Figure 1: Geological setting of northeastern Tibet. (a) The blue stars illustrate the epicenters of 5 earthquakes (M ≥ 5.9, Nos. 7-10, and 12) and 7 earthquakes (M ≥ 7.0, Nos. 1-6 and 11) in and around the Qilian-Haiyuan fault system, respectively, from 1920 to 2022. The blue beach balls illustrate the focal mechanisms of 11 historical earthquakes (Nos. 1-11). The red star and beach ball illustrate the 2022 Mw 6.6 Menyuan earthquake (No. 12) epicenter and focal mechanism, respectively. The earthquake parameters are summarized in Table S1 (Supporting Information Text S1). The black lines represent the active faults. The black and red solid bold lines present the Qilian-Haiyuan fault system and the Tianzhu Seismic Gap in this fault system, respectively. The bold green dashed lines represent the 1920 Mw 8.5 Haiyuan (No. 11) and 2022 Mw 6.6 Menyuan (No. 12 ) earthquakes ruptures. The light green triangles indicate the locations of cities with names. The upper right inset presents the location of the target area in a larger geological setting. (b) It illustrates the fault segments in the Tianzhu Seismic Gap in the area indicated by the white dashed line rectangle in (a). The yellow dots represent epicenters of the earthquakes (M > 3.0) from Jan 8, 2022 to Feb 8, 2021. HLHF: the Hala Lake fault; TLSF: the Tuolaishan fault; LLLF: the Lenglongling fault; JQHF: the Jinqianghe fault; MMSF: the Maomaoshan fault; LHSF: the Laohushan fault; HYF: the Haiyuan fault; NQLF: the north Qilian fault (Deng, 2007).
The 1920 Mw 8.5 Haiyuan ruptured the HYF segment (the eastern portion of the Qilian-Haiyuan fault system) by producing an approximately 230 km long surface rupture and caused more than 230,000 casualties based on the field investigation data (Liu-Zeng et al., 2007, 2015). After experiencing the recent 2022 earthquake and the 1920 Haiyuan earthquake, a seismic gap in the Qilian-Haiyuan fault system was left between these two earthquakes with the length of approximately 260 km, which was named as ”Tianzhu Seismic Gap”, including the LLLF, JQHF, MMSF, and LHSF (Gaudemer et al. 1995). This seismic gap has not experienced an earthquake of M > 7.0 for more than 800 years according to the historical earthquake documents (Gaudemer et al. 1995; Liu-Zeng et al., 2015; Xiong et al., 2018). So far the future seismic hazards of the Tianzhu Seismic Gap has been keeping unclear.
Earthquake interaction has significant influence on regional earthquake activities by increasing or decreasing Coulomb Failure Stress (ΔCFS) on target faults (Reasenberg and Simpson, 1992; Harris, 1998; Stein, 1999). Normally, earthquake activities are encouraged in the areas with increased stress, whereas earthquake activities are prohibited in the areas with decreased stress (Harris, 1998). This method has been successfully used in investigating the relation between main shock and aftershocks (King et al., 1994; Lin and Stein 2004; Toda et al., 2011), large earthquakes (Stein, 1999; King and Cocco, 2001; Stein 2003; Freed, 2005), and volcanoes and earthquakes (Nostro et al., 1998; Lara et al., 2004; Walter et al., 2005; Walter and Amelung, 2007; Eggert and Walter, 2009). Historical earthquake catalogue indicates that several large earthquake (M ≥ 7.0) occurred in and around the Qilian-Haiyuan fault system in northeastern Tibet during the past century. Taken as an example, the 1927 Mw 8.3 Gulang earthquake (No. 11 in Fgure1 ) rupture locates less than several tens of kilometers north to the Qilian-Haiyuan fault system (Deng et al., 2007, Xiao and He et al., 2015). This earthquake might have significant impact on the seismic activity of the Qilian-Haiyuan fault system due to its large magnitude and proximity. However, the stress transferred to the this fault system by those historical large earthquakes, as well as its influence on the 2022 Menyuan earthquake and the seismic activities and hazards of the Tianzhu Seismic Gap has not been well studied.
To investigate those questions above, we simulated the ΔCFS induced by 12 large historical earthquakes with available focal mechanisms in and around the Qilian-Haiyuan fault system during the period from 1920 to 2022 in this study. Space-time stress distributions before and after 2022 along the Qilian-Haiyuan fault system were evaluated to determine its control on the 2022 Menyuan earthquake, as well as their influence on the seismic activities and hazards of the Tianzhu Seismic Gap. This is important for understanding the reasons for earthquake generation and migration in the northeastern Tibet from the perspective of earthquake interaction, and also helpful for hazards prevention and disaster relief for the government of Gansu province, China.