Abstract:

On 21 May 2021, a great earthquake of M6.4 struck Southwest China. This catastrophic event caused extensive casualties, a large number of houses collapsed, traffic disrupted, and large bridges damaged in Yunnan Province. The epicentre of the Yunnan Yangbi earthquake is located near the NW trending Weixi-Qiaohou-Weishan Fault. After this earthquake, the Institute of Geophysics of China Earthquake Administration calculated the focal mechanism solution using the global network data, the result shows that the earthquake is a strike-slip faulting event with normal component. The result of the focal mechanism solution is consistent with the strike of the Weixi-Qiaohou Fault and the distribution of aftershocks. Therefore, the strike of seismogenic structure of this earthquake was determined to be NW. Based on the strong motion observation data of 21 strong motion seismographs and 304 seismic intensity meters, the earthquake ground motion intensity map of the 21 May, 2021 Yangbi, Yunnan earthquake was obtained using the deviation correction method of magnitude shift, considering the geological tectonic background of the seismogenic fault, the focal mechanism solution and the precise location of aftershocks of this event. A commonly used proxy V_S30, the time-averaged shear wave velocity to 30m depth, was used to account for the local site effect of ground motion in the calculation of ground motion intensity map. We used V_S30-based amplification terms, which depends on the amplitude and frequency of ground motion, to account for site amplification. The V_S30 data of the macroscopic site classification was estimated using the correlation between topographic gradient and V_S30. Ground motion prediction equations(GMPEs) was used to supplement sparse data in its interpolation and estimation of ground motions. The selection of GMPEs for ground motion estimation were the attenuation relation of peak acceleration in western China in the fourth generation seismic zoning map. The observations of the ground motion for this event show that the maximum horizontal peak ground acceleration is 720.3cm/s² on the Yangbi station, 7.9km from the epicentre. Horizontal peak ground acceleration at 14 seismic stations is greater than 45cm/s². A large number of remote observation records with small values of ground motion also revealed the attenuation characteristics of ground motion for this earthquake. Using strong motion observation data available, we computed an event bias that effectively removed the inter-event uncertainty from the selected GMPE. The deviation correction method of magnitude shift minimizes the systematic deviation between the observed and estimated data produced by ground motion prediction equation, and reduces the uncertainties of the ground motion estimation in the area without stations. After the ground motion observations were corrected(de-amplified) to “rock”, we flagged any data that exceed three times the sigma of the GMPE at the observation point as abnormal data. The bias was then recalculated using different earthquake magnitudes and the flagging was repeated until systematic deviation between the observed and estimated data produced by GMPE was minimal. The results of the earthquake ground motion intensity map show that the highest seismic intensity caused by Yangbi earthquake is Ⅷ. Cangshanxi Town in Yangbi County and Taiping Town are located in the seismic intensity Ⅷ area. The area of seismic intensity Ⅵ and above covers an area of about 6 500km², spreading northwest in general. Many roads including Expressway G56 and national highway G215, pass through the estimated seismic intensity Ⅶ area, which may cause road damage and traffic disruption following this earthquake. On the other hand, the reliability of small amplitude observations recorded by far-field simple intensity meters need to be evaluated further. Finally, the seismogenic tectonic setting, the focal mechanism solution and the aftershock distribution of the earthquake also play a macro-control role in the distribution of the earthquake ground motion intensity. The results of this paper can provide theoretical basis and reference for earthquake emergency response decision-making and provide input for earthquake disaster emergency assessment.

Key words: Yangbi earthquake, the earthquake ground-motion intensity, peak ground acceleration, earthquake disaster

摘要：

2021年5月21日在云南省漾濞县发生了6.4级地震。 文中利用收集到的21个强震仪和304个烈度计的强震观测数据, 综合考虑发震断层的地质构造背景、 震源机制解以及余震分布精定位结果, 采用震级平移的偏差校正方法快速计算获得了此次地震的地震动强度分布图。 本次地震的强震观测数据显示, 最大水平向峰值加速度为震中距7.9km的漾濞台所观测到的720.3cm/s², 其中14个台站观测到的水平向峰值加速度>45cm/s², 大量远距离、 小幅值的观测记录在一定程度上反映了本次地震的地震动衰减特征。 文中使用的震级平移的偏差校正方法能最小化台站观测值与衰减关系估计值的系统偏差, 从而降低缺少台站地区地震动参数估计的不确定性。 地震动强度图的结果表明, 漾濞地震造成的地震烈度最高为Ⅷ度, 漾濞县苍山西镇和太平乡位于Ⅷ度区内。 Ⅵ度及以上区域的面积约为6 500km², 总体呈NW向展布。 希望文中的结果能为地震应急反应决策提供理论依据和参考, 为地震灾害的应急评估提供地震危险性输入。

关键词: 漾濞地震, 地震动强度, 峰值加速度, 地震灾害