Six-Component (6C) Motions in a 50-Story Skyscraper in Kunming, Yunnan Province, China, Recorded During the Mw 7.7 Mandalay, Myanmar, Earthquake of 28 March 2025: What Caused the Beating in the Response?

The Mw 7.7 Mandalay, Myanmar, earthquake of 28 March 2025 caused devastation in Myanmar and severe damage to structures as far as Bangkok, 700 km away from the fault. The mainshock was followed 11 min later by a powerful Mw 6.7 aftershock, both of which were recorded in a densely instrumented 50-story skyscraper located in Kunming, Yunnan Province, China, 773 km northeast from the epicenter. While the ground motion at the building site and the structural response were small (∼10−4 rad peak interstory drift), the building vibration exhibited long duration (about 24 min) and beating, which was the most regular and persistent in the transverse response. Beating, observed in earthquake response of tall buildings, has been previously attributed to coupling of modal responses with close but not identical frequencies or of modal response(s) with a site resonance of similar frequency. In this article, it is shown for the first time that beating can be caused also by the input ground motion. The beating in the transverse response was caused by coupling of the fundamental transverse mode (fN,1=0.267 Hz) with narrowband energy in the ground motion (fg=0.235 Hz) carried to the site by surface waves, which resulted in beats of length TB=1/|fg−fN,1|≈31 s. The beating in the torsional response was caused by coupling of the fundamental torsional (fT,1=0.353 Hz) and longitudinal (fE,1=0.377 Hz) modal responses, resulting in beats of length TB=1/|fT,1−fE,1|≈41.7 s. Beating in the response of tall buildings during distant earthquakes is of concern because it amplifies the peak response of the lightly damped structure, which, combined with long duration, may cause fatigue and, upon repeated exposures, reduce its capacity to resist earthquakes.