The foreshocks and aftershocks of large earthquakes provide valuable information for studying earthquake nucleation, fault rupture processes, and postseismic deformation. We investigated the spatiotemporal evolution of the seismicity 60 days before and 30 days after the 2018 Mw 6.4 Hualien earthquake, which occurred near the Milun fault (MF) in eastern Taiwan. By applying the matched‐filter technique to continuous waveform data, we identified approximately two times more earthquakes than listed in the standard Central Weather Bureau earthquake catalog. The spatial distributions of the foreshock and aftershock hypocenters are consistent with one of the mainshock nodal planes. We found foreshocks migrated to the southwest toward the mainshock hypocenter, while the aftershock area expanded immediately after the mainshock, exhibiting a northeastward step‐like logarithmic expansion. Most of the foreshocks occurred in the vicinity of the mainshock hypocenter, and some of them had high waveform similarities, implying they occurred virtually in the same region and were driven by aseismic slip. Our results suggest the existence of a blind southeast‐dipping fault cutting the MF. The step‐like extension of aftershocks is most likely due to fault segmentation.

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