Statistical analysis of near-fault ground motion acceleration peak ratio and pulse characteristics

To further understand the significant vertical characteristics and velocity pulse characteristics of near-fault ground motions, a comprehensive study was conducted using 1 706 sets of strong ground motion records selected from the NGA-West2 database. This study aimed to investigate the overall distribution patterns of the vertical-to-horizontal acceleration peak ratio, designated as a_V/a_H, and to explore how this ratio varies with various ground motion parameters such as moment magnitude M_W, fault distance R_jb, site type, and fault type. Additionally, the study employed the multi-component velocity pulse identification method to obtain crucial pulse parameters, including the pulse period （T_p） and velocity pulse peak （PGVp）, for 140 groups of near-fault velocity pulse ground motions. The statistical patterns and empirical models were then established to assess the relationship between these pulse parameters and the moment magnitude, as well as the fault distance. The results revealed several intriguing insights. Firstly, it was observed that the near-fault acceleration peak ratio a_V/a_H exhibits a strong correlation with ground motion parameters. The probability distribution of a_V/a_H follows Frechet distribution, indicating that the occurrence of high a_V/a_H ratios is not random but rather follows a specific pattern. This finding indicates that the vertical characteristics of ground motion may be significant in the near-fault regions, especially under certain seismic conditions. In addition, it is also found that the a_V/a_H ratio tends to increase with moment magnitude increasing and fault distance decreasing, indicating that the ground motion with larger vertical acceleration is more likely to occur. Additionally, the study also observed that smaller equivalent shear wave velocities v_S30 and fault types such as strike-slip and reverse faults are associated with a higher likelihood of generating larger a_V/a_H ratios. These findings highlight the importance of considering vertical ground motion in seismic hazard assessments and earthquake engineering design, especially in areas with these specific ground motion characteristics. The study also focused on the pulse characteristics of near-fault ground motions. The analysis revealed that the pulse period increases with moment magnitude increasing, while the correlation with site conditions is relatively weak. This suggests that the pulse period is primarily influenced by the magnitude of the earthquake rather than local site conditions. Furthermore, a comparison of the empirical model of pulse period with the model proposed by Shahi and Baker revealed that the two models exhibit minimal differences in the range of large magnitudes. More importantly, the difference between the two models decreases as the moment magnitude increases, indicating that whether or not to distinguish between different pulse types has a negligible impact on the quantitative relationship between pulse period and moment magnitude. This finding suggests that the empirical model for pulse period can be reliably used to predict pulse periods in near-fault regions, regardless of the specific pulse type. Finally, the study found that the largest amplitudes of velocity pulse peaks primarily appear in regions with high magnitude earthquakes and in proximity to the seismogenic fault. This observation underscores the significance of vertical ground motion in these areas, particularly during large earthquakes. It also highlights the need for enhancing seismic hazard assessments and engineering designs that specifically address the vertical component of ground motion in these high-risk regions. In summary, this comprehensive study provides valuable insights into the significant vertical and pulse characteristics of near-fault ground motion. The findings emphasize the importance of considering earthquake magnitude, fault distance, site conditions, and fault type in assessing the potential for extreme vertical ground motion and quantifying the key parameters of near-fault velocity pulses. These results not only enrich our understanding of the complex behavior of near-fault ground motion, but also provide essential guidance for earthquake engineers and researchers in refining seismic hazard assessments, designing earthquake-resistant structures, and developing effective risk mitigation strategies. The study underscores the necessity of incorporating these nuanced ground motion characteristics into future seismic design codes and guidelines so as to ensure the safety and recoverability of the near-fault area.