Abstract:

On January 7, 2025, a M_S6.8 earthquake struck Dingri County, China. Strong earthquake observation records provide critical insights into ground motion, aiding in macro-intensity assessments, post-earthquake emergency responses, and loss estimations. These records help fill gaps in near-fault strong motion observations and capture the complexity of various near-fault vibrations associated with the earthquake source. Due to the lack of strong motion observation stations near the epicenter, no effective near-field long-period strong ground motions were obtained in this earthquake. The curved grid finite-difference method employs a traction mirror technique derived from the stress mirror method to process free-surface boundary conditions within a curved coordinate system accurately. This method has been extensively utilized for rapid earthquake disaster assessment and simulating strong ground motion. To evaluate the long-period ground motion and velocity pulse distribution of this earthquake, this study applies the curved grid finite-difference method, incorporating the strong earthquake rupture model of the Dingri earthquake and topographic data from the source area. The simulation results illustrate the wavefield propagation process and intensity distribution in the affected region. Furthermore, using a velocity pulse identification method, the study determines the velocity pulse distribution characteristics of the source area. The study accounts for the region’s undulating terrain by first linearly interpolating and downsampling the Shuttle Radar Topography Mission(SRTM)terrain data to align it with the computational grid. The velocity medium model, which significantly influences strong ground motion, is also interpolated and corrected to match the terrain, ensuring compliance with computational requirements. The accuracy and reliability of the simulation results are validated by comparing them with observed waveform and velocity wavefield data. The findings indicate that peak ground velocity(PGV)in the vertical(UD)component is significantly higher than in the east-west(EW)and north-south(NS). This phenomenon is attributed to the normal fault mechanism of the Dingri earthquake. Although some vertical ground motion records exist, near-fault vertical motion data remain scarce. Previous studies suggest that, in near-fault regions, the peak vertical acceleration-to-horizontal acceleration ratio is influenced by factors such as magnitude and epicentral distance, often exceeding the standard 2/3 ratio and sometimes surpassing 1. The maximum simulated intensity in this study is IX, with higher-intensity areas concentrated near the fault’s hanging wall, demonstrating a pronounced hanging wall effect. Due to local topographic influences, the intensity distribution appears irregular. However, the simulated intensity pattern aligns with observed intensity trends, confirming the validity of the long-period earthquake simulation results. Further analysis reveals that near-fault intensity distribution is closely linked to the rupture characteristics of the source area. In the hanging wall region, seismic wave propagation is significantly influenced by fault geometry and surrounding geological conditions. Additionally, the study indicates that intensity distribution varies considerably under different terrain conditions, particularly at the interface between mountains and basins, where seismic wave focusing may locally amplify intensity. The simulated velocity pulses of the EW, NS, and UD components primarily concentrate within the surface projection area of the fault. The EW and NS velocity pulse distribution ranges are narrower than that of the UD component. Since a normal fault caused the Dingri earthquake, velocity pulses induced by rupture directivity effects predominantly appear in the component perpendicular to the fault plane. Compared to other fault types, such as strike-slip faults, normal fault earthquakes are less likely to generate significant velocity pulses. Strike-slip and reverse fault earthquakes, in contrast, tend to produce stronger velocity pulses due to their rupture mechanisms. Normal fault earthquakes are relatively rare, and this event has heightened awareness of potential normal fault seismic hazards in the rift zone. Strengthening research on pulse-type ground motions in normal fault earthquakes is crucial for disaster mitigation. Future studies will collect geometric data of the causative fault and regional stress field information to conduct dynamic rupture simulations. Through numerical analysis, this research aims to further understand pulse-type ground motions in normal fault settings, particularly their spatial distribution and influence on source and site conditions. The findings will enhance our understanding of pulse-type ground motions in normal fault earthquakes and provide a scientific basis for assessing potential seismic impacts and developing disaster prevention strategies.

Key words: Dingri M_S6.8 earthquake, ground motion simulation, seismic intensity, curved grid finite-difference method, velocity pulse distribution

摘要： 2025年1月7日西藏日喀则市定日县发生 M_S6.8 地震。由于该地区强震动观测台站稀疏, 在此次地震中未能获得近断层速度脉冲。为评估此次地震的长周期地震动场及速度脉冲分布特征, 文中基于定日地震强震破裂模型, 结合震源区域地形数据, 采用曲线网格有限差分方法给出了定日地震波场传播过程和震源区域烈度分布模拟结果。在此基础上, 结合速度脉冲识别方法, 获得震源区域的速度脉冲分布特征。结果表明: 受震源的单侧破裂影响, 地震烈度的空间分布呈现出在破裂传播方向上烈度较高的特征, 最大烈度约为Ⅸ度。正断层破裂模式导致速度脉冲主要集中于断层上盘区域, 并以垂直于地表的方向为主。文中的模拟结果为研究正断型地震脉冲分布特征提供了新的视角, 也为未来类似地震的震害预测与防震减灾策略的制定提供了参考。

关键词: 定日M_S6.8地震, 地震动模拟, 地震烈度, 曲线网格有限差分方法, 速度脉冲分布