Determining the Source of the Explosive Sound Heard in Hokkaido, Japan, on 26 April 2021

Abstract

An explosive sound was heard over a wide area of Sapporo city, Hokkaido, Japan, on 26 April 2021. It was accompanied by minor shaking and was considered likely to be a sonic boom produced by a fireball. However, no luminous phenomenon was observed. We analyzed seismic data to identify the source of the sonic boom, and estimated the trajectory of the object from the associated signal arrival times. The source was located ∼100 km off the west coast of Hokkaido at an altitude of 50 km. The elevation angle was greater than 66° and the estimated velocity was higher than that of a fighter plane. Although the luminous phenomenon was not observed, the trajectory model suggested that the source of the sonic boom was not an aircraft and most likely a fireball. We compared trajectory models estimated from optical and seismic observations for past sonic boom events, and they were found to be in good agreement. Although our trajectory model is simplified and assumes that sound and the fireball have a constant speed, the model is highly accurate. The results show that seismic data can be used to estimate the trajectory of objects producing sonic booms, even when visual observations are unavailable.