Large air pressure changes triggered by P-SV ground motion in a cave in northern Taiwan

Abstract A barometer in the cave of the SBCB station records an unusual phenomenon of larger amplitudes in air pressure changes inside than those at the Xinwu station (outside). Accordingly, the comparison between the recorded data at the SBCB and Xinwu station can drive investigations of potential sources of the unusual phenomenon. Analytical results of phase angle differences reveal that the air pressure outside the cave at the Xinwu station often leads air pressure changes inside at the SBCB station at relatively low frequency bands. In contrast, the larger pressure changes at frequencies > 2×10-4 Hz inside the cave at the SBCB station lead smaller changes outside at the Xinwu station. To expose causal mechanisms of the unusual phenomenon, continuous seismic waveforms recorded inside the cave at the SBCB station are further conducted for examination. When the horizontal and vertical ground velocities of ground motion yield a difference in the phase angle close to 90°, amplitudes of the air pressure changes at the SBCB station are amplified accordingly. This suggests that the pressure-shear vertical ground motion can drive air pressure changes. Meanwhile, the results shed light on investigating the existence of acoustic waves near the Earth’s surface using a partially confined space underground due to that the assumptions of the waves can propagate upward in the atmosphere driving changes in the ionosphere.