# Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation
## 5 Results and Discussion
### Tectonic Implications  
In this study, we discussed the relationship between the velocity structures of Vs and Vp/Vs over time and the geometry of faults in order to determine the variations in Vs and Vp/Vs before and after the occurrence of the Chi-Chi earthquake sequences along the velocity profiles. Figure 12 shows the time-dependent perturbation profile in periods I and II. It can be seen from the section of CC*-EE* connection that in the \(\alpha\) area (hypocenter) during the Chi-Chi earthquake, Vp/Vs increased by approximately 4% and Vs decreased by approximately 4%. Figure 12: The AA* to EE* profiles analyzed in this study are displayed in the left panel. The profile across the Chi-Chi earthquake uses gray dotted line and red star to indicate the location of the CLF and the mainshock, respectively. The right panel shows the location of the AA* to EE* profiles. The profile CC* across the Chi-Chi earthquake are depicted with the thick red line. (**a**) Temporal variation profile of Vs in the periods I and II. (**b**) Temporal variation profile of Vp/Vs in the periods I and II. In the \(\beta\) area (near the hypocenter), \(\mathrm{Vp}/\mathrm{Vs}\) decreased by approximately 6% and \(\mathrm{Vs}\) increased by approximately 10%. The fluid-filled fractured region at the \(\alpha\) area (hypocenter) was assumed to lead to fissure expansion; therefore, the decline of \(\mathrm{Vs}\) was greater than \(\mathrm{Vp}\). Furthermore, in the south to north sections of \(\mathrm{Vs}\), \(\mathrm{Vs}\) changed along an eastward-tilted inclined plane. Moreover, \(\mathrm{Vs}\) changed more significantly south of the hypocenter compared with the north. This phenomenon occurred before the Chi-Chi earthquake and was concentrated south of the CLF. However, the variations in the \(\beta\) area were opposite to those in the \(\alpha\) area. In contrast, in this study, we discuss the velocity variation over a long time scale before the occurrence of the earthquake sequence; we found remarkable changes in periods I and II, such as the decrease in \(\mathrm{Vs}\) in the \(\alpha\) region and the increase in \(\mathrm{Vs}\) in the \(\beta\) region, while the hypocenter was located at the junction of the \(\alpha\) and \(\beta\) areas. This phenomenon indicated that the direction of the tectonic stress was from the south-east (approximately \(100^{\circ}\), [48]), resulting in the closure of the pores in the \(\beta\) area, which may allow the fluid to migrate toward the \(\alpha\) area. The decrease in the frictional strength of the fault in this area might be attributed to the vicinity of the \(\alpha\) area to the hypocenter, the increase in the pore density, and the possibility of fluid intrusion. These factors may have led to the occurrence of the Chi-Chi earthquake, suggesting it is a precursor phenomenon. From periods II and III, the time-dependent perturbation profile of \(\mathrm{Vs}\) is shown in Figure 13; it can be seen from the section of AA*-EE* connection in the low anomalous regions that \(\mathrm{Vs}\) significantly expanded from south to north, revealing that the northern section of the CLF has a larger slip displacement after the rupture, which leads to an east-dipping, low-\(\mathrm{Vs}\) anomalous region. Furthermore, the slip displacement of the southern section was smaller than that of the northern section, and \(\mathrm{Vp}/\mathrm{Vs}\) exhibited fewer anomalies compared with those in the northern section. This anomalous region is speculated to be the Chi-Chi earthquake rupture zone, which is consistent with the location of the CLF. Further analysis is required to explore the spatial variation of the earthquake rupture, the results of which would also be valuable for confirming the larger rupture zone in the northern section of the CLF compared to the southern section. In addition, the authors of [49] independently analyzed the Qs of earthquakes and found that Qs changed with time.