Assessment of Site Response and Building Damage Following the Mw 6.4 Copiapó Earthquake

On June 6, 2025, an inslab earthquake of Mw 6.4 occurred in the north‐central region of Chile, near the city of Copiapó, within the Atacama seismic gap. Despite its moderate magnitude, the event produced strong ground motions with recorded accelerations exceeding 0.4 g and widespread nonstructural damage. Due to these unusual consequences, we conducted a multidisciplinary field study, collecting in situ data, which provides an opportunity to characterize both local site conditions and structural behavior in a mature seismic gap region. Shallow shear‐wave velocity profiles and predominant site periods were derived from ambient‐vibration measurements to evaluate site amplification effects in Copiapó. Damage surveys revealed widespread nonstructural damage in designated evacuation zones, significantly impacting the functionality of numerous facilities. Finally, using ambient‐vibration measurements, we estimated the structural properties of nine buildings (five damaged and four undamaged) of a residential housing complex located within the affected area. Despite their similar architecture and design, we identified systematic differences in the dynamic properties of the damaged buildings compared to the undamaged ones. On average, damaged buildings presented approximately 20% larger fundamental vibration periods and 10% higher damping ratios than undamaged buildings. The findings of this field survey provide insight into the vulnerability of buildings under moderate seismic events, whose seismic demand remains below that expected during a major earthquake.