Characteristics of Strong Ground Motions from Four M-S >= 5.0 Earthquakes in the 2021 Yangbi, Southwest China, Seismic Sequence

A seismic sequence has struck the Yangbi county, west part of Yunnan Province in southwest China since May 18, 2021 . This region experienced the strongest ground shakings on the night of 21 May with the successive occurrence of four earthquakes with M-S >= 5.0 in a short time and more small earthquakes. Large numbers of strong-motion recordings obtained in the four earthquakes with M-S >= 5.0 (i.e. the M-S 5.6 foreshock, the M-S 6.4 mainshock, the M-S 5.0 and M-S 5.2 aftershocks) were analyzed in detail to characterize the source effects, distance decay, etc. The velocity pulse-like waveform at near-source station 53YBX and the significant asymmetry of distance decay along the SSE-NNW direction (i.e. much slower toward the SSE) in the M-S 6.4 mainshock seem to hint the occurrence of the asymmetrical source rupture. The source rupture directivity for the four events were investigated by fitting the azimuth-dependent residuals of the PGAs. The M-S 6.4 mainshock is characterized by the significantly asymmetrical bilateral rupture propagation, predominantly in the SSE direction with a rupture velocity of about 2.24 km/s. The PGA and PSAs from the four earthquakes were further compared with the predicted medians of the ASK14 and CY14 prediction models for global shallow crustal earthquakes. The negative inter-events residuals reflect the weaker source contributions of the Yangbi events to ground motions. The source contributions to short-period ground motions approximately show the dependence on earthquake type, that is, M-S 5.6 foreshock > M-S 6.4 mainshock > M-S 5.2 aftershock, consistent with the dependence of stress drop on earthquake type. We further noted that the inter-events values for the M-S 5.0 aftershock are far below those for the M-S 5.2 aftershock. This may be attributed to the differences in the spatial and temporal distance to the mainshock, that is, weak source contributions from the close aftershocks to the mainshock. The values of adjustment coefficient Delta c(3) are almost positive, indicating the weaker anelastic attenuations in the study region. Finally, we also found that the distance scaling of significant duration can be well described by both empirical models (AS16 model for predicting significant duration and the BT14 model for empirically illustrating the ground-motion path duration).