The distinction matters because meteotsunamis and true tsunamis are nearly indistinguishable at the coast. Both are gravity waves with periods ranging from a few minutes to a couple of hours, and both can flood harbors, snap mooring lines, and endanger lives. A meteotsunami begins as a modest sea-level perturbation of just a few centimeters, produced by atmospheric forcing such as gravity waves, pressure jumps, squall lines, or frontal passages. What turns it into a hazard is resonance. When the speed of a traveling atmospheric disturbance matches the speed of long ocean waves over a particular shelf or basin, energy is transferred efficiently into the sea through what physicists call Proudman resonance. The initial ripple then undergoes wave shoaling and topographic amplification as it enters bays and harbors, growing into a destructive surge. This is why devastating meteotsunamis strike only a limited number of sites worldwide, wherever bathymetry and coastal geometry conspire to amplify them.

Greece would seem an unlikely place to mistake weather for earthquakes. The Hellenic Subduction Zone, where the African plate dives beneath the southern margin of Eurasia, produces remarkably high seismicity and a long, well-documented history of genuine tsunamis, including the catastrophic wave generated by the magnitude 7.7 Amorgos earthquake of 9 July 1956. Yet the same region’s underwater landscape, a mosaic of deep trenches, isolated basins, and shallow plateaus, creates exactly the conditions in which atmospheric disturbances traveling at specific speeds can couple with the sea and amplify. Triantafyllou assembled descriptive accounts from Herodotus and Polybius, nineteenth-century travel narratives, newspaper archives, tide-gauge records from the Hellenic Navy Hydrographic Service, earthquake catalogs from the International Seismological Centre, and atmospheric reanalysis data from the European Centre for Medium-Range Weather Forecasts to build the case.

The most famous case in the study is also the oldest. In 479 BCE, during the Greco-Persian War, the historian Herodotus recorded that Persian forces besieging Potidaea in the northwest Aegean attempted to cross an exposed seabed after a great ebb tide lasting several hours. A sudden flood tide then returned, drowning those who could not swim, while the survivors were cut down by Potidaean boats. The episode has long been cited as one of the earliest documented tsunamis in Europe, and sedimentary layers near Potidaea have been attributed to a high-energy wave. But Herodotus made a crucial observation that modern researchers have seized upon: the ebb lasted for a long while before the flood arrived. A tsunami generated by an earthquake or landslide typically brings its first inundation within a short time of the withdrawal. A prolonged ebb, followed by a flood, points instead to a meteorological mechanism, possibly a storm surge or a seiche driven by atmospheric pressure changes. Herodotus also noted that similar flood episodes had occurred in the area before, a hint of recurring local conditions. The region was, after all, struck by a destructive supercell in July 2019 that killed seven people, a reminder that the atmosphere can be lethal there without any help from the underworld.

Two other historical cases tell similar stories. In 226 BCE, the historian Polybius described how the Macedonian king Antigonus Doson, sailing off Larymna in the North Evoikos Gulf, was surprised by a sudden ebb that left his ships aground until the water returned. No earthquake was reported, and the closed bay is known for abnormal tidal behavior; the study favors a meteorological explanation. More revealing is the destructive sea wave that struck Patras and coastal sites of the Corinth Gulf on 9 January 1821. Traveler Francois Pouqueville linked the wave to an earthquake of 6 January, and tsunami catalogs dutifully repeated that association for nearly two centuries. But careful reanalysis of the original narrative shows the earthquake occurred on 6 January and the wave on 9 January, three days later, during stormy weather, and the earthquake itself was a distant aftershock of a mainshock near Zakynthos in the Ionian Sea. The wave was a storm surge, not a tsunami, yet some modern catalogs still list it as seismic.

The instrumental period provides the strongest evidence, because tide gauges record what eyewitnesses cannot. Five episodes, in April 1928, February 1959, June 1961, June 1978, and May 1991, shared a striking property: their oscillations were recorded across spatial scales of up to roughly 600 kilometers, and their amplitudes attenuated with distance far more slowly than those of any known Greek seismic tsunami. Triantafyllou fitted an empirical attenuation law to the tide-gauge data and compared it with established laws for earthquake-generated tsunamis in Greece, including the 1956 Amorgos and 2020 Samos events. Seismic tsunamis in the region show attenuation exponents between 1.0 and 2.44, meaning wave heights fall off steeply with distance. The tsunami-like episodes yielded an exponent of just 0.22, an order of magnitude gentler. Meteotsunamis, generated by moving atmospheric sources and strongly modulated by local resonance, do not obey a single clean attenuation pattern at all, and that absence of pattern is itself a diagnostic fingerprint.

The 1978 case is the study’s forensic centerpiece, and it reads like a detective story. On 20 June 1978, a magnitude 6.22 earthquake ruptured the northern Greek mainland near Thessaloniki. Early the next morning, long-period sea oscillations appeared in tide gauges at Kavala in the north Aegean and Heraklion on the north coast of Crete, roughly 500 kilometers apart. The Kavala oscillation began about two hours after the earthquake, with an amplitude of about 15 centimeters and a period of about 42 minutes, and lasted nearly 23 hours. Heraklion’s oscillation started about seven hours after the quake, reached about 20 centimeters, rang at a period of about 34 minutes, and persisted for 17 hours. The timing seemed damning. But the earthquake’s causative fault ruptured inland, ruling out co-seismic seafloor displacement, and a hypothetical submarine landslide near Kavala cannot explain why the Heraklion wave arrived five hours after the Kavala one, or why it was larger rather than smaller.

Triantafyllou then tested a subtler seismic hypothesis: that long-period Rayleigh surface waves radiating from the earthquake had dynamically shaken the harbors into resonance, a phenomenon documented after great earthquakes worldwide, including seiches in Norwegian lakes triggered by the 2011 Tohoku event. Calculating the arrival times of the R1, R2, R3, and R4 Rayleigh phases at both harbors, and comparing the seismic wave periods of 15 to 40 seconds against the harbors’ natural resonant periods computed from Merian’s formula, she found that no phase arrival matched the onset of the sea oscillations, and no frequency matched the basins’ natural modes. Meanwhile, the same morning brought a catastrophic 6-meter meteotsunami to Vela Luka in the Adriatic, an event long since attributed by Croatian and Italian researchers to a mesoscale atmospheric disturbance riding a deep upper-level trough and a southwest jet stream over the Apennines. That same disturbance, moving from the Adriatic toward the Aegean, would have reached the north Aegean roughly two hours after the earthquake, purely by coincidence, pumping energy into Kavala’s higher-order resonant mode and later exciting the Cretan shelf near Heraklion. The hours-long durations of the oscillations, far beyond any single seismic pulse, sealed the meteorological verdict.

The remaining instrumental episodes reinforce the pattern. The May 1991 oscillation in Lakki Bay on Leros, which pushed fishing boats ashore, began with a gradual rise of mean sea level, the signature of an inverse barometer effect, and its 12-minute period matches the natural resonance of the funnel-shaped bay, the same mode that rang at about 15 minutes during the genuine 1956 Amorgos tsunami. Crucially, the 1956 seismic tsunami arrived at Lakki with a receding wave first, while the 1991 event began with a rise, a diagnostic contrast between seismic and atmospheric triggers. The January 1991 Evdilos oscillation, the 1961 sea rise driven by a barometric pressure drop and southerly winds, and the February 1959 sea drop of up to a meter under a powerful anticyclone and strong northerly winds all occurred during seismically quiet periods. Even the puzzling April 1928 episode, long listed as a tsunami despite beginning a week after the Bulgarian earthquakes and two days before the Corinth Gulf mainshock, fits the meteorological picture once its slow attenuation and basin-wide extent are considered.

The broader lesson is a warning about causality. Space-time coincidence, the study shows, is seductive but unreliable: an earthquake and a strange sea can share a morning without one causing the other. Meteotsunamis often strike under what researchers call good weather, calm skies concealing high-altitude atmospheric processes such as wave ducting and unstable mid-level layers that spawn small but resonant disturbances. Because witnesses see no storm and feel no shaking, earthquakes and submarine slides get blamed instead, polluting tsunami catalogs and skewing hazard assessments. As recent Mediterranean hazard studies have argued, meteotsunamis belong in risk planning alongside their seismic cousins. For Greece, a country whose tsunami history has been written partly by Herodotus and partly by tide gauges, separating the atmosphere’s quiet surges from the earth’s violent ones is not an academic exercise. It determines which coasts must prepare for the next great subduction quake, and which harbors, on some deceptively serene afternoon, will suddenly breathe.

Subject of Research: Discrimination between seismic tsunamis and meteotsunamis in Greece from antiquity to the instrumental era

Article Title: Distinction between seismic tsunamis and meteotsunamis recorded in Greece from antiquity up to the present

Article References: Distinction between seismic tsunamis and meteotsunamis recorded in Greece from antiquity up to the present. (n.d.). https://doi.org/10.1007/s11069-026-08425-x

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08425-x

Keywords: meteotsunami, tsunami, Greece, Aegean Sea, seiche, tide gauge, Hellenic Subduction Zone, Herodotus, atmospheric disturbance, harbor resonance, attenuation law, natural hazards

The research, led by Anjela A. Ilagan of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (DOST-PAGASA) and the University of the Philippines Diliman, together with Olivia C. Cabrera and Marcelino Q. Villafuerte II, tackles a persistent blind spot in Philippine hazard science. Most previous investigations of the country’s storm surges have concentrated either on individual catastrophic events or on numerical simulations of specific tropical cyclones. What was missing was a systematic, observation-based reconstruction spanning the whole archipelago and many decades. Because the Philippines is made up of more than seven thousand islands with an extraordinarily long and complex coastline, local bathymetry, shelf width, and coastal orientation can dramatically alter how high the sea rises when a typhoon makes landfall. A national picture assembled from actual measurements, rather than models alone, is therefore an essential foundation for hazard assessment.

The team’s approach hinged on a deceptively simple but technically demanding manipulation of tide gauge data. Tide gauges record the total water level at the coast, which is a mixture of the predictable astronomical tide, longer-term sea level variations, and the meteorological contribution from winds and pressure changes associated with storms. To isolate the storm signal, the researchers first removed the tidal component using harmonic analysis with the UTide software package, which fits the known tidal constituents to the observed record and allows the tide to be subtracted out. What remains is the residual sea level, the non-tidal part of the water level that carries the fingerprint of storm-driven water pile-up. The raw residuals were then smoothed with a low-pass filter to suppress high-frequency noise, ensuring that short-lived fluctuations unrelated to genuine surge events would not contaminate the detection.

Identifying storm surge events from residuals required a detection threshold, and here the researchers made a methodologically important choice. Rather than applying a single fixed cutoff across the entire country and record, they used an annually varying threshold, which accounts for the fact that baseline sea levels and seasonal conditions differ from year to year and from station to station. This approach, consistent with skew surge methods used by established sea level monitoring facilities, allows the detection algorithm to remain sensitive to genuine surge events even in years or locations where the background water level is naturally higher. Applying this threshold to the de-tided records across multiple coastal stations, the team identified 133 distinct storm surge events over the 77-year study period, the most comprehensive instrument-based storm surge inventory ever assembled for the Philippines.

The statistics that emerged from this inventory carry a message with direct implications for coastal safety. The majority of detected storm surges, measured as pure surge residuals, reached heights of only 20 to 50 centimeters, and only about 5 percent of events exceeded one meter. On its face, that might suggest that surges in the Philippines are generally modest. But the picture changes sharply when the researchers examined storm tide, the combined water level produced when the storm surge coincides with the astronomical tide. In approximately 39 percent of events, the storm tide exceeded one meter, nearly eight times the proportion seen for surge alone. The lesson is unambiguous: the timing of a typhoon’s arrival relative to the tidal cycle can transform a manageable rise in water into a genuinely dangerous coastal flood. A 40-centimeter surge arriving at high tide can inundate areas that the same surge at low tide would leave untouched.

The single largest surge in the record was measured at Pasacao, in the province of Camarines Sur, during Typhoon Molave in 2020, when the water level anomaly reached 211 centimeters, more than two meters above the expected tide. Molave, known locally as Quinta, swept across the Bicol region in late October 2020, and the Pasacao observation stands as a stark reminder of what even a storm that is not among the most intense on record can deliver when its winds align favorably with coastal geography. The study’s case analyses of Molave and Super Typhoon Goni, which struck the same general region just days later in early November 2020, highlighted how the orientation of the coastline relative to the storm’s track and the phase of the tide jointly determine the severity of coastal impacts. Two storms hitting within a week of each other, with different tracks and tidal timings, produced markedly different surge outcomes.

Mapping the geographic distribution of the 133 events revealed a pronounced regional pattern that reflects the underlying physics of surge generation. High storm surges clustered along the coasts of Catanduanes, Camarines Sur, Eastern Samar, Cagayan, Batanes, and Palawan. What these disparate locations share is a shallow and wide continental shelf, a bathymetric configuration that favors surge amplification. When storm winds push ocean water across a broad, shallow shelf, friction with the seabed constrains the water’s vertical escape and forces it to pile up at the coast, a mechanism that deep water adjacent to steep coasts does not permit. Eastern Samar’s presence in this list will resonate with anyone familiar with Haiyan, whose catastrophic surge in Leyte Gulf was amplified by exactly this kind of shelf-driven shoaling. Equally telling is where surges were largely absent: Mindanao, the large southern island, registered few significant events, a pattern consistent with its exposure to a less typhoon-frequented portion of the western North Pacific basin.

Beyond its scientific findings, the database itself may prove to be the study’s most consequential product. A validated, observation-based catalogue of historical surge events spanning nearly eight decades provides exactly the kind of ground truth that storm surge forecasters, hazard mappers, and model developers need. Numerical surge models, from the high-resolution unstructured-mesh systems used in operational hurricane forecasting to global reanalyses of extreme sea levels, depend on historical observations for calibration and validation. Extreme value statistics, the branch of statistics used to estimate the probability of rare events such as hundred-year surges, likewise require long, homogeneous observational records to produce trustworthy return-period estimates. By anchoring such analyses in real tide gauge measurements from Philippine waters, the new inventory reduces the reliance on extrapolations from other coastlines and improves the credibility of national hazard assessments, evacuation planning, and coastal infrastructure design.

The study also arrives at a moment when the stakes of surge forecasting in the Philippines are rising. Mean sea level is climbing globally, and even a modest long-term rise raises the baseline upon which every future storm surge builds, effectively converting yesterday’s moderate surge into tomorrow’s damaging flood. The researchers’ use of an annually varying threshold partly reflects this shifting baseline, and their 1947-to-2024 record offers a valuable window into how surge occurrences have unfolded across a period of both changing climate and changing observation technology. The authors acknowledge the National Mapping and Resource Information Authority’s Hydrography Branch for providing the tide gauge data, and the work was carried out without dedicated external funding, a testament to the value of careful analysis of existing national observational assets.

For a country that sits squarely in the most active tropical cyclone basin on Earth, with roughly twenty storms entering its area of responsibility in a typical year, the ability to look back through 77 years of measured sea level data is more than an academic exercise. It is a way of letting the ocean itself testify about which coasts are most exposed, how badly the tide can conspire with the wind, and what the worst-case scenarios look like when the next Molave or Goni forms over the warm waters of the Pacific. The 133 events now catalogued in this inventory are, in effect, 133 rehearsals for future disasters, and the researchers hope that forecasters, planners, and coastal communities across the archipelago will use them to be better prepared when the next rehearsal becomes the real thing.

Subject of Research: Historical storm surge events in the Philippines reconstructed from tide gauge observations

Article Title: Investigating historical storm surge occurrences in the Philippines from tide gauge observations

Article References: Ilagan, A. A., Cabrera, O. C., & Villafuerte, M. Q., II (2026). Investigating historical storm surge occurrences in the Philippines from tide gauge observations. Natural Hazards, 122(18), Article 625. https://doi.org/10.1007/s11069-026-08360-x

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08360-x

Keywords: storm surge, storm tide, Philippines, tide gauge, tropical cyclone, Typhoon Molave, Super Typhoon Goni, sea level, coastal flooding, hazard assessment, UTide, Natural Hazards