in Sichuan province has little effect on the occurrence of the Jiuzhaigou earthquake because this earthquake imparted coseismic Coulomb stress changes of merely （0.015~0.080）×105Pa and postseismic Coulomb stress changes of （-0.029~0.025）×105Pa; （4） The Jiuzhaigou earthquake alone just significantly loaded the northern segment of the Minjiang fault, the Tazang fault and the southern segment of the Huya fault by 0.09×105Pa, （0.14~2.03）×105Pa and 0.25×105Pa on average, respectively. On the other hand, when surrounding additional 31 M≥6 earthquakes are accounted for, there are many segments of the major active faults that were heavily loaded. The southernmost portion of the north Minjiang fault and the south Minjiang fault were raised by 5.6×105Pa and 9.8×105Pa, respectively. The southernmost portion of the Xianshuihe （N） fault was promoted by 23.0×105Pa. The northern portion of the Xianshuihe （S） fault was loaded by a coseismic Coulomb stress increment of 9.0×105Pa, and at its southernmost termination the coseismic Coulomb stress change was enhanced by 3.0×105Pa. The northern portion of the middle segment of the Longmenshan fault received （6.1~7.4）×105Pa on average and some portions in the middle segment were enhanced by （2.1~11.5）×105Pa. The eastern portion of the West Qinling Northern Frontal （East） fault was loaded by 4.4×105Pa. The northernmost portion of the northern segment of the Longriba fault, the northern portion of the Xiaojinhe fault and the northern portion of the Anninghe fault were loaded by positive Coulomb stress changes of 2.0×105Pa, 1.7×105Pa and 1.6×105Pa, respectively; （5） Owing to the postseismic relaxation of the viscous lower crust and upper mantle, postseismic Coulomb stress changes associated with all of the large earthquakes after 20 years since the Jiuzhaigou earthquake, heavily loaded the Xianshuihe fault, the middle segment of the Longmenshan fault, Tazang fault and the western segment of the Qinling Southern Frontal （East） by （1.0~3.0）×105Pa, 2.8×105Pa, （2.3~2.7）×105Pa and 0.9×105Pa, respectively. However, the spatial patterns of coseismic Coulomb stress changes induced by all of the large earthquakes are not dramatically altered as a result of the postseismic relaxation effect. The compounded Coulomb stress map exhibits that some faults are seriously imposed by Coulomb stress changes larger than 4×105Pa, including northern portion of the Daofu-Kangding segment of the Xianshuihe fault （S） and the southern portion of the Xianshuihe fault （N）, the northern portion of the middle segment of the Longmenshan fault, the southernmost portion of the north Minjiang fault and the whole south Minjiang fault, the Huya fault, the Tazang fault and the eastern segment of the West Qinling Northern Frontal. Accordingly, the aforementioned portions should be paid much more attention to in the near future because of likely much elevated likelihood of large earthquakes at these loci.