# Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation
## 1 Introduction  
Changes in the physical properties of a fault zone before an earthquake rupture have become an important issue that scientists attempt to explore. Generally, the most popular methods for the topic of earthquake precursors are ionospheric anomalies [1, 2, 3] and seismic activity [4, 5]. They can include variations in rock properties or pore pressures during the inter-seismic period, but these characteristics still cannot be directly observed around the seismogenic zone. Results of seismic velocity measurements reveal some information about the potential role of crustal rock strength and pore pressure in crustal dynamics. From a wave propagation simulation, it is known that Vp is inversely proportional to the crack density and Vs is proportional to the fractional volume of the fluid content in fractured rock [6]. In addition, for the velocity structures of Vp and Vs, the Vp/Vs ratio is correlated to Poisson's ratio; therefore, they are regarded as important parameters in research into crustal rock characteristics [6, 7]. Poisson's ratio reflects the porosity, degree of crushing, and hydraulic pressure of rock formation. Many factors such as the composition of rock, orientation of cracks, pore pressure, temperature, and fluid saturation would affect the Vp/Vs ratio. Since most seismic events occur in active fault zones, the Vp/Vs ratio can be used to determine if pressure changeswill cause cracks to affect the velocity structure, which may result in gas or fluid migration and further large changes in pore pressure [8; 9]. Therefore, the relationship between the characteristics of a fault zone and the distribution of the \(\mathrm{Vp/Vs}\) ratio can be explored through three-dimensional (3D) tomography inversion. Furthermore, velocity structures can also be utilized to detect the attitude of subsurface formation and a brief overview of interface properties and features. Based on the above advantages, the results obtained in this study can be used to further understand the spatiotemporal evolution of fault materials. Recently, some studies about seismic velocity variation have been conducted on the basis of the rupture of rocks caused by faulting [10], migration of magmatic fluids accompanying volcanic activity [11; 12], and dilatancy and fluid migration before earthquakes [13], among others. The authors of [6] also described the variations in \(\mathrm{Vp}\) before and after the 1999 Chi-Chi earthquake and the corresponding variations in \(\mathrm{Vp/Vs}\). Although there were no significant changes in \(\mathrm{Vp}\), after the occurrence of the Chi-Chi earthquake, deeper regions of the low-velocity anomaly appeared to have further expanded. The \(\mathrm{Vp/Vs}\) model showed a substantial change reflecting an outward shift of the fluid-filled fractured source region. These changes are attributed to a localized stress change caused by the slip of the Chi-Chi rupture. The authors of [14] stated that the relationship between the velocity structure and time can be applied in earthquake forecasting and prediction, volcano warning systems, geothermal exploration, oil and gas reservoir assessment, and carbon dioxide capture and storage. However, the study of spatiotemporal variations of subsurface velocity structures is still a challenging work. For the Taiwan region, from studies using 3D velocity models, similar results were discovered [15; 16; 17; 18]; however, temporal variation was not considered in these 3D models. In 1999, the disastrous Chi-Chi earthquake \(\mathrm{M_{L}}\) 7.3 occurred in central Taiwan and greatly affected Taiwanese society. In addition, although considerable research has been conducted to better understand the changes in the velocity structure in response to seismic events in central Taiwan, few studies have emphasized the significance of the velocity structure in relation to time. Therefore, this study aims to determine the impact of the Chi-Chi earthquake in central Taiwan before and after the event in order to understand the behavior of subsurface velocity structures.