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.