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Abstract The Hualien city is located near the pole of rotation induced by the along-strike transition from Ryukyu subduction to the collision between the Luzon arc and the Chinese continental margin in northeastern Taiwan (Rau et al., J Geophy Res 113: B09404, 2008). Such a tectonic regime creates varied seismogenic structures that are responsible for the ruptures of the 1951 M L 7.3 Hualien-Taitung earthquake sequence (Chen et al., J Geophy Res 113: B02304, 2008), the 2018 M w 6.4 Hualien earthquake sequence (e.g., Rau and Tseng, Terr Atmos Ocean Sci 30: 281–283, 2019), the 2021 M L 6.2 Shoufeng earthquake sequence, and numerous and persistent moderate-to-small size earthquake swarms in the Hualien area. This special issue was introduced by the several month-long 2021 episode of moderate-to-small size earthquake swarms occurred in the Hualien area (Figs. 1 and 2, Central Weather Bureau, 2022). The 2021 Hualien earthquake episode includes small-to-moderate-size earthquakes that release minute strain energy on the small asperities and larger events (M > 5.5) that ruptured nearby asperities. Figure 3 shows the spatiotemporal distribution of the Hualien earthquake sequence between April 1 and December 31 of 2021. The 2021 Hualien earthquake sequence provides us a great opportunity to examine the earthquake physics, characteristics, seismogenic processes, and the early warning algorithm of an earthquake swarm, which are the key elements for the understanding of generation of earthquakes. In this special issue, we have collected six papers that detailed various aspects of the 2021 Hualien earthquake sequence, which provide us a better understanding of the nature of the Hualien earthquake swarms. Fig. 1 The crustal (≤ 40 km) Hualien earthquake swarm activities observed from April to December of 2021. Star symbols show the epicenters of the largest three earthquakes in this time period. Rainbow color represents the focal depth of each event. The radius of each circle is proportional to the observed ML Fig. 2 The temporal distribution of seismicity in the upper 40 km beneath this area observed from April to December of 2021. Several swarm activities can be identified, in which intense seismicity happened within a short time interval. The main earthquake swarms occurred from April to July of 2021. Numbers indicate the days after April 1st, 2021 Fig. 3 The spatiotemporal distribution of the Hualien earthquake swarm activities from April to December of 2021. The symbol color represents the time in days after April 1st, 2021