7.1 Determination of rupture mode
For rupture mode 1, it was found that there was significant slip generated in the area between Yingxiu and Hongkou region. This is due to the constraint of GPS data (Yin, 2022). When this region ruptured, it would take 5-14s. From the contribution to the synthetic record, it is not allowed to generate significant dislocations. Figure 5 shows the synthetic records of the far-field and near-field in 51WCW, 51PXZ, and 51CDZ stations. The corresponding far-field synthetic record is about 13-38s, but the waveform of the synthetic record is reversed from the observation record, especially at the maximum amplitude of the observation record at 23s and 32s, which is very obvious.
The unilateral rupture of the southern section of the BCF from Yingxiu to Longmen area resulted in a contradiction between the synthetic and observation records at the far-field station, which had a destructive effect on the second band of the synthetic records (Fig. 5). In order to offset the composite records in this area, rupture mode 1 generated larger dislocations in the low dip area below Longmenshan area, while the dislocations on the PFG were correspondingly smaller.
However, from the perspective of the effect of synthesized records, the dislocation between Yingxiu and Hongkou area resulted in the synthesized records of the second band of some stations in rupture mode 1 being worse than mode 2, with a waveform fitting residual of 7% greater (Table 1). From the synthetic records of the near-field stations corresponding to this region, it can be seen that when a unilateral rupture occurs, the synthetic record corresponds to the position before the second wave packet of the southwest station, which generates larger amplitude than the observed record between 0 and 20s in southwest stations. For rupture mode 2, the synthetic record excited by dislocations in this area corresponds to the record of the second wave packet at the southwest station. So, the amplitude of the second wave packet synthesized by the 51WCW, 51PXZ, and 51CDZ stations is larger than that the mode 1, which is more in line with the observed records (Fig. 6).
Table 1
Residual of two types of rupture modes
Rupture mode | Near-field | Far-field |
1 | 0.774 | 0.412 |
2 | 0.739 | 0.384 |
7.2 Inversion results
The inversion results are shown in Fig. 7–11, and Table 2. From the results, it can be seen that the three data synthetic records match well with the observed records. The obtained seismic moment is 1.058×1021N • m, and the entire rupture duration is about 100 seconds. There are 5 asperities on the fault plan, the positions of the main asperities are similar to existing research results (Wang et al. 2008; Shen et al. 2009; Feng et al. 2010; Hartzell et al. 2013). Among them, there are 4 asperities on Beichuan fault and 1 asperity on PGF fault. The seismic moments of the southern section of the Beichuan Fault, the northern section of the Beichuan Fault, and the Pengguan Fault are 0.491×1021N·m, 0.424×1021N·m and 0.143×1021N·m, respectively, accounting for 46%, 40%, and 14% of the total seismic moments (Table 2).
Table 2
Fault | Maximum slip/m | Seismic moment /1021N•m | Total seismic moment /1021N·m | Residual near-field | Residual far-field |
PGF | 9.4 | 0.143 | 1.058 | 0.739 | 0.384 |
BCF1-4 | 11.8 | 0.491 |
BCF5 | 10.1 | 0.424 |
In the southern section of the Beichuan Fault, the slip is concentrated in the low dip area below Longmenshan Town, distributed within a range of 35km along the strike and 24km along the dip. The maximum slip is 11.0m, and the static stress drop is 15.0Mpa. The slip on the fault plane is mainly characterized by thrust. In the high dip area, it is concentrated between Longmenshan and Yingxiu area, about 35km along the strike and 12km along the dip, with a maximum slip of 12.0m. It is located on the lower side of Hongkou, with a static stress drop of 23.6Mpa. There are also significant dislocations occurring in the high dip area from Yuejia Mountain to Qingping, with a range of 30km along the strike and 12km along the dip, with a maximum slip of 11.7m. It is located on the lower side of Yuejia, with a static stress drop of 25.7Mpa, and the form of dislocations is mainly thrust. The maximum slip rate in the surrounding area of the 51SFB station on the fault is 74cm/s (Liu and Li, 2009), with an average slip rate of 60cm/s, which is equivalent to the 80cm/s horizontal PGV of the station. In the vicinity of the 51MZQ fault, the maximum slip rate reached 85cm/s (Liu and Li, 2009), which is equivalent to the 100cm/s horizontal PGV of the station. The maximum slip around the initial rupture point is 4.0m, mainly characterized by thrust slip. The average slip speed in this area is 56cm/s, which is equivalent to the 46cm/s horizontal PGV of the 51WCW station (Liu and Li, 2009). Inversion based on near-field records of 1999 Taiwan Chi-Chi earthquake indicates that in areas with higher slip rates on the fault plane, surrounding stations often observe larger horizontal PGV (Wu, et al. 2001). The Wenchuan earthquake also showed that the large PGV observed at the station is related to the slip rate on the fault plane.
The slip of the northern section of the Beichuan fault is concentrated near Beichuan, with a strike of about 30km and a dip of 21km. The maximum slip is 10.0m, located near the surface of Beichuan area, with a static stress drop of 13.2Mpa, mainly characterized by thrust displacement. In the area north of Beichuan, slip is concentrated near Nanba, distributed within a range of 55km along the strike and 21km along the dip, with a maximum slip of 10.0m. It is located 20km north of Nanba and 12km along the dip, with a static stress drop of 13.2Mpa. On the south side of Nanba, it is mainly characterized by thrust displacement, and on the south side of Nanba, it transforms into strike slip.
The slip on the Pengguan fault is concentrated on the lower side of the Bailu, within a range of 30km along the strike and 12km along the dip, with a maximum slip of 9.4m and a static stress drop of 17.5Mpa, mainly characterized by thrust slip. There is also slip near the surface in the northeast section of the Pengguan fault, with a maximum value of 8m, located below the Hongkou, mainly characterized by thrust slip.
The entire rupture process is divided into four stages. The first stage starts from 0 to 12 seconds, corresponding to slip in the surrounding area of the initial rupture zone. The seismic moment release is about 6% of the total value. In the second stage from 2 to 40 seconds, it can be seen that the main energy of the Wenchuan earthquake is released during this stage, releasing 52% of the total energy. During this process, the southern section of the Beichuan fault and the Pengguan fault rupture. The third stage lasts from 40 to 60 seconds, during which a rupture occurs near Beichuan, releasing 17% of the total energy. The fourth stage, from 60 to 92 seconds, releases 22% of the total energy, which is another concentrated stage of energy release during this earthquake.