Seismoacoustic Analysis of Possible Vent‐Drying Sequence During Phreatomagmatic Activity on 13 July 2021 at Semisopochnoi Island, Alaska

Abstract Pulse‐like infrasound accompanying volcanic phenomena sometimes suggests interaction with water at the crater. We focus on pulse‐like infrasound observed at Semisopochnoi Island in the Aleutian Arc, Alaska, during the phreatomagmatic activity on 13 July 2021. Overflight observations confirmed a small, ephemeral water lake on the crater floor in June 2021. We examined temporal variations in pulse‐like infrasound activity, seismoacoustic amplitudes, and spectral characteristics. Pulse‐like infrasound activity gradually became intermittent and waned at the end of the day. Seismic‐infrasonic amplitude ratio decreased, while the infrasound frequency index increased with time. These results suggest less interaction with water due to drying out of the vent. Satellite observations also show increased ground surface temperatures inside the crater from 13 to 14 July. Numerical simulation suggests a significant topographic effect on the waveform and spectrum of the pulse‐like infrasound, which should be considered before interpreting the source process.