Large air pressure changes in a cave in northern Taiwan

Two barometers were installed outside (Xinwu station) and inside a cave (SBCB station) to investigate the potential source of the unusual larger pressure changes in the cave. Analysis of phase angle differences reveal that the air pressure outside the cave often lead air pressure changes inside. However, the larger pressure changes at frequencies > 2×10-4 Hz inside the cave lead smaller changes outside. On the other hand, continuous seismic waveforms recorded inside the cave suggest that the ground motion drives changes in air pressure when the difference in the phase angle is close to 90° between the horizontal and vertical ground velocities. This study proposes a potential mechanism of the pressure-shear vertical ground motion driving air pressure changes and helps us understand the origin of acoustic waves propagating upward from the near surface to the atmosphere.