Abstract
Acoustic-gravity waves are generally considered to be one of the major factors that drive changes of the total electron content in the ionosphere. However, causal mechanisms of couplings between sources in the lithosphere and responses in the atmosphere and the ionosphere are not fully understood, yet. 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 > ~ 5 × 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 are further conducted for examination. When the horizontal and vertical ground velocities of ground vibrations yield a difference in the phase angle close to 90°, coherence values between the air pressure changes and ground vibrations become large. This suggests that the pressure-shear vertical ground vibrations 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 into the atmosphere driving changes in the ionosphere.
Highlights
Acoustic-gravity waves are generally considered to be one of the major factors that drive changes of the total electron content in the ionosphere
We down sampled the air pressure data retrieved from the SBCB station to a sampling interval of 1 min for fairly comparing with them obtained fdrom the Xinwu station utilizing the Magnitude-Square Coherence (MSC) index
This study proposes an efficient method to document the physical evidence of the P-SV type ground vibrations triggering changes in air pressure near the Earth’s surface
Summary
We assessed the coherence of the amplitude within a particular frequency band using the Magnitude-Square Coherence (MSC) index[17]. The differences in the phase angles of a particular frequency band were computed to understand the leading (or lagging) of air pressure changes at the Xinwu and SBCB station. This suggests that variations in air pressure inside the cave at the SBCB station lead them outside at the Xinwu station. The coherence values are not obvious, the relatively-high values suggest that changes of the air pressure in the cave are probably dominated by the ground vibrations at the SCBC station in these particular frequency bands. These interactions could be dominated by P-SV ground vibrations, which are unclear and not fully understood in the world
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