Abstract:
[Objective] On 21 May 2021, an M_S6.4 earthquake struck Yangbi County, Dali Prefecture, Yunnan Province — the largest event near the Weishan Basin segment of the Weixi–Qiaohou fault since 1976. The mainshock produced no surface rupture and did not occur on any known active fault. Despite numerous previous studies, the precise seismogenic structure and causative faults of this typical foreshock–mainshock–aftershock sequence (including seven M_S  ≥ 4.0 foreshocks and 22 M_S ≥ 4.0 aftershocks) remain controversial; some results are poorly constrained or even mutually contradictory. Furthermore, the occurrence of moderate-to-strong earthquakes surrounding the source area has increased significantly in recent years. This study thus aims to elucidate the rupture behaviour and seismogenic environment of the entire earthquake sequence. [Methods]Using pre-existing focal mechanism solutions of minor earthquakes as reference events, we adopted the relative centroid relocation method and time–frequency source characterisation method to calculate rupture directivity parameters for ten M_S ≥ 4.0 events. In addition, we calculated the radiation efficiency for all M_S ≥ 3.0 earthquakes based on waveform recording from local seismic station. [Results] Our results demonstrate distinct along-strike segmentation of rupture directivity across the Yangbi sequence. On the northwestern segment of the sequence, five earthquakes feature NW-striking fault planes. On the southeastern segment, four earthquakes exhibit faults either NW or NE. The largest foreshock (M_S 5.6) and the largest aftershock (M_S 5.2) both rupture toward the NE, indicating that they did not occur on the same fault as the M_S 6.4 mainshock. Combined with previous relocation and geodetic results, we interpret that these two events occurred on conjugate faults. The M_S 6.4 mainshock ruptured toward the NW, consistent with the dominant rupture azimuth of the NW segment, suggesting that the mainshock primarily ruptured the NW-trending master fault, whereas conjugate faulting in the SE segment constitutes an important component of the entire sequence. For the M_S 4.4 foreshock, the relative centroid method yields a rupture direction toward the NW, while the time–frequency source method gives a direction toward the SE. This event is therefore interpreted as a frequency-dependent bilateral rupture. We further calculated and corrected the radiation efficiency values for all M_S≥ 3.0 events. The efficiency results show a similar along-strike segmentation: the radiation efficiency differs systematically between the NW and SE segments. Integrating rupture directivity patterns, radiation efficiency measurements and regional geological constraints, we infer that the segmentation in rupture directivity is primarily caused by differences in fault frictional properties between the NW and SE segments. [Conclusions] The Yangbi earthquake sequence exhibits distinct segmentation in rupture directivity, with the NW segment dominated by NW-striking master fault rupture and the SE segment characterized by conjugate faulting. The M_S 4.4 foreshock corresponds to a frequency-dependent bilateral rupture event. The segmentation in radiation efficiency further supports the interpretation of different fault frictional regimes along the fault strike. [Significance] These findings deliver new constraints on the source characteristics and seismogenic structure of the Yangbi earthquake sequence. They improve our knowledge of the regional tectonic setting and earthquake nucleation cycle, and offer valuable seismological evidence for future seismic hazard assessment across the study area.