# Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation
## Appendix A  
Results of velocity structure model  
In general, the seismic velocity is affected by several geological factors, including lithology, fluid saturation, pore density, geometry of the crack, and the degree and type of rock cementation [50]. The effect of shallow layer is the most prominent; the pressure increases with depth due to the weight of the rocks above the layer, which subsequently leads to the closure of the majority of the pores [51].  
This study was divided into three time periods to explore the variations in Vp, Vs, and Vp/Vs in the study area; the results are shown in Figures 13-14. We divided the time period 1 January 1991-31 December 2003 into three periods to study the change in the velocity structure before and after the Chi-Chi earthquake that occurred on 21 September 1999 (UTC+8). Each period is discussed in the following paragraphs.  
In the period I (1 January 1991-31 December 1995), the lateral variations in Vp (Figure 13a), Vs (Figure 13b), and Vp/Vs (Figure 13c) in the study area were not significantly different from those at depth. However, at a depth of 10-45 km, especially at a depth of 15 km or less, the Hsuehshan Range presented a significantly low Vp and Vs zone, which may be related to the plastic deformation caused by the plate extrusion (plate collision); these findings were consistent with the previous studies [15; 16; 17; 18]. In addition, there was a higher in Vs and a lower in Vp/Vs at depths of 5-20 km in the LF and LVF.  
In the period II (1 January 1996-19 September 1999), which was before the occurrence of the Chi-Chi earthquake sequence, Vp (Figure 13a) showed no significant variation compared to that in the first period. However, Vs (Figure 13b) began to change significantly before the earthquake. We found that there was lower Vs value in the source area, resulting in a higher Vp/Vs; this may be due to the cracks generated in this area and the intrusion of the fluid causing pore expansion. Instead, Vs was higher around the source area, which led to a lower Vp/Vs. This may be attributed to the closure of the pores in this region. At depths of 15-35 km, the low-Vs zone in the WF was still visible.  
In the period III (1 January 2001-31 December 2003), Vp (Figure 13a), Vs (Figure 13b), and Vp/Vs (Figure 13c) were recorded after the episode of the Chi-Chi earthquake sequence. The results showed significant lateral variations at depths less than 15 km, and the tectonic stress was assumed to be readjusted after the large earthquake. One interesting phenomenon at depths of 5-25 km was the lower Vp anomaly in the northern and WF and HR compared to that in the first and second periods. Furthermore, the abnormal range of Vp was larger than the range in Vs. At a depth of 5 km, relatively low Vp/Vswas concentrated in the CLF. A high Vp/Vs mainly concentrated in the south and north sides. At a depth of 10 km, relatively low Vp/Vs was observed westward and concentrated between the CHF and SKF; moreover, the range of the low Vp/Vs tended to expand. The low Vp/Vs was located further to the central of the LF at depths of 5-20 km. The high Vp/Vs was located further to the south side of the CLF and north side of the LVF at depths of 5-35 km.  
Figure 13: This study is divided into three periods to explore the Vp, Vs, and Vp/Vs in the study area. Yellow triangle represents the station. The results of Figure 14 represent the second period. (**a**) Vp at different depths. (**b**) Vs at different depths. (**c**) Vp/Vs at different depths.