Abstract: The regional characteristics of seismic ground motion are of importance for its reliable prediction, and the resulting seismic hazard mitigation and seismic risk controllability. The Jiashi region in Northwest China, which is located in the boundary between the western segment of the South Tianshan and the northwest Tarim Basin, is subject to high seismic hazard due to the active seismicity. This region has frequently suffered from the moderate to strong earthquakes. For the purpose of investigating the regional characteristics of the ground motion in Jiashi region, we separated simultaneously the earthquake source spectra, propagation path attenuation, and site responses from the horizontal S-wave spectra in the 366 three-component strong-motion recordings with high quality after manual detection and selection based on the nonparametric spectral inversion technique. The datasets were recorded at 25 strong-motion stations in 46 earthquakes of M3.0~6.4 from July 2007 to February 2020 occurring in the Jiashi region, mainly including both Jiashi seismic sequences in 2018 and 2020.
We thus developed the region-specific empirical models for the Jiashi region, including the path attenuation, the path duration, and the linear site response models. At the same time, the source parameters theoretically depicting the omega-square source spectral model, including seismic moment M_W, corner frequency f_c, and stress drop Δσ were retrieved from the inverted source spectra for the 46 earthquakes considered using the grid-searching method. Their reliability was further verified from the normalized fitting residuals between the inverted and theoretical source spectra varied with f_c, and the f_c-M_W plots. We determined that M_W=5.893, f_c=0.362Hz, and Δσ=6.684MPa for the M_S6.4 Jiashi earthquake in 2020. According to the inverted path attenuation curves against hypocentral distance less than 120km, the ground motions generally show the slower path attenuation at local distances, while the faster path attenuation at regional distances was clearly found, which could be ascribed to the significant anelastic attenuation in the Jiashi region. The path attenuation in the Jiashi region was approximately modeled by the empirical expression, simultaneously including the hinged bilinear geometrical spreading model and the anelastic attenuation term in the function of quality factor Q. The yield Q values were regressed as 60.066f^0.988 at frequencies of 0.254~30Hz. The strong anelastic attenuation may be attributed to the prominent interaction of the seismic wave propagating within the high inhomogeneous crust. The transition distance R₁=60km, and the geometrical spreading exponents n₁=0.30 and n₂=0.59 before and after R₁ well defined the preferred hinged bilinear geometrical model. The site responses show the dependence on the V_S30 values, and the higher amplifications are generally predominant at sites with the lower V_S30 values, and vice versa. Compared with the linear site amplification model proposed by Syehan and Stewart(2014) for the NGA-West2 project, the local sites in the Jiashi region show the very weak amplification effects on the high-frequency ground motions at 10~30Hz, while the good consistencies at low-to-intermediate frequency bands occur. On the basis of the SS14 model, the modified model applicable for the Jiashi region was proposed in this study. The path durations, defined by an effective 95%~5%significant duration, were calculated for the strong-motion recordings(a total of 502)in the Jiashi region, and were applied for the development of the empirical model. The prolonged path durations were observed in those ground motions recorded in the Jiashi region, significantly higher than those observed in the Sichuan region.
Furthermore, the yield regional characteristics were preliminary examined by the ground motion simulations. The stochastic finite-fault method was further applied to reproduce ground motions in the M_S6.4 Jiashi earthquake with these regional characteristics under consideration. The kinematic source models were first generated stochastically by the filtering method in the two-dimensional wavenumber domain. These stochastic source models can well represent the source rupture process in physical and its sufficient variability. We then verified the reliability of these regional characteristics from the good agreements of the ground motion intensity measures between the observations and the simulations, including the peak ground accelerations and velocities, and the spectra accelerations. Moreover, the key role of the regional characteristics in ground motion simulation was clearly showed by the significant deviations of the long-period ground motions at large distances between the observations and the simulations based on the path attenuation models in the Sichuan-Yunnan region. Finally, the ground motion maps were developed for the M_S6.4 Jiashi earthquake according to the regional characteristics and a best-performing source kinematic rupture model.
The region-specific ground motion empirical models are very helpful for reliably predicting ground motions, and mitigating seismic hazards in the Jiashi region with the active seismicity.

Key words: Jiashi M_S6.4 earthquake, source parameters, path attenuation, regional characteristics, ground motion simulation

摘要： 为研究伽师地区地震动的区域特征, 提高地震动预测的可靠性, 文中利用伽师地区近10a收集的强震动观测记录, 采用非参数谱反演方法分离了地震动的震源、 传播路径及场地影响, 建立了反映伽师地区地震动区域特征的路径衰减、 路径持时、 场地线性反应等经验模型, 并给出了伽师地区46次地震的震源参数(矩震级、 拐角频率和应力降)。 文中确定的伽师M_S6.4地震的矩震级为5.893、 拐角频率为0.362Hz、 应力降为6.684MPa。 伽师地区地震动的近场路径衰减较慢, 而远场更快的路径衰减与显著的非弹性衰减有关, 文中将伽师地区的路径衰减近似地描述为双段线性几何扩散经验模型和品质因子Q=60.066f^0.988 表示的非弹性衰减项的叠加; 伽师地区地震动的路径持时较长, 明显高于四川地区; 对比SS14场地线性反应模型, 伽师地区局部场地对10~30Hz频段地震动的线性放大效应很弱, 其他频段的场地线性反应与SS14模型接近, 在此基础上提出了适用于伽师地区的SS14调整模型。 文中进一步将伽师地区地震动区域特征应用于伽师M_S6.4地震的地震动模拟, 震源破裂过程采用随机产生的震源运动学破裂模型表示, 通过对比观测和模拟记录的峰值和反应谱, 检验了区域特征的可靠性, 最后给出了伽师M_S6.4地震的地震动场。

关键词: 伽师M_S6.4地震, 震源参数, 路径衰减, 区域特征, 地震动模拟