# Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation
## 3 Data and Methods  
In this research, we utilized the massive volume of the seismic events compiled by the Central Weather Bureau Seismic Network (CWBSN) from January 1991 to December 2003. We divided this time span into three segments to investigate velocity structures in different time periods, namely, 1 January 1991-31 December 1995; 1 January 1996-19 September 1999; 1 January 2001-31 December 2003 (hereinafter, period I, period II, and period III, respectively) (see Figure 1b). We excluded the days immediately before and after the Chi-Chi earthquake because of complicated source effects that make some seismic events unsuitable to be adopted as a point source; therefore, these data could not be used for tomography [32]. This study suggests that a large earthquake caused by stress  
Figure 1: **(a)** Fault setting in central Taiwan and seismic stations used in this study. The black arrows indicate the near-field GPS horizontaloseismic displacement. **(b)** Seismic distributions at different time periods. In the period I, there were 6075 events and 93,870 ray paths within the study area. In the period II, there were 5970 events and 96,862 ray paths within the study area. In the period II, there were 8234 events and 135,391 ray paths within the study area. EP: Eurasian Plate; PP: Philippine Sea Plate; WF: Western Foothills; HR: Hsuehshan Range; CR: Central Range; CHF: Changhua Fault; CLF: Chelungpu Fault; STF: Shuangung Fault; SKF: Shuilikeng Fault; LF: Lishan Fault; LVF: Longitudinal Valley Fault. adjustment makes the subsurface structure unstable, so the data for the year after the Chi-Chi earthquake was eliminated. In this study, we used the travel times of P- and S-waves from earthquakes located inside the study area. Our study area stretched from 120.0\({}^{\circ}\) E to 122.0\({}^{\circ}\) E in longitude and from 23.5\({}^{\circ}\) N to 24.5\({}^{\circ}\) N in latitude. Earthquakes with magnitudes equal to or greater than 1.0 were considered, and data with poor location quality were excluded. Twenty-five stations were chosen in the study area, and at least 10 readings of each earthquake event were recorded, as the amount of observation of the matched earthquake event arrival affects the quality of the location of the earthquake; fewer observations result in poor location quality. We applied seismic travel time data for tomography inversion by local tomography software [33]. The local tomography software was modified according to the bending-ray tracing algorithm by [34]. First, we utilized the Goal function [35] to detect the initial locations of the earthquakes. A straight path was drawn from the hypocenter to the station to locate the turning point at the midpoint and create a new path in sequence. Then, the corresponding travel time residue was calculated and the one with the minimum value was set as the ray path [28]. Finally, travel-time inversion was performed using the LSQR algorithm [36,37] to measure the P- and S-wave velocity anomalies and for obtaining the source parameters (four parameters for each seismic event). Furthermore, these results are fed back on a 3D grid for performing iterative inversion calculations until the iteration number or the travel-time residual value was less than the threshold value. The study area was divided into numerous grids to invert the velocity variation. The grid was centered (121\({}^{\circ}\) E, 24\({}^{\circ}\) N) on the positive and negative sides of the x-, y-, and z-axes to 200 km and had dimensions of 5 \(\times\) 5 \(\times\) 5 km\({}^{3}\). As a single-grid azimuth may cause artifacts [38], this study averaged the results from four angles--0\({}^{\circ}\), 22\({}^{\circ}\), 45\({}^{\circ}\), and 67\({}^{\circ}\)--to reduce any grid dependency. We integrated different combinations of Vp and Vs damping values to obtain the optimal model through the trial and error process. Finally, we selected the appropriate Vp and Vs damping parameters that would give results with the stable travel-time residual (RMS).