4.2. Future hazards on the Tianzhu Seismic Gap
Our results demonstrate strong stress contrast among different fault segments in the Tianzhu Seismic Gap (Figure 5) after 2022, which was mainly controlled by five historical earthquakes: the 1990 Mw 6.2 Tianzhu (No. 7), 2016 Mw 5.9 Menyuan (No. 9), 1920 Mw 8.5 Haiyuan (No. 10), 1927 Mw 8.3 Gulang (No. 11), and 2022 Mw 6.6 Menyuan (No.12) earthquakes.
Stress increase was observed in the segment (with the length of approximately 18 km) between the 2016 Mw 5.9 (No. 9) and 2022 Mw 6.6 (No. 12) Menyuan earthquakes with the peak ΔCFS value of approximately 375.15 kPa in the western portion of the Tianzhu Seismic Gap. This stress value (375.15 kPa) is much higher than the typical stress threshold of 10 kPa (Reasenberg and Simpson, 1992), indicating its increased seismic hazards.
The eastern portion of the Tianzhu Seismic Gap (starting from the middle of the JQHF to the east end of the LHSF with the length of approximately 149 km) was also stress loaded due to the 1920 Mw 8.5 Haiyuan (No. 10) and 1927 Mw 8.3 Gulang (No. 11) earthquakes (Figures 4 and 5). The peak value of the increased ΔCFS in the LHSF (with the length of approximately 60 km), MMSF (with the length of approximately 54 km), and eastern portion (with the length of approximately 35 km) of the JQHF were approximate 786.68 kPa, 1035 kPa, and 216.7 kPa, respectively. These stress values are much higher than the typical stress threshold of 10 kPa (Reasenberg and Simpson, 1992), suggesting increased seismic hazards potential in the eastern Tianzhu Seismic Gap. Our stress results are sensitive to the model parameters. By considering the uncertainty from all cases by model parameterization (see Section 4.1), we found that the maximum ΔCFS increase after 2022 ranged from 516.55 kPa to 1755 kPa, from 419.86 kPa to 3359.4 kPa, and from 10.66 kPa to 655.50 kPa, in the LHSF, MMSF, and eastern JQHF, respectively. Compared with a typical stress release of 1000–2000 kPa for small and medium earthquake and approximately 5000 kPa for large earthquakes (Zielke and Arrowsmith 2008), the loaded stress with these values are large enough to bring those fault segments close to their failure criteria. Although the 1990 Mw 6.2 Tianzhu earthquake did occurred in the west of the LHSF, this earthquake was not large enough to fully release the accumulated energy in the LHSF due to its relatively low magnitude (Mw 6.2) and limited rupture length of f approximately 20 (Liu-Zeng et al., 2007). If the whole eastern portion of the Tianzhu Seismic Gap (with the length of approximately 149 km, including the eastern JQFH, MMSF, and LSHF) ruptures, there might be a future earthquake with a magnitude greater than 7.4, thus releasing accumulated energy at a fault slip rate close to approximately 4–6 mm/a over the past centuries (Wang et al., 2020). Furthermore, paleoearthquake investigations have suggested that 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; Wang et al. 2017; Xiong et al., 2018). Subsequently, more attention should be paid to the east portion of the Tianzhu Seismic Gap.
Note that these two stress increased zones in the Tianzhu Seismic Gap mentioned above are separated by a stress shadow zone (starting from the east of the 2016 Mw 5.9 (No. 8) to the middle of the JQHF) with the maximum ΔCFS decrease of -396.24 kPa. This stress shadow with relatively large negative ΔCFS value may act as a stress barrier to prohibit the future earthquake in this fault segment to some extent from the perspective of fault interaction (Mallman and Parsons, 2008). By preventing the whole Tianzhu Seismic Gap being ruptured in one event, it may consequently decrease the possibility of generating a future large earthquake of magnitude more than Mw 7.7 in Tianzhu Seismic Gap. The role of stress shadows on limiting earthquake rupture extent is not unique in the Tianzhu Seismic Gap of the Qilian-Haiyuan fault system. This phenomenon was also previously observed in some other earthquake zones, such as the northeast unilateral rupture of the 2008 Mw 7.9 Wenchuan earthquake in eastern Tibet (Liu et al., 2018, 2020), and the limitation of the 2018 Mw 7.5 Palu earthquake rupture extent in the Palu-Koro fault in Central Sulawesi, Indonesia (Liu et al., 2021).
Our findings of stress shadow in the middle of the Tianzhu Seimic Gap are congruent with those of Xiong et al. (2018) who calculated the ΔCFS of the Tianzhu Seismic Gap caused by 5 historical earthquakes (M>7.0) around the Qilain-Haiyuan fault system. However, the contribution by each historical event to the stress change on different fault segments in the Tianzhu Seismic Gap was not clear by Xiong et al. (2018). By comparing the stress change individually by 12 historical events (Nos. 1-12) (Figures 2 and 3), we found that the stress shadow zone in the eastern LLLF and the western JQHF are mainly controlled the 1927 Mw 8.3 Gulang (No. 11) and 2016 Mw 6.6 Menyuan (No. 9) earthquake. Note that the maximum ΔCFS decrease of -396.24 kPa in this stress shadow zone suggested in this study is much lower than the estimates of -190 kPa by Xiong et al. (2018). Our results show that the 2016 Mw 6.6 Menyuan (No. 9) earthquake had the major contribution to the stress show by decreasing ΔCFS with the peak value of -335 kPa on the LLLF. However, this contribution by the 2016 Mw 6.6 Menyuan earthquake (No. 9) was neglected by Xiong et al. (2018), since this earthquake was excluded in the earthquake catalogue use in his ΔCFS calculation. This is the reason for why the maximum ΔCFS decrease of -396.24 kPa in the eastern LLLF estimated in this study is much lower than that of -190 kPa by Xiong et al. (2018). It also indicates that it is important to use a complete earthquake catalogue when estimating the seismic hazards in the Tianzhu Seismic Gap by calculating earthquake-induced stress change.