基于经验格林函数法的2021年 MS6.4 漾濞地震近场强震动模拟

doi:10.3969/j.issn.0253-4967.2023.04.004

摘要/Abstract

摘要：

由于震中附近强震台分布不均, 文中联合使用近场2个国家强震动观测台网的强震台和7个云南地震预警台网烈度台的强震记录, 采用经验格林函数法构建了特性化震源模型, 并利用此模型对近场强震动进行了模拟。结果表明: 烈度台的记录可与强震台的记录联合, 共同作为强震动模拟的对象, 但应注意区分二者的有效频带; 在0.20~30.0Hz频带, 在53YBX台处的NS分量上, 模拟结果的伪加速度反应谱较好地再现了0.1s处的峰值, 在EW分量上, 合成的速度波形虽然幅值较低, 但较好地再现了速度脉冲波段; 在53DLY台处的合成波形较好地再现了约2s的长周期地震动; 在0.50~30.0Hz频带, 合成波形和反应谱与信噪比较高的烈度台的记录较为一致。文中确定的用于模拟强震动的特性化震源模型由一个强震动生成域构成, 其面积和相应的短周期范围内加速度震源谱的水平段幅值与地震矩的关系均遵循经验标度律。

关键词: 经验格林函数法, 强震台, 烈度台, 强震动模拟, 强震动生成域

Abstract:

Both short(0.1s)and long(1~2s) periods ground motions were observed at the near-field strong-motion stations during the mainshock of the 2021 M_S6.4 Yangbi earthquake, Yunnan. As there are only two strong-motion stations observed by the National Strong Motion Observation Network System of China in the near-field, the strong ground motion records at seven intensity stations observed by the Local Earthquake Early Warning Network of Yunnan Province are combined to construct the characterized source model with the empirical Green's function method. The broadband strong ground motions in the near-field are synthesized by using the characterized source model. We firstly select out an M_W4.3 foreshock as the empirical Green's function event. Then, we calculate the observed source spectral ratios at seven stations assuming that the source spectrum obeys to the ω^-2 law. The results show that both the observed source spectral ratios in the frequency of 0~0.2Hz at strong-motion stations and 0~0.5Hz at intensity stations deviate from the ω^-2 law assumption. Thus, we determine the size of the sub-fault by fitting the observed source spectral ratio averaged at two strong-motion stations to the theoretical source spectral ratio in the frequency of 0.2~30.0Hz. Taking two strong-motion stations and four intensity stations as the target stations, we adopt the simulated annealing method to determine the optimal parameters required for ground motion synthesis, such as proportional parameters N and C, the position of rupture starting point within the strong motion generation area, rupture velocity, and rise time, when the misfit function between synthesized and observed ground motions reach the minimum at the target stations. We fix the rupture starting point at the hypocenter of the mainshock, and determine N=4, C=3.69, rupture velocity of 2.5km/s, and rise time of 0.56s. In this study, the characterized source model used for the strong ground motion simulation consists of one strong motion generation area which is 5.4km along the strike direction and 5.4km along the dip direction, and with the stress drop of the strong motion generation area of 12.8MPa. In general, the ground motions synthesized by the characterized source model and those optimal parameters are comparable with the observed ground motions at the target stations. Further, we apply those optimal parameters to synthesize ground motions at other three intensity stations which are not taken as the target stations. In consideration of the different instrumental responses in the strong-motion and intensity stations, we impose different frequency band of bandpass filters on both the observed and synthesized ground motions, i.e. the frequency band is 0.2~30.0Hz for the ground motions at the strong-motion stations, while the frequency band is 0.5~30.0Hz for the ground motions at the intensity stations. The ground motions of the mainshock in the 53YBX strong-motion station are characterized by the large amplitude of the response spectrum around 0.1s which obviously exceeds the level of design response spectrum for rare earthquake, and the pulse(~1s)is observed in the east-west direction. The synthesized ground motions in this study are in good agreement with the above characteristics, except that the amplitude of the synthesized ground motions around 1s is smaller than the observed one, which may be caused by the small amplitude of the empirical Green's function. Moreover, the long-period(~2s)ground motions in the 53DLY strong-motion station are reproduced by the synthesized ground motions. On the other hand, the synthesized ground motions in the frequency band of 0.5~30.0Hz agree well with the observed ground motions in the intensity stations with sufficiently large signal-to-noise ratios. Finally, we examine the relationships between two source parameters and the seismic moments. The strong motion generation area and the flat amplitude of the acceleration source spectrum in the short period range are found to have linear relations with the seismic moment, which is consistent with the empirical scaling relationships. In the future work we will continue to examine the applicability of the empirical scaling relationships for prediction of strong motions by analyzing more earthquakes with different magnitudes in China.

Key words: empirical Green's function method, strong-motion station, intensity station, strong ground motion simulation, strong motion generation area

吴浩, 入倉孝次郎, 林国良. 基于经验格林函数法的2021年 M_S6.4 漾濞地震近场强震动模拟[J]. 地震地质, 2023, 45(4): 864-879.

WU Hao, IRIKURA Kojiro, LIN Guo-liang. THE EMPIRICAL GREEN'S FUNCTION-BASED SIMULATION OF NEAR-FIELD STRONG GROUND MOTIONS OF THE 2021 YANGBI EARTHQUAKE(M_S6.4), YUNNAN, CHINA[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(4): 864-879.

图/表 10

图 1 主震、前震和余震(MS>3.0)的震中及其周边台站分布

Fig. 1 Distribution of epicenters of mainshock, foreshocks, and aftershocks(MS>3.0), and location of seismometers.

表 1 主震和前震的震中位置及震源机制解

Table 1 Epicenter locations and focal mechanisms of the mainshock and foreshock events

发震时刻	北纬/(°)	东经/(°)	震源深度/km	M_W	走向/(°)	倾角/(°)	滑动角/(°)
2022-05-21 21:48:35	25.685	99.870	6.76	6.1	137.7	75.0	-163.0
2022-05-20 20:56:02	25.649	99.930	8.0	4.3	159.0	45.0	-115.0

图 2 各台站的观测震源谱比与最佳拟合震源谱比

Fig. 2 Observed source spectral ratio vs. best-fitting source spectral ratio.

表 2 各参数的搜索范围及其最优解

Table 2 Search ranges of parameters and their best solutions

参数	C	N	i	j	V_r/km·s^-1	t_r/s
搜索范围	3.0~5.3	3~6	1~N	1~N	(0.6~0.9)V_S	0.05~0.3
最优解	3.69	4	2	3	0.71V_S	0.14

图 3 目标台站的观测波形(黑线)与合成波形(红线)的对比

Fig. 3 Comparisons of waveforms(acceleration, velocity, and displacement)between observed(black) and synthesized(red) ground motions at six target stations.

图 4 目标台站的观测波(黑线)与合成波(红线)的伪加速度反应谱对比

Fig. 4 Comparisons of pseudo-acceleration response spectra(with 5%damping ratio)between observed(black)and synthesized(red)ground motions at six target stations.

图 5 非目标台站(L0101、 L2202、 L3001)的观测波形(黑线)与合成波形(红线)的对比

Fig. 5 Comparisons of waveforms(acceleration, velocity, and displacement) between observed(black) and synthesized(red) ground motions at L0101, L2202 and L3001 stations.

图 6 非目标台站(L0101、 L2202、 L3001)的观测波(黑线)与合成波(红线)的伪加速度反应谱的对比

Fig. 6 Comparisons of pseudo-acceleration response spectra(with 5%damping ratio) between observed(black)and synthesized(red) ground motions at L0101, L2202 and L3001 stations.

图 7 强震动生成域与凹凸体的相对位置关系断层的滑动量数据来自文献(Wang et al., 2022)

Fig. 7 Locations of strong motion generation area and asperity.

图 8 震源参数的标度律 a 凹凸体或强震动生成域面积与地震矩的标度关系; b 加速度震源谱短周期处的水平段幅值与地震矩的标度关系

Fig. 8 Empirical scaling relationships of source parameters.

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