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The 2018 Mw 6.4 Hualien earthquake generated a large peak-to-peak velocity of over 2 m/s, with a period of 3 s at the south end of the Milun fault, which resulted in the collapse of five buildings. To investigate the shallow subsurface soil structure and evaluate possible effects on the ground motion and building damage, we performed microtremor measurements in the Hualien basin. Based on the velocity structure jointly inverted from both Rayleigh-wave dispersion curves and microtremor horizontal-to-vertical spectral ratio data, we found that the shallow subsurface structure generally deepens from west to east. Close to the Milun fault, the structure becomes shallower, which is consistent with faulting during the 2018 earthquake and the long-term tectonic displacement. There is no significant variation for the site conditions in the north–south direction that can explain the large peak ground velocity in the south. As a result of the dense measurements in the heavily damaged area, where three high-rise buildings totally collapsed, these locations have the average S-wave velocity of the upper 30 m (AVS30) values and are relatively high compared to the more distant area from the Meilun River. This is somewhat unusual, because lower AVS30 values indicating softer ground conditions are expected close to the river. We did not find any characteristic subsurface soil structure that may contribute to the building collapses. The large 3 s pulse was probably generated by source effects, rather than subsurface soil amplification.

On 5 February 2016, a moderate earthquake occurred in southwestern Taiwan with M L 6.4 and a focal depth of 16.7 km. This earthquake caused damage to a few buildings and 117 casualties. A low‐cost earthquake early warning (EEW) system ( P ‐alert) is in operation for the purpose of EEW and for providing near‐real‐time shake maps. During this event, a detailed shaking map was generated by the P ‐alert system within 2 min after the earthquake occurrence, and the high shaking regions strongly correlated with the locations in which the damage and casualties occurred. In the field, individual P ‐alert devices also serve as onsite EEW systems using P ‐wave information. The individual P ‐alert provided a 4–8 s lead time before the arrival of violent shaking in the damaged regions. For regional EEW, both the Central Weather Bureau (CWB, official agency) and the P ‐alert system responded very well. Currently, regional warnings in Taiwan are only provided to cities at epicentral distances of 50 km or more by the CWB. For cities within a 50‐km epicentral distance, the P ‐alert system could be useful for providing onsite EEW. The performance of the P ‐alert network during this earthquake proves the efficiency of this real‐time, low‐cost network in terms of early warning (regional and onsite), near‐real‐time shake maps, rapid reports, and strong‐motion data for research purposes.