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Observation and Theory of Strain–Infrasound Coupling during Ground-Coupled Infrasound Generated by Rayleigh Waves in the Longitudinal Valley (Taiwan)


 In this study, changes in atmospheric pressure recorded by absolute microbarometers operating in the Longitudinal Valley (Taiwan) during the passing seismic waves from strong earthquakes (Mw≥6) are systematically analyzed during the 2007–2019 period. Using a continuous wavelet transform analysis, local infrasound signals are detected for 23% of the events (21 events out of 89), with Mw ranging from 6.0 to 9.1 at a radial distance of 15 to about 4000 km from the central Longitudinal Valley. Infrasound signals are observed in the period range from about 1 to 20 s; they have maximal amplitudes ranging from 0.4 to 20 Pa and initiate predominantly during the passage of Rayleigh waves. The atmospheric pressure response to dilatational strain waves during seismoacoustic disturbances is investigated using collocated borehole strainmeter stations, and dynamic interactions between signals are characterized using a sliding windowed time-lagged cross-correlation analysis. The infrasound response shows a phase shift of −60° to −100°, with respect to the dilatation strain signal with a coupling factor of 0.002–0.006  Pa/nϵ for most of the cases (62%). Whereas acoustic pressure fluctuations are generated instantaneously by the vertical seismic velocity, the phase delay is related to the intrinsic nature of the dilatational strain. Observational strain–infrasound coupling parameters are in close agreement with theoretical estimates in the case of ground-coupled acoustic signals generated by Rayleigh waves. The study represents the first attempt to analyze ground-coupled infrasonic waves with strain waves and illustrates the potential of collocated strainmeter–microbarometer stations for basic seismoacoustic studies.