Study on the rupture directivity of the 2021 Yangbi earthquake sequence

This study used the strong-motion recordings of the M_S6.4 Yangbi earthquake sequence in Yunnan Province to establish the prediction equations of ground motion parameters, and then estimated the focus rupture directivities characteristics for nine Yangbi earthquakes with abundant recordings in a good spatial station coverage based on the rupture directivity effect fitting method. The results indicated that the rupture directivity effects are observed in four of these earthquakes （i.e., the 1st, 4th, 8th, and 9th earthquakes）. The four earthquakes are all characterized by the bilateral ruptures with various predominant rupture directions （i.e., southeast for the 4th and 8th events, northwest for the 1st event, and southwest for the 9th event）, which illustrates the rupture complexities of the Yangbi earthquake sequence. Since the estimated rupture velocity （about 2.2 km） of the mainshock （the 4th event） is very slow, its rupture directivity effects are not strong and mainly affect the peak ground velocity. However, the rupture directivity effects for the other three earthquakes are very strong and the rupture velocities are faster than that of the mainshock. The dependency of the rupture directivity effects on the period was also observed in the four earthquakes.