# Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation
## 5 Results and Discussion  
### Temporal Evolution of a Velocity Structure  
The pore pressure is inversely proportional to the seismic wave velocity, evidenced by oil field detection and laboratory experiments [39]. Moreover, many experiments have examined the effects of fluid-filled cracks on seismic velocity [40; 41; 42; 43]. Therefore, it is generally accepted that Vp is inversely proportional to the crack density and that Vp/Vs is proportional to the fluid saturation in the crack. In addition, its correlation with the rock crack and pore pressure was discussed. Fortunately, there have been studies using more realistic observational data to build fracture models [44] and there are detailed discussions on the differences between various fracture models.  
First, variations in Vp, Vs, and Vp/Vs in the \(\alpha\) zone (source area, see Figure 10b) before the occurrence of the Chi-Chi earthquake sequence in periods I and II are discussed and are shown in Figure 10.  
The results indicate that Vp decreased by approximately 2%, Vs decreased by approximately 4%, and Vp/Vs increased by approximately 4%. In the \(\beta\) zone (around the source area, see Figure 10b), Vp increased by approximately 4%, Vs increased by approximately 10%, and Vp/Vs decreased by approximately 6%. The results demonstrate that the  
Figure 10: (**a**) Temporal variation in Vp in the periods I and II. (**b**) Temporal variation in Vs in the periods I and II. (**c**) Temporal variation in Vp/Vs in the periods I and II. See the detail description in the text.  
subsurface structure had already changed before the impending Chi-Chi earthquake, and the reason for the significant decrease in Vs in the \(\alpha\) zone compared with Vp is possibly the proliferation of cracks and fluid intrusion. The authors of [45] also speculated using a magnetotelluric technique that the seismogenic mechanism of the Chi-Chi earthquake may be associated with fluid intrusion. The velocity structure variation in the \(\beta\) zone was opposite to that in the \(\alpha\) zone, presumably due to the closure of the crack, resulting in an increase in Vs. Figure 10b,c clearly show that, at depths of 5-10 km, the hypocenter of the Chi-Chi earthquake occurred in the boundaries of the areas of high and low Vp/Vs. However, no significant difference was noticed at depths greater than 15 km.  
From periods II and III, the variations in Vp, Vs, and Vp/Vs after the earthquake sequence are shown in Figure 11.  
The results demonstrate that Vp and Vs varied within \(\pm\)10% in the vicinity of the CLF. These variations were more complicated in the shallow crust (equal to or less than 15 km). Note that in Figure 11a, the result shows a significant decrease in Vp at a depth of 5-15 km in the northern segment of the CLF. The distribution of the low Vp anomaly is in good agreement with the result of maximum slip distribution in the GPS calculation [46; 47], because the slip vector in the GPS calculation is the combined effect from the surface observation. However, from the temporal variations in the velocity structure, we can estimate the possible slip pattern beneath the subsurface. When the depth was less than 15 km, the lateral variation in Vp was more prominent than in the lower crust, which reflected changes in rock cracks after the earthquake occurred. No more lateral change was noted for depths greater than 25 km, which indicated that the lower crust was less  
Figure 11: (**a**) Temporal variation in Vp in the periods II and III. (**b**) Temporal variation in Vs in the periods II and III. (**c**) Temporal variation in Vp/Vs in the periods II and III. See the detail description in the text.  
affected by the Chi-Chi earthquake. At a depth of more than 15 km, the variations in Vs and Vp/Vs near the hypocenter reduced after the earthquake, and the area with obvious velocity variation was located in the area north of the CLF to LF.