Megathrust earthquakes in subduction zones often go unreported because they are rare and the historical record is short. On the Ryukyu subduction zone of southwestern Japan, unlike neighboring Nankai Trough, the history and future potential of great interplate earthquakes are not well known. While the geodetic measurements on the islands suggest that the plate coupling is very weak, recent observations of slow seismic events as well as offshore geodetic measurements imply the presence of coupled patches along the megathrust. Furthermore, the historical and geological studies indicate evidence of great tsunamis. Here, we use fossil microatolls in Ishigaki island to reconstruct the relative sea level in the Holocene. The coral record reveals several relative emergence episodes clustering between 5-4 and 3-2 thousand years ago (ka). Elastic modeling shows that the observed motions can correspond to coseismic uplift associated with megathrust earthquakes. The clusters of megathrust events suggest possible supercycles of earthquakes with a recurrence interval of more than 2 ka. Such results imply a strong seismic hazard for the upcoming centuries. The devastating 1771 Meiwa earthquake and associated tsunami may mark the onset of the most recent seismic supercycle.

This work shows how the VARION (Variometric Approach for Real-Time Ionosphere Observation) algorithm, able to estimate the TEC variations in real-time, can be used to shed the light on the mechanism of Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) and thus to be used for ionospheric monitoring of natural hazards events. In particular, we present the methodology of Total Variometric Approach (TVA) was developed to estimate the tsunami genesis and to support classic methods for tsunami warning system. TVA is based on the joint application of VADASE (Variometric Approach for Displacements Analysis Standalone Engine) and VARION to estimate ground shaking, co-seismic displacements and TEC disturbances, using the same real-time GNSS data stream. Furthermore, we present an oceanic-ionospheric joint analysis after the 2022 Tonga eruption. We report on the reversed amplitude of the two phenomena in the oceans and in the ionosphere. The sea-surface perturbation caused by the Lamb wave was not significant, while ionospheric perturbation was considerable. In contrast, the regular tsunami waves manuscript reached ~1m around New Caledonia-New Zealand and ~2m along the Chilean coastline; however, the associated ionospheric perturbation was quite small, half the size of the Lamb wave perturbation. We finally explore the possibility to increase the amount of available data for a denser ionospheric monitoring, applying VARION also to observations coming from geostationary satellites, from ship-based GNSS receivers and from smartphones.

The 2022 Tonga event highlight the necessity to have more and more knowledge about the activity of volcanoes. To this point, it is well known that volcanoes explosion can trigger ionospheric perturbation detectable through the Global Navigation Satellite System (GNSS) signal [1]. The VARION (Variometric Approach for Real-Time Ionosphere Observation) algorithm has been successfully applied to detection of ionospheric perturbations in several real-time scenarios [2, 3]. VARION, thus, estimates sTEC (slant total electron content) variations starting from the single time differences of geometry-free combinations of GNSS carrier-phase measurements. The aim of this work is to analyse some Etna explosions occurred in 2021 with the VARION algorithm in order to better study the coupling between volcanoes and ionosphere. This study can pave the way to a real-time ionospheric monitoring of Etna volcano. [1] Manta, Fabio, et al. "Correlation between GNSS‐TEC and eruption magnitude supports the use of ionospheric sensing to complement volcanic hazard assessment." Journal of Geophysical Research: Solid Earth 126.2 (2021): e2020JB020726. [2] Ravanelli, Michela, et al. "GNSS total variometric approach: first demonstration of a tool for real-time tsunami genesis estimation." Scientific Reports 11.1 (2021): 1-12. [3] Savastano, Giorgio, et al. "Advantages of geostationary satellites for ionospheric anomaly studies: Ionospheric plasma depletion following a rocket launch." Remote Sensing 11.14 (2019): 1734.

A total solar eclipse occurred at the ascending node of the Moon's orbit on 2 July 2019, with an eclipse magnitude of 1.0459. The totality was visible from the southern Pacific Ocean east of New Zealand to the Coquimbo Region (Chile) and Central Argentina at sunset, with the maximum of 4 min 32 s visible from the Pacific Ocean. The recent Great American Eclipse, 21 August 2017, clearly showed that the ionosphere is strongly affected by the totality. In order to explore the ionospheric signature of the 2019 eclipse, we use data of ∼110 GNSS stations seeing multiple GPS and GLONASS satellites to visualize the eclipse signature on the total electron content (TEC) in the southern hemisphere. The location of the South American GNSS stations, at the end of the path of totality, right before sunset, makes the eclipse signature in the ionospheric TEC act like an early sunset with 15 TEC‐Unit variations, corresponding to 40% of background ionization. The effect reaches only four TEC‐Units, corresponding to 25%–40% of background ionization, for the more westerly observations, where we clearly highlight the effect of the eclipse in the ionosphere and the following recovery to normality. By applying the omega‐k analysis, we can find TIDs with wavelengths of around 100 km and larger than 200 km and with periods of 20–50 min, which supports the results of the Great American Eclipse. Using different grids, we can also prove the robustness of the omega‐k analysis.

Global Navigation Satellite System (GNSS) is used in seismology to study the ground displacements as well as to monitor the ionospheric total electron content (TEC) perturbations following seismic events. The aim of this work is to combine these two observations in one real-time method based on the Total Variometric Approach (TVA) to include the GNSS real-time data stream in future warning systems and tsunami genesis estimation observing both, ground motion and TEC. Our TVA couples together the Variometric Approach for Displacement Analysis Stand-alone Engine (VADASE) with the Variometric Approach for Real-Time Ionosphere Observation (VARION) algorithms. We apply the TVA to the Mw 8.3 Illapel earthquake, that occurred in Chile on September 16, 2015, and we demonstrate the coherence of the earthquake ground shaking and the TEC perturbation by using the same GNSS data stream in a real-time scenario. Nominally, we also highlight a stronger kinetic energy released in the north of the epicenter and visible in both, the ground motion and the TEC perturbation detect at 30 s and around 9.5 min after the rupture respectively. The high spatial resolution of ionospheric TEC measurement seems to match with the extent of the seismic source. The GNSS data stream by TVA of both the ground and ionospheric measurement opens today new perspectives to real-time warning systems for tsunami genesis estimation.

One of the main issues in GNSS ionosphere seismology is to localize the exact height of the single thin layer (Hion) with which the ionosphere is approximated. Hion is generally assumed to be the altitude of the maximum ionospheric ionization (hmF2), i.e., in the ionospheric F-layer. In this sense, Hion is often be presumed from physical principles or ionospheric models. The determination of Hion is, therefore, fundamental since it affects the coordinates of the ionospheric pierce point (IPP) and subsequentely of the sub-ionospheric pierce point (SIP). In this work, we present a new developed methodology to determine the exact localization of Hion. We tested this approach on the TIDs (Travelling ionospheric disturbances) connected with the 2011 Tohoku-Oki earthquake and tsunami [1]. In detail, we computed the slant Total Electron Content (sTEC) variations at different Hion (in the range from 100 to 600 km) with the VARION (Variometric Approach for Real-Time Ionosphere Observation) algorithm [2,3], then we interpolated the different pattern in sTEC values related to different waves detected in the ionosphere (AGWepi, IGWtsuna and AWRayleigh) finding the mean velocity value of these waves. Subsequentely, the minimized difference between the estimated propagation velocity and the values from physical models fix us the correct Hion. Our results show a Hion of 370 km, while ionopshere model IRI 2006 located the maximum of ionospheric ionization at an height of 270 km. This difference is important to understand how a different Hion can impact on the location of the sTEC perturbation, affecting the shape and the extent of the source from TEC observations. References [1] https://earthquake.usgs.gov/earthquakes/eventpage/official20110311054624120_30/executive [2] Giorgio Savastano, Attila Komjathy, Olga Verkhoglyadova, Augusto Mazzoni, Mattia Crespi, Yong Wei, and Anthony J Mannucci, “Real-time detection of tsunami ionospheric disturbances with a stand-alone gnss receiver: A preliminary feasibility demonstration, ”Scientific reports, vol. 7, pp. 46607, 2017. [3] Giorgio Savastano, Attila Komjathy, Esayas Shume, Panagiotis Vergados, Michela Ravanelli, Olga Verkhoglyadova, Xing Meng, and Mattia Crespi, “Advantages of geostationary satellites for ionospheric anomaly studies: Ionospheric plasma depletion following a rocket launch,”Remote Sensing, vol. 11, no. 14, pp. 1734, 2019

Since 2011 there has been an ongoing debate about the possibility of short‐term earthquake prediction using total electron content (TEC) ionospheric monitoring by the Global Navigation Satellite System (GNSS). Heki (2011), https://doi.org/10.1029/2011gl047908 initiated this debate when he published results for the 2011 Tohoku event reporting a TEC enhancement 40 min before the earthquake; several later papers by Heki and coworkers have made similar claims for other earthquakes. If correct, Heki's methods might contribute to short‐term earthquake prediction. However, Heki's claims have been strongly criticized as being due to a decrease in the background TEC after earthquakes–the so called ionospheric hole–rather than an enhancement before. Depending on the choice of reference curve to be subtracted from the raw data to infer the “anomaly,” the data analysis can produce either a hole or an enhancement. We show that the choice of reference curve ‐calculated by Heki with a polynomial fit‐is strongly affected by the degree of the polynomial, as well as by the selection of the time window. We also show using synthetic examples that even if there is actually no signal before the event, Heki's methods can lead to spurious precursory signals (i.e., signals with non‐zero amplitude before the event) after the reference curve is subtracted. It thus appears likely that the reported TEC enhancements are artifacts.

The largest tsunamis are generated by seafloor uplift resulting from rupture of offshore subduction-zone megathrusts. The rupture of the shallowest part of a megathrust often produces unexpected outsize tsunami relative to their seismic magnitude. These are so called ‘tsunami earthquakes’, which are difficult to identify rapidly using the current tsunami warning systems, even though, they produce some of the deadliest tsunami. We here introduce a new method to evaluate the tsunami risk by measuring ionospheric total electron content (TEC). We examine two M w 7.8 earthquakes (one is a tsunami earthquake and the other is not) generated in 2010 by the Sunda megathrust, offshore Sumatra, to demonstrate for the first time that observations of ionospheric sounding from Global Navigation Satellite System (GNSS) can be used to evaluate the tsunamigenic potential of earthquakes as early as 8 min after the mainshock.

Despite the global threat posed by large‐scale eruptions to communities, to the climate, and to the consequent impacts on the world economy, many active volcanoes still lack of adequate ground‐based instrumentation. Satellite‐based remote sensing has been used to complement volcano monitoring and risk assessment for volcanic ash, but this technique is often limited by weather conditions. In this work, we explore the ionospheric total electron content (TEC) perturbations measured by GNSS to provide additional information and complement conventional monitoring systems. To this end, we measure the GNSS TEC perturbation associated with the acoustic‐gravity waves generated by 22 volcanic explosions. We introduce a new metric—the Ionospheric Volcanic Power Index (IVPI)—to quantify the energy transferred to the ionosphere by volcanic explosions. We evaluate the IVPI against several well‐established metrics from seismic and infrasonic volcano monitoring as well as satellite remote sensing. Our results show that the IVPI successfully correlates with the Volcanic Explosivity Index (VEI) for events larger than VEI 2. Moreover, the IVPI shows strong correlation with both the acoustic source power and the ash plume height, from which depends the style of volcanic activity. Moderate correlation between IVPI and peak ground velocity (PGV) requires further study in order to evaluate the role of different parameters (seismic magnitude, attenuation, style of faulting, crustal structure, etc.). Our results suggest that ionospheric monitoring by GNSS TEC can help to characterize volcanic eruptions, opening new exciting avenues for continuous volcano monitoring and warning systems by remote sensing.

Most tsunamis occur after large submarine earthquakes, particularly in the Pacific Ocean. However, following the 2004 tsunami in the Indian Ocean, tsunami hazard awareness was significantly raised at the global scale, and warning systems were developed in many other regions, where large tsunamis are rarer but can also produce large catastrophes. Here we first review the basic physics of a tsunami, from its triggering to its coastal impact, and we offer a review of the geophysical and sea-level data that can describe the various processes operating during a tsunami. Global Navigation Satellite System (GNSS) data have a key role in better describing the ground deformation following a tsunamigenic earthquake close to the coast. The GNSS observations complement seismological data to constrain the rupture model rapidly and robustly. Interferometric Synthetic Aperture Radar (SAR) also contributes to this field, as well as optical imagery, relevant to monitoring elevation changes following subaerial landslides. The observation of the sea-level variations, in the near field and during the propagation across the ocean, can also increasingly benefit from GNSS data (from GNSS buoys) and from robust satellite communication: pressure gauges anchored on the seafloor in the deep ocean contribute to warning systems only by data continuously transmitted through satellites. The sounding of ionospheric Total Electron Content (TEC) variations through GNSS, altimetry, or a ground-based airglow camera, is a promising way to record tsunami initiation and propagation indirectly. Finally, GNSS, optical and SAR imagery are essential to map and quantify the damage following tsunami flooding. Satellite data are expected to contribute more to operational systems in the future provided they are reliably available and analysed in real time.

On the 21 August 2017 the eclipse shadow drastically changed the state of the ionosphere over the United States. This effect on the ionosphere is visible in the total electron content measured by Global Navigation Satellite Systems (GNSS). The shadow moved with the supersonic speed of ~1,000 m/s over Oregon to ~650 m/s over South Carolina. In order to exhaustively explore the ionospheric signature of the eclipse, we use data of total electron content from ~3,000 GNSS stations seeing multiple Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS) satellites to visualize the phenomena. This tremendous dataset allows high‐resolution characterization of the frequency content and wavelengths—using an omega‐k analysis based on 3‐D fast Fourier transform—of the eclipse signature in the ionosphere in order to fully identify traveling ionospheric disturbances (TIDs). We confirm the generation of TIDs associated with the eclipse including TIDs interpreted as bow waves in previous studies. Additionally, we reveal, for the first time, short (50–100 km) and long (500–600 km) wavelength TIDs with periods between 30 and 65 min. The sources of the revealed short wavelength TIDs are co‐located with the regions of stronger gradient of the EUV related to sunspots. Our work confirms and describes physical properties of the waves observable in the ionosphere during the Great American Eclipse.

Surface waves emitted after large earthquakes are known to induce atmospheric infrasonic waves detectable at ionospheric heights using a variety of techniques, such as high frequency (HF) Doppler, global positioning system (GPS), and recently over-the-horizon (OTH) radar. The HF Doppler and OTH radar are particularly sensitive to the ionospheric signature of Rayleigh waves and are used here to show ionospheric perturbations consistent with the propagation of Rayleigh waves related to 28 and 10 events, with a magnitude larger than 6.2, detected by HF Doppler and OTH radar respectively. A transfer function is introduced to convert the ionospheric measurement into the correspondent ground displacement in order to compare it with classic seismometers. The ground vertical displacement, measured at the ground by seismometers, and measured at the ionospheric altitude by HF Doppler and OTH radar, is used here to compute surface wave magnitude. The ionospheric surface wave magnitude (M-s(iono)) proposed here introduces a new way to characterize earthquakes observing the signature of surface Rayleigh waves in the ionosphere. This work proves that ionospheric observations are useful seismological data to better cover the Earth and to explore the seismology of the Solar system bodies observing the ionosphere of other planets.

The detail and nature of earthquakes are still challenging from traditional technique observations, e.g., seismometers and strong motion accelerographs. Nowadays, the ionospheric total electron content (TEC) can be obtained from ground-based global navigation satellite systems (GNSS) and space-borne GNSS Radio Occultation, which can be used to investigate the seismo-ionospheric disturbances and may provide insights on the earthquake. In this paper, GNSS ionospheric seismology is presented and reviewed, including methods, observation results and characteristics. Case studies of the 2008 Wenchuan earthquake and 2011 Japan earthquake are presented using ground-based GNSS observations. Significant co-/post-seismic ionospheric anomalies are found from continuous GNSS observations near the epicenters, showing that the seismic ionospheric total electron content (TEC) disturbances were derived mainly from the main shock. The detailed pattern and evolution of the ionospheric disturbance are revealed by denser GNSS observations. Some simulations explore the nature of the ionospheric perturbation, highlighting that acoustic-gravity waves are generated close to the epicenter, and that surface Rayleigh waves and tsunamis generate in the atmosphere/ionosphere acoustic and gravity waves respectively. These waves are induced by solid-Earth/ocean and atmosphere coupling at the ground or ocean interface with the atmosphere propagating upward until the ionosphere create strong perturbation in plasma density and plasma velocity.

The catastrophic seismic events of the last decades push forward the necessity to explore new techniques for source estimation, oceanic tsunami tracking, as well as tsunami warning systems. Early observations of the Rayleigh wave signature in the ionosphere by Doppler sounder were able to measure lithospheric properties, sounding the atmosphere at 200 km of altitude. After the Sumatra event (26 December 2004), the successful tsunami detection by altimeters validates the possibility of tsunami detection by ionospheric sounding. Today, the catastrophic tsunamigenic earthquake in Tohoku (11 March 2011) strongly affirms the potential of ionospheric sounding to visualize the vertical displacement of the ground and ocean: the Japanese GPS network, GEONET, imaged the source extent 8 min after the rupture; it also visualizes the radiation pattern and Rayleigh waves over the entire Japan, including the oceanic region overlooking the rupture; in the far field, the airglow camera located in Hawaii showed the internal gravity wave forced by the tsunami propagating in a zone of 180 ×180 km2 around the island. The ionospheric sounding potential of 2D visualization could extend the present vision of seismology. This work highlights the actual capability and the potential improvement suggested by ionospheric seismology. 9 Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universit Paris Diderot, France 9.1. ORIGINS In early history, Aristoteles [524] described his pneumatic theory that the cause of earthquake is the pneuma, and indirectly the atmosphere: indeed, the wind, heated by the Sun, gets inside the Earth‐cavities where it produces strong pressure variations resulting in the earthquake. The simplistic (and unrealistic) vision of Aristoteles [524] contains the visionary idea about the unicity of the Earth: solid Earth and the fluid parts of the planet, nominally the ocean and the atmosphere/ionosphere, continuously exchange energy. Today, the continuous excitation of normal modes, observed in the solid Earth by seismometers and usually called the “hum,” is the clear proof of this hypothesis [Nawa et al., 1998; Suda et al., 1998; Tanimoto et al., 1998; Roult and Crawford, 2000; Kobayashi et al., 2001]. Several observational works show that the source of the hum is located mainly in the ocean [Rhie and Romanowicz, 2004; Webb, 2007] and partially in the atmosphere [Nishida et al., 2000]. In particular, the atmospheric component of the hum seams to excite normal modes 0S29 and 0S37. An additional proof of the coupling between the solid Earth and the atmosphere is the volcanic explosion of Mount Pinatubo in 1991. The atmospheric explosion was detected worldwide by the global network of seismometers in the form of a bichromatic signal with energy mainly located at 3.68 mHz and 4.40–4.65 mHz, corresponding to normal modes 0S27–29 and 0S34–37 [Kanamori and Mori, 1992; Zurn and Widmer, 1996; Watada and Kanamori, 2010]. In essence, the atmospheric energy excited by the volcanic explosion or by the atmospheric dynamics (humsource) is transferred inside the solid Earth where it propagates at the surface as Rayleigh waves, detectable by seismometers. AQ1 0002577616.INDD 169 7/30/2015 1:06:07 AM 170 SUBDUCTION DYNAMICS Successive theoretical works [Watada, 1995; Lognonne et al., 1998] strongly support this coupling observations and explain that the energy can be transferred in two ways: from the solid Earth to the atmosphere and vice versa. Normal modes computed for the Earth with ocean and atmosphere [Lognonne et al., 1998] clearly show that Rayleigh waves produce acoustic waves in the atmosphere (see section 2); in the same way, tsunamis produce internal gravity waves in the overlying atmosphere (see section 4). Here we indicate the first with AWRayleigh and the second IGWtsuna (Figure 9.1). The first observational evidence of the coupling between solid Earth and the fluid envelopes of the planet is an indirect consequence of the Cold War: the continuous monitoring to detect nuclear explosions via their signature in the atmosphere and in the solid Earth pushed scientists and engineers to compare atmospheric/ionospheric observations (barometers, Doppler sounders, and backscattered radars) with data from seismometers. This data matching from different instruments revealed that acoustic‐gravity waves are generated not only after nuclear explosions but also after seismic events [Row, 1967]. As a consequence of the rich spectrum of energy characterizing the seismic rupture, the ground displacement at the epicenter (Figure 9.1) generates, simultaneously, acoustic and gravity waves in the atmosphere above the epicenter (AGWepi). After the Alaska earthquake in 1964 (Mω 9.2), the Berkeley barometer detected two unexpected signals: the first was correlated with the arrival time of Rayleigh wave (AWRayleigh), the second was essentially an acoustic‐gravity wave propagation from the epicenter (AGWepi) [Bolt, 1964]. The atmospheric waves propagate through the low neutral atmosphere [Donn and Posmentier, 1964] and up to the ionosphere where they are detected by ionospheric sounding [Davies and Baker, 1965; Leonard and Barnes, 1965; Row, 1966]. The 1964 Alaska earthquake opened the era of ionospheric seismology. 9.2. IONOSPHERIC SEISMOMETERS Many observations followed the 1964 Alaska event and clearly showed that the signature of Rayleigh waves was detectable by ionospheric monitoring, mainly using Doppler sounders. AGWepi IGWtsuna AWrayleigh Altimeter Vi

The catastrophic seismic events of the last decades push forward the necessity to explore new techniques for source estimation, oceanic tsunami tracking, as well as tsunami warning systems. Early observations of the Rayleigh wave signature in the ionosphere by Doppler sounder were able to measure lithospheric properties, sounding the atmosphere at 200 km of altitude. After the Sumatra event (26 December 2004), the successful tsunami detection by altimeters validates the possibility of tsunami detection by ionospheric sounding. Today, the catastrophic tsunamigenic earthquake in Tohoku (11 March 2011) strongly affirms the potential of ionospheric sounding to visualize the vertical displacement of the ground and ocean: the Japanese GPS network, GEONET, imaged the source extent 8 min after the rupture; it also visualizes the radiation pattern and Rayleigh waves over the entire Japan, including the oceanic region overlooking the rupture; in the far field, the airglow camera located in Hawaii showed the internal gravity wave forced by the tsunami propagating in a zone of 180 ×180 km2 around the island. The ionospheric sounding potential of 2D visualization could extend the present vision of seismology. This work highlights the actual capability and the potential improvement suggested by ionospheric seismology. 9 Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Universit Paris Diderot, France 9.1. ORIGINS In early history, Aristoteles [524] described his pneumatic theory that the cause of earthquake is the pneuma, and indirectly the atmosphere: indeed, the wind, heated by the Sun, gets inside the Earth‐cavities where it produces strong pressure variations resulting in the earthquake. The simplistic (and unrealistic) vision of Aristoteles [524] contains the visionary idea about the unicity of the Earth: solid Earth and the fluid parts of the planet, nominally the ocean and the atmosphere/ionosphere, continuously exchange energy. Today, the continuous excitation of normal modes, observed in the solid Earth by seismometers and usually called the “hum,” is the clear proof of this hypothesis [Nawa et al., 1998; Suda et al., 1998; Tanimoto et al., 1998; Roult and Crawford, 2000; Kobayashi et al., 2001]. Several observational works show that the source of the hum is located mainly in the ocean [Rhie and Romanowicz, 2004; Webb, 2007] and partially in the atmosphere [Nishida et al., 2000]. In particular, the atmospheric component of the hum seams to excite normal modes 0S29 and 0S37. An additional proof of the coupling between the solid Earth and the atmosphere is the volcanic explosion of Mount Pinatubo in 1991. The atmospheric explosion was detected worldwide by the global network of seismometers in the form of a bichromatic signal with energy mainly located at 3.68 mHz and 4.40–4.65 mHz, corresponding to normal modes 0S27–29 and 0S34–37 [Kanamori and Mori, 1992; Zurn and Widmer, 1996; Watada and Kanamori, 2010]. In essence, the atmospheric energy excited by the volcanic explosion or by the atmospheric dynamics (humsource) is transferred inside the solid Earth where it propagates at the surface as Rayleigh waves, detectable by seismometers. AQ1 0002577616.INDD 169 7/30/2015 1:06:07 AM 170 SUBDUCTION DYNAMICS Successive theoretical works [Watada, 1995; Lognonné et al., 1998] strongly support this coupling observations and explain that the energy can be transferred in two ways: from the solid Earth to the atmosphere and vice versa. Normal modes computed for the Earth with ocean and atmosphere [Lognonné et al., 1998] clearly show that Rayleigh waves produce acoustic waves in the atmosphere (see section 2); in the same way, tsunamis produce internal gravity waves in the overlying atmosphere (see section 4). Here we indicate the first with AWRayleigh and the second IGWtsuna (Figure 9.1). The first observational evidence of the coupling between solid Earth and the fluid envelopes of the planet is an indirect consequence of the Cold War: the continuous monitoring to detect nuclear explosions via their signature in the atmosphere and in the solid Earth pushed scientists and engineers to compare atmospheric/ionospheric observations (barometers, Doppler sounders, and backscattered radars) with data from seismometers. This data matching from different instruments revealed that acoustic‐gravity waves are generated not only after nuclear explosions but also after seismic events [Row, 1967]. As a consequence of the rich spectrum of energy characterizing the seismic rupture, the ground displacement at the epicenter (Figure 9.1) generates, simultaneously, acoustic and gravity waves in the atmosphere above the epicenter (AGWepi). After the Alaska earthquake in 1964 (Mω 9.2), the Berkeley barometer detected two unexpected signals: the first was correlated with the arrival time of Rayleigh wave (AWRayleigh), the second was essentially an acoustic‐gravity wave propagation from the epicenter (AGWepi) [Bolt, 1964]. The atmospheric waves propagate through the low neutral atmosphere [Donn and Posmentier, 1964] and up to the ionosphere where they are detected by ionospheric sounding [Davies and Baker, 1965; Leonard and Barnes, 1965; Row, 1966]. The 1964 Alaska earthquake opened the era of ionospheric seismology. 9.2. IONOSPHERIC SEISMOMETERS Many observations followed the 1964 Alaska event and clearly showed that the signature of Rayleigh waves was detectable by ionospheric monitoring, mainly using Doppler sounders. AGWepi IGWtsuna AWrayleigh Altimeter Vi