2024年1月23日新疆乌什MS7.1地震动态触发山西左云MS3.0地震的可能性

Possibility of the January 23,2024 Wushi,Xinjiang MS7.1 earthquake dynamically triggering the Zuoyun,Shanxi MS3.0 earthquake

  • 摘要: 北京时间2024年1月23日2时9分4秒新疆阿克苏地区乌什县发生MS7.1地震,3时59分33秒山西大同市左云县发生MS3.0地震,两次地震在发震时间上间隔较短。为了厘清新疆乌什MS7.1地震是否对山西左云MS3.0地震有触发作用,本文选取了山西北部地区四套四分量钻孔应变仪记录的采样率为100 Hz的应变波形数据,从体应力、最大剪应力、动态库仑应力变化和应变震级等进行了定量分析。结果显示:体应力幅值为0.96 kPa,最大剪应力幅值为1.37 kPa,口泉断裂(走向N35°E,倾角50°,正断层)上库仑应力变化接近2.11 kPa。计算得到的动态库仑应力变化值较小,远未达到动态应力触发的阈值(0.01—0.1 MPa),表明1月23日新疆乌什MS7.1地震对山西左云MS3.0地震的触发的可能性较小,山西左云MS3.0地震为正常背景活动起伏的可能性较大。

     

    Abstract: Dynamic stresses associated with crustal surface waves resulted from the main shock are capable of triggering seismicity at remote sites on certain conditions. Stress changes caused by a large earthquake may affect the occurrence of subsequent earthquakes. Therefore, the trend of subsequent earthquakes is a matter of concern. At 02:09:04 on January 23, 2024, the MS7.1 earthquake occurred in Wushi, Aksu Prefecture, Xinjiang Autonomous Region. The depth of the epicenter is 22 km, and the average elevation within 10 km of the epicenter was about 3179 m. Immediately after the earthquake, the China Earthquake Networks Center (CENC) organized experts to analyze the earthquake, their results show that the earthquake was located in the Tianshan seismic zone, and Maidan fault is nearest to the epicenter with a distance of 6 km. Preliminary results of the focal mechanism show that the earthquake provided a thrust-type rupture. At 03:59:33 the MS3.0 earthquake occurred in Zuoyun, Datong City, Shanxi Province. The Shanxi Earthquake Agency activated the Ⅳ emergency response to the background, cause and impact of the Zuoyun MS3.0 earthquake immediately. Due to the short interval between Zuoyun MS3.0 earthquake and the Ms7.1 earthquake in Wushi, Xinjiang, it is easy to make seismologists doubt whether the MS7.1 Wushi earthquake triggered the MS3.0 Zuoyun earthquake. Wushi is located in the western part of the South Tianshan seismic belt, to the east of the Pamir Arc Tectonic Belt, and is situated in the convergence area of three large tectonic units, namely the Tarim, South Tianshan and Pamir, and it belongs to the Kashgar-Wucha seismic tectonic zone of the South Tianshan seismic tectonic belt. The Shanxi rift zone, located in the Trans-North China Orogen (TNCO) of the North China Craton (NCC), is well known for hosting large intraplate earthquakes in Chinese mainland. Both may be affected by stress changes due to uplift of the Tibetan Plateau. To explain the above question, in this paper selected data from four sets of four-component borehole strain gauges with a 100 Hz sampling rate deployed in Youyu station, Yingxian station, Huairen station, and Yangqu station, respectively, of northern Shanxi. We quantitatively calculated volumetric stresses, maximum shear stresses, dynamic Coulomb stress variations as well as strain magnitude by high-pass filtering process, taking into account local elastic modulus parameters. The results show that the amplitude of body stress is 0.96 kPa, the value amplitude of maximum shear stress is 1.37 kPa, and the variation of Coulomb stress on the Kouquan fault is close to 2.11 kPa with N35°E of striking, 50° of dipping, -90° of slipping, which is located in the vicinity of Huairen station and is the northwestern boundary fault of the Datong Basin, and is the main fault near the epicenter of the Zuoyun. Therefore, we can determine the magnitude of the dynamic stress in the Zuoyun area produced by the Xinjiang Wushi MS 7.1 earthquake by calculating the Coulomb stress change on the Kouquan fault. Triggered seismicity in these areas was initiated during the passage of the Love and Rayleigh waves. However, the exact triggering mechanism for earthquakes is still not clear. Coulomb failure models based on a frictional strength threshold offer an explanation for cases of rapid-onset triggered seismicity that develop during the peak dynamics stress caused by surface waves. The computed dynamic Coulomb stress change are small and far from the threshold of dynamic stress triggering (0.01—0.1 MPa), suggesting that the Wushi MS7.1 earthquake’s triggering the Zuoyun MS3.0 earthquake is highly unlikely. Moreover, we counted the activity frequency of earthquakes with magnitude M≥3.0 in the northern Shanxi region statistically in the past ten years, and the maximum and minimum of annual frequency are 8 and 2, with an average of 3.5. Zuoyun MS3.0 earthquake is the first one so far this year, so it is likely a normal background activity undulation. Finally, we discussed the relationship between strain amplitude and strain magnitude, i.e., the strain magnitude can be obtained from the strain amplitude, and vice versa. It is undoubted that an earthquake of MS7.8 or higher in Wushi would bring the change in Coulomb stress on similar faults in the Shanxi region to the dynamic trigger threshold of 10 kPa. The MS7.1 Wushi earthquake could only bring the Coulomb stress on similar faults in the area within about 1121 km from the epicenter to the dynamic triggering threshold 10 kPa. Even if the value of the Coulomb stress change reaches the threshold, an earthquake may not be triggered if the stress state has not already reached the critical edge of the danger. However, directly observed four-component borehole strain data as an important information to study dynamic stress-triggered earthquakes, it will provide a new way to researches on earthquake triggering.

     

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