The M_S6.4 earthquake occurred in Yangbi County, Dali Prefecture, Yunnan Province on May 21, 2021, the epicenter is located at the southwest boundary of Sichuan-Yunnan rhomboid block, where Weixi-Qiaohou-Weishan Fault meets Honghe Fault. According to data released by the China Earthquake Networks Center, the Yangbi M_S6.4 earthquake is the only earthquake in recent years with M_S>6.0 in the epicentral region. The National Strong-Motion Observation Network System(NSMONS)of China has built strong motion stations with relatively large density in Yunnan and Sichuan Provinces, NSMONS has obtained a large number of high quality strong ground motion acceleration recordings during this earthquake. In the process of earthquake, the different characteristics of strong ground motion often lead to the different characteristics of building structure damage in the epicentral region. In-depth analysis of ground motion observation characteristics is helpful to deepen the understanding of earthquake damage.
In this study, the NGA-West 2 data processing flow and reasonable and reliable high-pass filtering frequency selection criteria were used to process 29 sets of strong ground motion acceleration recordings of the earthquake, we obtained reliable peak ground acceleration(PGA), peak ground velocity(PGV), and 5%damped acceleration response spectra(SA). We drew the spatial distribution maps of PGA and PGV in the E-W, N-S, and U-D directions, compared the observed ground motion PGA, SA with the calculated values of the ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, and analyzed the amplitude and time-frequency characteristics of the observed ground motions, and a comparative analysis was performed between the spectral acceleration recorded by the near-field stations with the design spectra of the code for seismic design of buildings in China. Combined with the on-site earthquake damage investigation, the main reasons for the lighter structural damage in the meizoseismal area were analyzed.
The results show that the maximum horizontal PGA and PGV of the Yangbi M_S6.4 earthquake recorded by the station both locate near the epicenter and the horizontal ground motion attenuates the slowest along the north-northwestern direction, which is basically the same as the long axis direction of isoseismals of seismic intensity map released by Yunnan Earthquake Agency. However, the vertical ground motion attenuates the slowest along the near north-south direction. The actual observation values on the soil site in the Yangbi earthquake are in good agreement with the calculated values of the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, while the observation values on the bedrock site are smaller than the calculated values of the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, which indicates the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region derived from the transfer method may over-predict the observation values on the bedrock site. According to the time-frequency diagram obtained by using the wavelet transform, the energy recorded in the EW and NS directions of the station 53YBX, which is the nearest station to the epicenter, is mainly concentrated in 8~15Hz, and the corresponding period range is 0.07~0.13s, while the energy recorded in the UD direction is mainly concentrated in 20Hz, and the corresponding period range is about 0.05s. When the period is smaller than 0.14s, the spectral accelerations in the EW and NS directions of the station 53YBX is significantly higher than the basic ground motion design spectra and rare ground motion design spectra of the code for seismic design of buildings in China; the remarkable period of the acceleration response spectra is 0.1s, the reaction spectrum decreases rapidly when the period is greater than 0.1s, the spectral acceleration corresponding to the superior period in the N-S direction of station 53YBX is 3.87 times the value of the rare ground motion design spectra platform. Most buildings near the epicenter are the one-story old timber frame structures which were built in the 1980s and 1990s, with a natural period of 0.1s, and the 2~3-story brick and concrete frame structures which were built in recent years. According to the analysis of above ground motion characteristics of the Yangbi M_S6.4 earthquake, the most serious damage of this earthquake is the one-story old timber frame structure, while the 2~3-story brick and concrete frame structure has little damage or very light damage. This phenomenon should be related to the characteristics of the structure itself and the disrepair of the structure, the extremely rich ground motion component with a period of 0.1s, and the relatively less ground motion component with a period of more than 0.14s.

The research on the exploration method of velocity structure of the site soil layer that is efficient, economic and easy for promotion and application is of great significance considering the importance of shear wave velocity structure in shallow underground for prediction and prevention of geological hazards. With no dependence on special hypocenter, no need for destructive drilling and a wide range of detectable depths, microtremor array applies to densely populated cities and plain areas, and has become one of the new research focuses in the field of geophysical exploration at home and abroad in recent years.
In the study of inversion record of wave velocity profile on shallow soil layer by using array observation records, surface wave dispersion(DC)or microtremor horizontal-to-vertical spectral ratio(MHVSR)inversion is generally carried out separately at present, but the velocity structure of inversion is often of obvious multi-solution. The dispersion curve mainly constrains the shear wave velocity of the loose sedimentary layer while the predominant frequency estimated from the peak value of MHVSR mainly constrains the thickness of the overburden. In addition, various results of the Rayleigh-wave dispersion curve indicate that the calculated frequency range of the phase velocity is higher than the predominant frequency. In view of this, a joint inversion method of DC and MHVSR is developed, and a new inversion strategy is proposed in this paper. Different from the existing inversion methods, in this paper, firstly the Rayleigh-wave dispersion curve is obtained from the data of microtremor array by Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method(F-K), the results are compared with the theoretical fundamental order and one high order Rayleigh wave dispersion curve calculated from the borehole data, and the measured dispersion curve is fitted. Secondly, the dominant frequency and its corresponding amplification coefficient of the site are analyzed based on the Microtremor Horizontal-to-Vertical Spectral Ratio(MHVSR)recorded from a single station. As a result of the correlation between the dominant frequency and the thickness of the overburden, the depth of the site bedrock is determined, and then the initial velocity structure of the site is obtained by the improved half-wavelength method. Finally, the best velocity structure of the site is determined by the joint inversion of DC and MHVSR, and the S-wave transfer functions caused by the vertical incidence of the inversion model and the measured borehole model obtained by different inversion methods are compared.
The advantages of the inversion method in this paper lie in two aspects. On the one hand, in the extraction of surface wave dispersion curve, the comprehensive application of Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method (F-K) widens the frequency range of extracting dispersion curve by a single method. On the other hand, in the determination of initial velocity structure, the problem of relying on certain prior information of the current other inversion methods is better solved with the improved half-wavelength method.
In this paper, the effectiveness and stability of the new inversion strategy are verified by a theoretical example and an array observation example. It is observed that the MHVSR of the single DC inversion model is different from that of the theoretical example model after the peak frequency(especially in the high frequency segment)under the initial model, and the DC of the single MHVSR inversion model is different from that of the theoretical example model in the lower frequency segment. However, the joint inversion of the two makes up for the high frequency difference of the MHVSR of the DC inversion model and the low frequency of the DC of the MHVSR inversion model, thus greatly reducing the multi-solution of the inversion model and reflecting well the site characteristics(amplification effect and predominant frequency)when the inversion model approaches the real site. Compared with the traditional seismic and electromagnetic exploration methods, the joint inversion method based on the microtremor array records in this paper is of more practical value in acquiring the velocity structure of shallow site.