## 4 Near-Fault Ground Motion Records: Comprehensive Data Compilation and Analysis  
The seismic response analysis of the bridge structure incorporates five near-fault ground motion records that were carefully selected to represent characteristic earthquake events recorded close to causative faults. These ground motions exhibit distinctive features typically associated with near-fault effects, including forward directivity pulses and fling-step displacement patterns, which are particularly critical for long-span bridge structures.  
The selected ground motion records were obtained from well-documented seismic events, with source-to-site distances not exceeding 10 km and moment magnitudes (Mw) ranging from 6.7 to 7.7, and were downloaded from the NGA2 database [42]. Table 1 presents the key characteristics of these records, including the recording station information and relevant ground motion parameters, such as the peak ground acceleration (PGA).  
Figure 7: Eigenperiods and corresponding eigenmodes for the bridge under consideration: fixed-base pairs vs. soil-bridge interaction.  
Each selected record demonstrates pronounced velocity pulses in the fault-normal component, with pulse periods ranging from 1.0 to 4.5 s, within the critical period range for the studied bridge structure. The velocity pulses are characterized by peak ground velocities (PGV) ranging from 55 to 125 cm/s, reflecting the severe near-fault effects typical of such recordings.  
The response spectra analysis reveals a significant amplification in the long-period range, particularly between 1.0 and 5.0 s, which coincides with the essential periods of the bridge structure. To ensure compatibility with the design earthquake, which has a peak ground acceleration (PGA) equal to 0.30 g = 2.943 m/s\({}^{2}\), all the examined records were scaled according to this target. Figure 8 depicts the response spectra of these records, their mean spectrum, and the design spectrum according to Eurocode 8 [37]. It was found that the mean response spectrum follows the EC8 [37] design spectrum, especially for the fundamental period of the bridge, i.e., for T\({}_{1,\text{fixed}}\) = 4.245 s and T\({}_{1,\text{ssi}}\) = 4.657 s, for rigid and flexible soil, respectively.  
Special attention was paid to the examination of the corresponding vertical component of ground motions, as near-fault records often exhibit strong vertical accelerations that can significantly influence the behavior of the bridge. The elastic response spectra were computed for a 5% damping ratio, covering a period ranging from 0.0 to 5.0 s.  
This comprehensive analysis of near-fault ground motion characteristics provided the foundation for subsequent sections examining the bridge's seismic response, particularly when combined with train loading and soil-structure interaction effects. The selected records effectively capture the critical features of near-fault ground motions that can significantly influence the dynamic response of long-span bridge structures.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Earthquake/Date** & **Magnitude (M\({}_{\text{w}}\))** & **Station** & **Component** & **PGA (g)** \\ \hline Cape Mendocino 25 April 1992 & 7.2 & 89156 Petrolia & NS & 0.662 \\ Chi-Chi, Taiwan 20 September 1999 & 7.7 & NSY & EW & 0.145 \\ Northridge 17 January 1994 & 6.7 & 24088 Pacoima Kagel & EW & 0.433 \\ Kobe 16 January 1995 & 6.9 & KJMA & NS & 0.821 \\ Northridge 17 January 1994 & 6.7 & 24279 Newhall & NS & 0.590 \\ \hline \end{tabular}
\end{table}
Table 1: Near-fault ground motion records used in this study.  
Figure 8: Response spectra and the mean spectrum of near-fault records vs. EC8 design spectrum.