Meteotsunamis or meteorological tsunamis are tsunami-like sea-level oscillations that take place in closed or semi-closed water bodies in bays or lakes with periods ranging from minutes to several hours1, As the generation of these tsunamis is related to one of the natural oscillation modes of the bay or the lake, their period and amplitude are a function of the size, depth and the configuration of the coastline2. The temporal and spatial characteristics of meteotsunamis and seismic tsunamis are similar, and shifting atmospheric disturbances, which are usually caused by sudden atmospheric pressure and/or wind changes, are the significant factors that will induce oscillations in water bodies. Atmospheric energy is transferred to a body of water more efficiently and in a more concentrated manner when the propagation speed of the atmospheric disturbance is approximately equal to the local free wave speed. This process is even more efficient if the water depth is within the optimal range3. Stronger atmospheric disturbances usually generate larger scale oscillations. Meteotsunamis are associated with frontal passages4, cyclones5,6, atmospheric gravity waves7, and mesoscale convective systems8,9, including derechos10, and have been reported worldwide1, 11. Importantly, their impacts on human communities and infrastructure are often severe 11 due to their high wave runup and strong associated currents12,13,14,15. Even meteotsunamis with moderate heights (~0.3 m) generate hazardous currents16,17. Owing to the ubiquitous nature of atmospheric disturbances, associated meteotsunamis can add to the risk posed by seismic tsunamis18 or can increase the risk to regions not traditionally recognized as seismic-tsunami-prone19. However, our quantitative understanding of the associated risks and frequency, generation, and propagation mechanisms related to meteotsunamis is limited20,21.
A typhoon is defined as a tropical cyclone (TC) when it develops in the Northwestern Pacific Basin, which has been recognized as one of the most active tropical cyclone areas in the world22. In Japan, typhoon measurements have been recorded since 195123. In Tokyo Bay, meteotsunamis induced by typhoons and the sea surface currents induced by meteotsunamis have been respectively measured with a tide gauge24 and High Frequency Radar (HFR)25. Results from these measurements are consistent with each other, and have differentiated two oscillation modes (OM) in Tokyo Bay, more specifically, OM1 (usually 2-3 hours duration) and OM2 (usually 5-6 hours duration). It has been interpreted that OM1 occurs when the tsunami is confined in the northern part of Tokyo Bay, whereas OM2 is associated with the tsunamis that occur throughout the entire longitudinal length of Tokyo Bay25.
Muography is similar to x-ray imagery, but it utilizes the strong penetration capability of high-energy muons (> a few tens of GeV) and their relativistic effect. Since the number of muons that pass through gigantic bodies reflects the interior spatial distribution of density, this distribution can be mapped by identifying where these muons passed through the object and subsequently creating a plot of the number of penetrating muons on a 2-dimensional plane. The origin of these high-energy muons is galactic cosmic rays (GCR) which are accelerated by high-energy events such as supernovas in our galaxy. The GCRs mainly consist of protons and alpha particles. These charged particles are generally accelerated to close to the speed of light. Although the galactic magnetic field is mainly aligned with the spiral galaxy, there is also a random component. The direction that cosmic rays travel is strongly affected by this random component of the galactic magnetic field. As a result, cosmic rays travel for millions of years (depending on their energy) before arriving at the Solar system. Consequently, by the time they arrive here, their initial direction of origin is completely lost as they have obtained an isotropic distribution of arrival directions. These cosmic rays interact with the Earth's atmosphere, and muons are generated. Muography takes advantage of the characteristics of muons, particularly their penetrative nature and universality, for a wide variety of applications, including visualizing the internal structure of volcanoes, tunnels, natural caves, and cultural heritage. So far, applications have focused in Africa26,27, the Americas27,29,30, Asia31,32,33,34,35,36,37,38,39,40,41, and Europe42,43,44,45,46,47,48.
The Tokyo-Bay Seafloor Hyper-Kilometric Submarine Deep Detector (TS-HKMSDD) consists of a linear array of several particle detectors located inside the underwater tunnel called the Tokyo Bay Aqua-Line (TBAL). The first and second segments of TS-HKMSDD were respectively installed in March 202123 and June 2021. The current total length of TS-HKMSDD is 200 m with a total active area of 3 m2. This article reports the results found thus far, including a meteotsunami-like periodical oscillation in muon flux as observed with TS-HKMSDD right after the typhoon approach to Tokyo.