29 June 2011
The mini-tsunami on the south coast of England – a submarine landslide?
Posted by Dave Petley
A fascinating story has appeared in various news outlets in the Uk over the last few hours. On Monday morning this week the southwest coast of the UK was affected by a highly anomalous marine event. This is covered quite well in a BBC news article, which describes “rivers changing direction, fish leaping out of water and hair standing on end due to static” and brief tidal anomalies of 0.2 m in Newlyn, Cornwall, 0.3 m in Plymouth and 0.4 m in Portsmouth. The event was also caught on video in the Yealm estuary near to Plymouth:
The various news reports ascribe this event to a submarine landslide. Now, this is quite an interesting event actually. The Google Earth map below shows the seafloor topography in this area and the locations that news reports suggest experienced the “tsunami”:
A few things to note here. First, the locations that this was reported are widely spaced Second the largest event was reportedly at Portsmouth (which is both a substantial distance to the east and sheltered by the Isle of Wight. Third, the seas offshore the south coast of England are really quite shallow. Fourth, the continental slope, which is the most likely location for a submarine landslide, are well to the west.
To me this is a somewhat strange event that requires some deeper investigation. I wonder if the tsunami was observed on the coast of northwest France – if this was a continental slope event then it should have been, but I have seen no reports to suggest this.
Certainly something happened to cause this event on Monday, but I think the jury is still out as to what it might have been, and where.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
No quakes recorded on 27th or anything that could explain this. Here is the BGS recent records .
http://www.earthquakes.bgs.ac.uk/recent_events/recent_events.html
Some slight interference on the Jersey records but not big enough event I think to cause such displacement. Very interesting. There was a quake at Newton Abbot 4 days prior to this. That also was a strange one. First time I have seen one in that area. Newton Abbot situated at the end of the River Teign estuary. Most of the rivers in this area are Rias. All these rivers come down from Dartmoor a large Granite Boss.
However like you I feel maybe this is possibly something that happened on the edge of the continental shelf but shouldn’t this amount of displacement have caused helicorders to have recorded something?
While I do not have the weather data to verify it, I suspect the tsunami was a meteotsunami — caused by a squall line off of the shore. When the squall is traveling near the same speed in the shallow water which is about 100 km/hr, it can excite the long wave. This is rare, but has happened a few years ago in Maine, and 1992 in Daytona Beach.
I am not seeing anything on any of the nearby tide gauges. For example http://www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Newlyn&span=2 . It seems strange given the reported size of the wave.
It seems that on the “Le Conquet” station, in Britany there is a small signal at about 7:00. Data visible at
http://www.ioc-sealevelmonitoring.org/station.php?code=leco
There are pockmarks on the continental shelf which may be indicative of past methane hydrate dissociation. There is methane hydrate formation on the continental slopes across the globe. If there was no earthquake and a submarine landslide is suspected a fair sized methane hydrate dissociation could very well be the culprit. If you do not know what methane hydrate is, it is time to learn about it because the governments of the world involved in the Joint Industry Project are not going out of their way to educate the public about this powerful and abundant fire ice. Some scientists think a globe wide methane hydrate dissociation/eruption event may have acted as a catalyst in at least 1 past extinction event in the geological record. The warmer the ocean gets the more of these chunks of methane hydrate (fire ice) will break off and cause this type of phenomena. The more methane reaches the atmosphere the more the atmosphere warms (positive feedback loop)
Here is a map of methane hydrate deposits across the world. I am not asserting that I know, only that I believe this could explain why the landslide could have happened with no seismic activity detected.
http://newenergyandfuel.com/http:/newenergyandfuel/com/2008/04/23/a-breakthrough-in-fuel-supplying-from-methane-hydrates/world-gas-hydrates-locations/
When a chunk of gas hydrate breaks off the ocean floor or continental shelf the gas impregnated ice floats upward, the lattice of ice compressing the gas melts, the gas decompresses and expands to 164 times it’s compressed volume and displaces seawater. Like dropping a pebble into a pond only from the bottom up not the surface down. (and the pebble expands as it floats up) Crude analogy I know but best I can do as an unofficial researcher. I am not a scientist, just a concerned citizen of the 3rd stone from the sun.
The signal is clear (albeit small) on all SW tide gauges of the UK national network. Magnitude of the non-tidal residual is around 10-20 cm and seiching at the gauge sites carries on for about 24 hours. The most likely cause, I would say, is mesoscale meteorology (e.g. squall line, downdraft on 50km scale) coupled with resonance of a particular water body. Interesting.
According to Spotlight (local Plymouth news) it was a sitch and fairly common on the Yealm – so much so that boats on the harbour out of the water are chained down so they don’t float off when this happens. It was on the iPlayer
[…] Video of a ‘Mild tsunami’/surge – possibly caused by an underwater landslide – moving up-river in SW UK . http://www.bbc.co.uk/news/uk-england-devon-13959478 (via @BGSwebEd) Dave Petley on the Landslide blog has more: it’s still not clear what caused it. https://blogs.agu.org/landslideblog/2011/06/29/the-mini-tsunami-on-the-south-coast-of-england-a-subma… […]
Is it normal for peoples hair to stand on end in the thunderstorm style for a tsunami? I don’t buy they were frightened by the 1 foot wave…
Personally, I woud take the reported “… hair standing on end due to static” with a grain of salt. Any evidence controverting Dan’s report that it could have been a tidal bore?
[…] en el blog del geólogo Dave Petley que la semana pasada hubo un minitsunami en la costa suroeste de Inglaterra. Aunque llegó a salir en la BBC yo no me enteré. De hecho, ni siquiera sabía que existieran este […]