'Mega-tsunami' trof de oostkust van Groenland in september 2023

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650 Feet High: The Megatsunami That Rocked Greenland’s East Coast

Seismologists measure tremors from distances up to 5000 km away.
On September 16, 2023, a massive wave struck a fjord on Greenland’s east coast, leaving evidence of flooding up to 200 meters (650 feet) high in some areas. Researchers, led by Angela Carrillo Ponce from the German Research Centre for Geosciences (GFZ), have analyzed seismic data from earthquake monitoring stations worldwide and uncovered another remarkable event. The megatsunami triggered a standing wave that oscillated back and forth in the narrow, uninhabited Dickson Fjord for over a week. The international team has published their findings in the journal

Rockslide as triggering event
The tsunami was triggered by a large landslide. Earthquake measuring stations up to 5,000 kilometers away registered the shaking caused by the landslide as a short signal. However, there was also a very long-period (VLP) signal that was recorded by the seismometers for more than a week. Angela Carrillo Ponce, who works as a doctoral student in the “Physics of Earthquakes and Volcanoes” section of the GFZ, says: “The mere fact that the VLP signal of a wave sloshing back and forth triggered by a landslide in a remote area of Greenland can be observed worldwide and for over a week is exciting. That’s why we in seismology have been most concerned with this signal.” Fortunately, the researcher adds, no people were harmed. Only a military base, which was without personnel at the time of the tsunami, was devastated.

Analysis of the seismic signals – shock waves that travel thousands of kilometers in the earth’s crust – showed that a so-called standing wave formed in the fjord after the landslide. Initially, the parts of the flank that fell into the water triggered a giant wave that spread through the entire fjord to the offshore island of Ella, more than 50 kilometers away. Near the point where the rockslide entered the fjord, the maximum height was more than 200 meters, along the coast an average of 60 meters. Parts of the wave apparently spilled back from the steep banks in the narrow fjord and a standing wave began to form, which undulated back and forth for more than a week. However, this wave measured only around 1 meter in height.


Depending on the frequency range filtered out, the rockfall triggering the tsunami can be seen as a single peak (top), the standing wave sloshing back and forth as an undulating pattern in the recordings (middle, with several hours depicted) or the overall signal of the rockfall and the tsunami over the course of a week with strongly decreasing intensity of the oscillations (bottom).

Standing wave persisted unusually long
Such standing waves and the resulting long-period signals are already known in research. Such VLP signals are normally associated with large break-offs from glacier edges. “In our case, we also registered a VLP signal”, says Angela Carrillo Ponce, “the unusual thing about it was the long duration”. What was particularly impressive was that the data from seismic stations in Germany, Alaska, and other parts of North America were of very good quality for the analysis. A comparison with satellite images confirmed that the cause of the first seismic signals corresponded well with the strength and direction of the rockfall that triggered the megatsunami. In addition, the authors were able to model the slow decay and the dominant oscillation period of the VLP signals.

This gives the researchers hope that they will be able to detect and analyze other similar events from the past. It is obvious that the retreat of glaciers, which previously filled entire valleys, and the thawing of permafrost are leading to increased landslides. Climate change is accelerating the melting of glaciers and could therefore increase the risk of megatsunamis.

Buried by a 330 Foot High Wall of Water; The Greenland Megatsunami Which Just Occurred

The Wave - Official Trailer

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Op woensdag 14 augustus 2024 12:53 schreef Starhopper het volgende:

[..]
Jah! Dat is een paar duizend jaar geleden toch eens gebeurd? Dankzij de instorting van een fjord in Noorwegen.

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Aarde trilde negen dagen lang door enorme tsunami in Groenland

In het oosten van Groenland stortte vorig jaar een 1.200 meter hoge bergtop in, als gevolg van stijgende temperaturen. Het veroorzaakte een megatsunami die negen dagen lang duurde en seismische golven over de hele aarde stuurde.

De bergtop stortte in op 16 september 2023 doordat de smeltende gletsjer eronder de rots niet meer kon dragen. De aardverschuiving kwam in het Dickson-fjord terecht en veroorzaakte een 200 meter hoge golf. Het water zat als het ware vast in het afgelegen fjord en klotste daardoor dagenlang heen en weer.

De tsunami in het Dickson-fjord duurde negen dagen. Over de hele wereld werden trillingen gemeten, maar onderzoekers hadden geen idee waar die vandaan kwamen. Een jaar lang deden 68 onderzoekers van veertig instituten in vijftien landen onderzoek naar de mysterieuze schokgolven.

"Toen collega's het signaal vorig jaar voor het eerst opvingen, zag het er heel anders uit dan een aardbeving. We noemden het een ongeïdentificeerd seismisch object", zegt onderzoeker Stephen Hicks tegen de BBC. "Het signaal was negen dagen lang elke negentig seconden te zien".

De onderzoekers gebruikten onder meer seismische data om te achterhalen waar het signaal vandaan kwam. Dat leidde ze naar het oosten van Groenland. Daar komen doorgaans geen enorme landverschuivingen en tsunami's voor, zegt hoofdonderzoeker Kristian Svennevig tegen The Guardian.

De gletsjer die de bergtop ondersteunde, smolt vanwege de stijgende temperaturen door klimaatverandering, stellen de onderzoekers. Uiteindelijk was het ijs zo dun dat het de rots ter grootte van 25 keer het Empire State Building niet langer kon dragen.

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Mega tsunamis in Greenland fjord confirmed as source of nine-day global seismic signal

A persistent, ultra-low frequency seismic vibration was detected worldwide in September 2023 and traced to Dickson Fjord, East Greenland, after two large landslides triggered tsunamis and a long-lasting seiche. For the first time, researchers directly observed this standing wave using NASA’s SWOT satellite mission, providing new insights into the connection between global seismic signals and surface water motion in remote coastal environments.

Seismic sensors around the world picked up a persistent, ultra-low frequency vibration at 10.88 millihertz in September 2023 that lasted for 9 days and came back a month later. These very-long-period (VLP) seismic signals were eventually traced back to a remote fjord in East Greenland, where two massive landslides had caused tsunamis.

The real source of the vibration was confirmed to be a seiche bouncing back and forth inside the fjord, an inference based on satellite observations and seismic correlation.

First direct evidence of a fjord seiche
For the first time, scientists have obtained direct evidence of a fjord seiche using satellite observations from NASA’s Surface Water and Ocean Topography (SWOT) mission. The research team from the University of Oxford combined satellite data, seismic records, and Bayesian machine learning to verify a natural event that had previously only been suggested by models or indirect evidence.

A seiche is a standing wave that can form in enclosed or semi-enclosed basins like lakes, harbors, or fjords.

Seiches are usually short-lived and localized, but the one in this case study persisted for more than nine days and produced a seismic signal strong enough to be detected worldwide.

Seiche characterization via SWOT and seismic correlation
Following the landslide on September 16, 2023, the SWOT satellite made several passes over Dickson Fjord, including one about 12 hours after the event. Its Ka-band Radar Interferometer (KaRIn) captured detailed measurements of the water’s surface, revealing gentle but consistent tilts characteristic of a standing wave.

The research team used a Bayesian regression model to estimate the maximum cross-channel slope at approximately 1.83 ± 0.59 m per km (9.68 to 3.11 feet per mile), an independent estimate that aligned with earlier analytical and numerical predictions.

Researchers matched the SWOT satellite data with filtered seismic signals from the International Institute seismic station at Alert, Canada (II.ALE), which sits about 1300 km (808 miles) away from the fjord. The timing and size of the vertical ground movement in the seismic data lined up with the surface slopes captured by SWOT. This allowed them to estimate the initial amplitude of the seiche at approximately 7.9 m (26 feet), inferred from a combination of satellite and seismic data.

A few weeks later, in October, a second landslide hit the same area, though this one was smaller and produced a weaker signal. SWOT passed over the fjord again within approximately half a day, giving the team another chance to measure the wave. This time, with more accurate timing, they were able to pin down the maximum cross-channel slope more precisely at about 1.37 ± 0.13 m per km (7.25 – 0.69 feet per mile).

The result from the second time around lined up well with their earlier estimate and helped further solidify the initial findings.

To make sure the wave wasn’t just the result of tides or wind, the team dug into tidal models and checked data from a local weather station, but the patterns didn’t match. Tides and wind-driven currents like Ekman transport were ruled out based on tidal modeling and local weather data, which didn’t match the observed cross-channel slopes. The spatial pattern and timing of the water’s motion lined up too well with the concept of a seiche for it to be anything else.

Expanding the role of satellite altimetry
Another great finding of this research was how satellite altimetry, which is traditionally used for slow-moving ocean trends, can now resolve fast, local events in coastal environments.

“This study is an example of how the next generation of satellite data can resolve phenomena that has remained a mystery in the past. We will be able to get new insights into ocean extremes such as tsunamis, storm surges, and freak waves. However, to get the most out of these data we will need to innovate and use both machine learning and our knowledge of ocean physics to interpret our new results,” commented Professor Thomas Adcock, one of the study’s co-authors from the University of Oxford’s Department of Engineering Science.