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.