Analysis of strong ground motion characteristics and directivity effect in the near-field for the May 21, 2021 MS6.4 Yangbi earthquake

Using the 158 sets of strong ground motion recordings obtained from the National Strong Motion Observation Network System (NSMONS) of China and the Local Earthquake Early Waring Network of Yunnan Province, we analyse the characteristics of observed near-field strong ground motion amplitude, acceleration response spectra and the spatial distribution of spectral accelerations at different periods for the MS6.4 Yangbi earthquake. The spectral accelerations of the near-field stations were compared with the NGA-West2 prediction models, and the on-site earthquake damage investigation results were combined to verify the directivity effect of the near-field strong ground motion of the MS6.4 Yangbi earthquake. The research results show that the peak ground acceleration (PGA) within 30 km in the near-field shows a difference in directionality from north-south (NS) to east-west (EW), and the maximum difference in PGA is 1.86 times; The spectral accelerations near the predominant period of the horizontal acceleration response spectra (SA) are generally higher than the basic design spectra. The SA corresponding to the predominant period in the NS direction of 53YBX station is 3.87 times the value of the rare design spectra platform; The spatial distribution contours show that the horizontal SAs observed in the rupture forward (south-east of epicenter) are significantly larger than that in the rupture backward, which is more obvious in a short period, and the difference gradually decreases with the increase of the period; The short-period SAs observed in the rupture forward is greater than the median predictions of four NGA-West2 prediction models, while the SAs observed in the rupture backward is smaller than the median predictions of four NGA-West2 prediction models throughout the period, showing a significant directivity effect; The earthquake damage investigation results also show that the damage of buildings in the near-field shows a more severe directivity difference in NS direction than EW direction. © 2021, Science Press. All right reserved.