(Seismological Bureau of Sichuan Province, Chengdu 6 1004 1)
The 900-kilometer-long main strike-slip active fault zone in western Sichuan shows strong sinistral faulting, which is one of the main seismogenic zones in southwest China. In this paper, the seismic potential of this fault zone is quantitatively evaluated by combining geological and historical seismic data. The average slip rate of the fault is recalculated or estimated, and the zone is divided into 16 segments according to the geometric shape of the fault and the time-space image of historical earthquake rupture. According to the estimated coseismic slip, historical and prehistoric earthquake time data, the author estimates the average recurrence time of each fault segment by using time prediction and updating model. Furthermore, the probability of segment rupture earthquake in the future is calculated by using the time-varying probability risk assessment model. The main results are as follows: ① By 2026, six of the 16 fault segments have high cumulative earthquake probability (> 0.45), and all these six fault segments are located in the vacant segment along the fault zone that has not been broken for at least 100 years; ② Because the average recurrence time of most of these six segments is long, or the time of death is short relative to the average recurrence time, not all of them are high probability occurrence conditions in the next 30 years (1996 ~ 2026); ③ The comparison of seismic probability of different sections shows that Ganning-Kangding (section 8-1 1) and Shimian-Xichang (sections 14 and 15) should belong to the relative danger zone of this fault zone in the future.
Keywords earthquake potential active fault; Western Sichuan
1 Introduction
The fault zone studied in this paper runs through the whole western Sichuan from northwest to southeast (figure 1), with a total length of about 900km. It consists of four faults, namely Ganzi-Yushu fault, Xianshuihe fault, Anninghe fault and Zemuhe fault. Since the late Quaternary, these faults have shown strong left-lateral strike-slip movement, and then the average slip rate and its standard deviation have been recalculated or estimated. Fig. 3 shows the recalculated or estimated average slip rate. In Figure 3, reliable geomorphologic dislocation and sediment dating data can be obtained at nine locations, so the calculated average slip rate and its standard deviation can be obtained. The average slip rate and its uncertainty range (figures in brackets) of the other three positions in Figure 3 are the results of reasonable speculation.
The map 1 index map of main strike-slip fault zones in western Sichuan shows the regional relationship between the studied fault zone and other main active faults in Chinese mainland.
Figure 3 shows that the slip rate of Ganzi-Yushu fault and Xianshuihe fault is relatively high, reaching 10 ~ 14 mm/a, while the slip rate of Anninghe and Zemuhe faults is only 5.5 ~ 6.5 mm/a.. As can be seen from Figure 1, there are many secondary branch faults around the Anninghe and Zemuhe faults. A possible reasonable explanation is that this secondary branch fault disperses the horizontal movement of the fault block, thus reducing the sliding rate of the main faults along the Anning River and Zemuhe River.
Fig. 2 block diagram of technical route for quantitative evaluation of seismic risk of active faults by segments.
Temporal and spatial images of historical earthquakes and their rupture
For this fault zone, except for two parts, there are historical seismic data in other parts. A set of plans in Figure 4 depicts the spatial distribution of historical sources in five periods from the early18th century to the present. The scale of each earthquake source is defined according to the scope of the severely damaged area during the earthquake.
In recent 250 years, there have been 2-3 historical earthquakes in the Luhuo-Daofu part of the fault zone (Figure 4), which is the part with the highest slip rate (13 ~ 14 mm/a) in the whole fault zone (Figure 3). Therefore, the higher the slip rate along the fault zone, the higher the recurrence rate of the earthquake.
If the focal length is taken as the corresponding rupture length and these rupture lengths are taken as a function of time, the spatio-temporal image of historical earthquake rupture can be obtained (Figure 5). Figure 5 illustrates:
(1) There is a time-space domain near Manigango in this fault zone, and there is no earthquake record in the literature, which means that for this fault zone, except for an earthquake (which occurred around 1506), the age is determined according to rough archaeological methods:
Proceedings of the 30th International Geological Congress Volume 5 Seismic Geology of Modern Lithospheric Movement
Where μ (=-0.0 1) is the average value of the distribution. The total uncertainty σN consists of two parts: data uncertainty σd and internal uncertainty σ1in the recurrence period;
Proceedings of the 30th International Geological Congress Volume 5 Seismic Geology of Modern Lithospheric Movement
The data uncertainty σd comes from the uncertainty of estimating the average recurrence interval Tm, and the inherent uncertainty σ 1(=0.2 1) comes from the above general distribution.
7 magnitude estimation of future rupture earthquakes
For strike-slip fault segments, the magnitude of future rupture earthquakes is roughly estimated by a set of selected empirical relations:
Proceedings of the 30th International Geological Congress Volume 5 Seismic Geology of Modern Lithospheric Movement
Assuming that a future characteristic event will rupture a fault segment with a length corresponding to L, four estimated values of the magnitude of this event can be obtained from the above four formulas. Take the average of these estimated values as the best estimated value of future earthquake magnitude.
The Zhuwo section numbered S4 is the only non-strike-slip fault section among all 16 fault sections (see Figure 6), which is located in the southeast edge of Ganzi Laya District. 1967 [1, 18] When an earthquake with the magnitude of 6.8 occurred, the fault section showed a normal fault in NE direction. Therefore, the future magnitude of this section is estimated by the global normal fault seismic relationship [1 1]:
Proceedings of the 30th International Geological Congress Volume 5 Seismic Geology of Modern Lithospheric Movement
Analysis of Eight Probability Earthquake Potential
Table 2 lists the calculated probability and predicted characteristic magnitude of future rupture earthquakes. Because of the adoption of two recursive models with predictable time and renewable time, the probability values obtained are slightly different. Therefore, the average of the probability values obtained by these two models is taken as the final result.
Fig. 7 shows the calculated probability. As can be seen from Figure 7, by 2026, there will be six high fault segments with cumulative probability greater than or equal to 0.45. These six segments are all in the positions of seismic gaps identified according to the spatio-temporal images of historical rupture (see Figure 5 and Figure 7). However, not all these six segments have the conditions of high probability in the next 30 years. In fact, if the recurrence time from a fault segment to the next earthquake is very long, for example, more than 300 years, then in a relatively short time interval, for example, DT=30 years, the calculated conditional probability will not be very high, no matter whether the death time since the last earthquake is long or short. This is somewhat different from the tectonic environment of plate boundary. The high activity of plate boundary faults makes the average recurrence interval of large earthquakes or giant earthquakes often only a few decades or between 100 and 200 years [8].
In the intraplate environment of Chinese mainland, it may be better to use not only conditional probability but also cumulative probability to analyze the long-term seismic potential of fault profile. For example, the conditional probability of S2 and S6 in the next 30 years is the same (Pc=0. 16), but the cumulative probability of S2 (by 2026, F=0.7 1) is much higher than that of S6 (by 2026, F=0. 19), so that.
Table 2 calculates the conditional probability PE (1996 ~ 2026) and the cumulative probability f (up to 2026) of the 16 fault section.
According to the comparison of probability values of different fault segments, from the viewpoint of long-term prediction, the author puts forward two areas (see Figure 6 and Figure 7), which should be considered as the main danger areas in the next 30 years. The former includes fault section S8-S 1 1, while the latter includes fault sections S 14 and S 16.
9 discussion
This paper evaluates the seismic potential of the main strike-slip active fault zones in western Sichuan. Here I think the following points should be emphasized:
(1) This study is only a preliminary effort, and the results are obviously uncertain, mainly due to the uncertainty of geological data. These geological data include fault slip rate, coseismic average slip, paleoseismic dating and the time of several paragraphs.
(2) The uncertainty of the result is also caused by the uncertainty of the model. The general recurrence time distribution of characteristic earthquakes [7] is established for the seismic data at the plate edge, and it is still a question whether this distribution is suitable for the intraplate tectonic environment like Chinese mainland. In the absence of other options, the result obtained by using this model is only an approximation.
(3) No matter how rough the results of this paper are, they are still useful for the long-term seismic risk assessment in the study area. The fact that all fault segments with high cumulative probability indicate long-term earthquake-missing gaps means that, although it is impossible to obtain accurate earthquake occurrence probability by using uncertain data and models, at least information about which fault segment has higher or lower earthquake potential than other fault segments is obtained.
The probability map representing the future earthquake potential of this fault section calculated in Figure 7 shows that the thin line on the surface is the boundary of Sichuan Province, and the thick line represents the studied fault zone. The height of the cylinder is proportional to the probability value.
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