When an object moves through the atmosphere, it induces pressure fluctuations. If the scale of the object’s movement spans several hundred meters to several kilometers, the period of the pressure fluctuation will range from several seconds to several tens of seconds, determined by the ratio of the scale to the speed of sound. These pressure fluctuations manifest within the frequency range between audible sound (> 20 Hz) and infrasound (< 20 Hz and > 3 mHz). The infrasonic waves, characterized by their low-frequency nature, present a promising avenue for long-distance remote sensing, particularly in the monitoring of geophysical events with the potential for widespread destruction, such as earthquakes and tsunamis. We developed sensors of both membrane and microphone types to monitor infrasound in lower or higher frequency, respectively. These sensors were installed on the Pacific side of Japan to observe infrasound mainly generated by tsunamis. The key advantage lies in the faster propagation speed of Lamb waves ( \(\sim\) 310 m/s) compared to that of tsunami waves ( \(\sim\) 200 m/s), making them a valuable precursor for timely detection. By combining with a small MEMS 3-axis accelerometers, this study explores the feasibility of identifying a sudden atmospheric pressure surge within the infrasonic frequency range, following a significant earthquake. The observed surge serves as a crucial indicator of the imminent arrival of large tsunami waves along the shoreline situated between the earthquake’s epicenter and the observation site. The practical implication of this detection methodology is profound, as it could potentially trigger early warning systems, leading to timely and targeted evacuations. These infrasound sensors effectively detected infrasound produced not only by tsunamis but also by volcanic eruptions, meteorite explosions, and so on. This research focuses on the significance of monitoring geophysical events in the infrasound frequency range by demonstrating the development of our infrasound sensors and the natural phenomena captured by these instruments. The study presents observational results based on three types of sensors (INF01LE, INF03, and INF04LE), each exhibiting a flat response within specific frequency ranges: 0.003 to 6.25 Hz, 0.1 to 100 Hz, and 0.1 to 100 Hz, respectively. These sensors have been rigorously tested across various natural phenomena to validate their effectiveness in capturing infrasonic waves. The findings of this research contribute to the growing body of knowledge on infrasonic wave coming from various geophysical disaster perspectives.
