Wasilewski (2014) and Tan et al. (2008) reported that the amplification effect of air pressure changes in deep mines can be caused by variations of the environments on the Earth’s surface due to the partially confined interior of tunnels. However, the large air pressure reported in this study is observed in the shallow cave with short aisles. The large air pressure is mainly limited within the relatively-high frequency band of > 2 × 10− 4 Hz. Those are significantly different with the reports in Wasilewski (2014) and Tan et al. (2008). This suggests that variations of the environments outside the caves are not the major factor of the large air pressure observed inside in this study. Otherwise, the air can be squeezed out from the cave. However, the pressure variations caused by air blowing are hardly monitored at the Xinwu station about 25 km away due to that the difference of 0.5 mb becomes smaller with the propagation via dispersion.
Previous studies (Artru et al. 2004; Hao et al. 2013; Liu et al. 2016) reported that changes in air pressure can be triggered by the arrival of propagating Rayleigh-like (Pressure-Shear vertical; P-SV) waves. Thus, beside the air blowing in the atmosphere, large-scale ground motion forces the ground and perturbs the air that can be one of the candidates for resulting the lags. The large-scale ground motion amplifies variations in air pressure changes inside the cave due to the confinement of the surrounding rocks and influence surface air pressure. In other words, relatively-large variations should result from activities inside the cave or beneath the ground that shows the possible connection between changes in ground motion and air pressure changes.
To examine the connection, continuous seismic waveforms (i.e., seismic data) were also analyzed in this study to understand how ground motion triggers air pressure variations. We computed the coherence and the phase angle difference varying with frequencies between the vertical ground velocity (Fig. 2a) and the air pressure inside the cave (i.e., both at the SBCB and Xinwu stations). A low coherence close to 0.2 in most of the frequency bands (Fig. 2b) suggests that, in typical condition, changes in air pressure are generally uncorrelated to ground motion. However, ground motion leads to changes in air pressure inside the cave (red circles in Fig. 2b), which can be observed in the frequency bands (e.g., close to 4 × 10− 4–7 × 10− 4 Hz, 1.5 × 10− 3–4 × 10− 3 Hz, 6 × 10− 3–7 × 10− 3 Hz in Fig. 2d) that exhibits the higher coherences (> 0.35). This finding suggests that ground motion can drive changes in air pressure in the cave. Meanwhile, the changes in the cave were amplified due to the surrounding materials. We thus investigate whether air pressure at the Xinwu station changes accordingly or dissipates due to dispersion.
We distinguished changes in air pressure outside the cave that are driven by ground motion using the same method (i.e., coherence and phase angle differences). Similarly, changes in air pressure outside the cave are almost uncorrelated with ground motion, except for several specific frequencies close to 3 × 10− 4–9 × 10− 4 Hz, 1.5 × 10− 3–4 × 10− 3 Hz, and 6 × 10− 3–7 × 10− 3 Hz (Figs. 2c and 2e). In these frequency bands, we can find that ground motion leads changes in air pressure outside the cave exhibiting the higher coherence (> 0.35).
One of the characteristics associated with P-SV waves is the 90° phase angle difference between the horizontal and vertical components. We computed the maximum horizontal amplitude as the horizontal component (Fig. 2a) by using the East-West and North-South ground velocities utilizing the method proposed by Tanimoto et al. (2006). We determine the ground motion with the differences of the phase angle between the horizontal and the vertical component are from 75° to 105° and from − 105° to -75° as P-SV waves. We integrated coherences that were larger than distinct thresholds ranging from 0 to 0.35 and counted the numbers associated with the P-SV waves for the different criteria. The P-SV motion ratios (number of the P-SV waves at each frequency grid / total number of the frequency grids) are proportional to the coherence, and increase to double the average (i.e., 0.017 = 60/360) for a threshold of 0.35 (Fig. 2f). In short, both air pressure and ground motion share frequencies that are dominated by the existence of the P-SV ground motion.
We try to evaluate air pressure changes dominated by variations of the volume of the cave through the ideal gas law (Clapeyron, 1834). We assumed that a total number of moles and temperature of air inside the cave are constant, while ground vibrations trigger changes in air pressure without break and/or damage. The volume of the cave in this study is approximately 270.00 (= 1.5 in width x 1.8 in height x 100 in length) m3. If the P-SV ground vibrations contribute changes of 0.5 mb in air pressure, the volume reduces to approximately 269.87 m3, accordingly, for maintaining the product of the air pressure and the volume. If the reduction of the volume is mainly contributed by the vertical component of the vibrations, the ground in the cave uplifts about 10− 3 meters. The comparable results between the observation and the model suggest the large air pressure changes in a cave can be attributed to the P-SV-type ground vibrations.
If the P-SV ground motion can drive changes in air pressure, the question is how often the interaction can be detected. The interaction of events by using both the P-SV ground motion and a coherence value > 0.35 at each particular frequency can be determined. The total number of interaction events was generally maintained at 20 during the study period of 930 days (from January 1, 2015 to July 19, 2018; in Fig. 3). This finding suggests that interactions permanently occur every day. These interactions can be dominated by the P-SV microseisms triggered by the interaction between oceanic waves and land (e.g., Cessaro 1994; Kimman et al. 2012).