关键词: 2021年漾濞地震, 随机有限断层法, 地震动模拟, 地震动影响场

Abstract:

In this study, we simulated the strong ground motion from the M_S6.4 earthquake that occurred in Yangbi, Yunnan Province on May 21, 2021, with stochastic finite fault method. The peak ground acceleration(PGA)distribution within the range of (25.25°~26.15°N, 98.5°~100.8°E) of 920grid points was synthesized and the impact field of this earthquake ground motion was also obtained. According to the information of strong ground motion records released officially by Institute of Engineering Mechanics, China Earthquake Administration, it can be seen that stations are sparse in the area near the epicenter, therefore, we chose 4 strong motion stations(53YBX, 53YPX, 53BCJ, 53LKT)with horizontal peak ground acceleration(PGA)greater than 10cm/s² in the range of 150km of epicentral distance to simulate the strong ground motions and obtained the synthetic acceleration time series and acceleration response spectrum(PSA)with a damping ratio of 5%for the four stations. Simultaneously, we compared the synthetic results with observed ones and found that the value of synthetic peak ground acceleration and the shape of accelerograms fit well with the records of stations 53YBX, 53YPX, and 53BCJ. Besides, the simulated acceleration response spectrums for the above three stations are consistent with the observed ones, and the average logarithmic error is between ±0.5, which suggests a good agreement for both high and low frequency. However, for station 53LKT, although the value of synthetic peak ground acceleration fits well with the observed one, the shape of acceleration time series has some differences with the observed, and the fitting degree is not as good as other three stations. Besides, there is a better agreement for high frequency than low frequency in the acceleration response spectrum, the simulated result is higher than the observed in low frequency for station 53LKT. The reason for this phenomenon is complex and may be associated with the site condition and so on, so further study is needed for the specific reasons. For the above mentioned four stations, the results show that there are some differences in duration between the synthetic acceleration time series and the recorded data. The cause for such differences may be described as follows: In the stochastic finite fault method, a time window is added to white Gaussian noise to control its shape and make sure that the windowed white Gaussian noise is similar to the real acceleration time series, and also, the path duration is expressed by a simplified theoretical duration, thus there are something different between the windowed white Gaussian noise and the real acceleration time series. Therefore, the simulation results cannot reflect the complex propagation process of seismic waves well. The result of PGA distribution shows a maximum peak ground acceleration of 875cm/s² located near the epicenter. The value of simulated maximum peak ground acceleration is beyond the range of 186~372cm/s², which is the peak ground acceleration range corresponding to intensity Ⅷ. The peak ground acceleration of 720.3cm/s² recorded by station 53YBX is also beyond the range of 186~372cm/s². To a large extent, the cause of this phenomenon may be related to the topography of the region of Yangbi County. About 98.4% of the area of Yangbi County is mountainous area. Consequently, mountain topography is the most widely distributed terrain in this county, and geological disasters are frequent. However, most buildings around the epicenter are not high, the intensity obtained from the damage degree could not reflect the value of peak ground acceleration clearly. Other earthquakes in Yunnan also have similar phenomena. For example, the Ludian M_S6.5 earthquake that happened in Yunnan in 2014 had a maximum intensity of Ⅸ, but the actual peak acceleration recorded by strong motion station reached 949.1cm/s², which is also beyond the peak ground acceleration range corresponding to intensity Ⅸ. The impact field of ground motion obtained in this study is approximately(25.25°~26.15°N, 99.3°~100.5°E), which is consistent with both the predicted ground motion influence area for Yunnan Yangbi M_S6.4 earthquake from Institute of Geophysics, China Earthquake Administration and the area shown in the seismic intensity map issued by Yunnan Earthquake Agency for the Yangbi M_S6.4 earhtquake, which suggest the effectiveness of the simulation results. Although the simulation results are consistent with the observed ones on the whole, there are still a few stations that have a little deviation. The main reason is that some parameters in the process of simulation are obtained by empirical formula and the site conditions are not well reflected, these are the aspects that need to be improved in the process of simulation. And it’s necessary to establish a more accurate model in order to realize accurate simulation and forecast the strong ground motions in the future. Using stochastic finite fault method to simulate ground motion can fill the gap of strong motion records to some extent and the synthetic ground motion results of this study can provide a scientific basis for post-earthquake relief, post-disaster reconstruction, and seismic design in this area.

Key words: 2021 Yangbi earthquake, stochastic finite fault method, ground motion simulate, impact field of ground motion