A small tsunami that struck the south coast of England on 27 June was most likely caused by weather conditions and not a submarine landslide or earthquake, according to the British Geological Survey (BGS).

The tsunami occurred in South West England, between Penzance and Portsmouth, along approximately 200 miles of coastline. It struck the Yealm (see below video) at about 10.30 on 27 June; wave heights were 0.5 - 0.8 metres. Tide data indicates an east to west progression of the wave, confirmed by observations at St Michael's Mount, where the water built up on the east side of the causeway (20 cm higher than on the west) before it was overtopped.

According to the Tidal Gauge Anomaly measure, which records the difference between the forecast tide and actual tide, the wave was higher by 20 cm in Newlyn, 30 cm in Plymouth and 40 cm in Portsmouth. The water levels dropped and withdrew from the coast before the wave came in (Marazion, Cornwall). Video evidence shows the wave as a bore in the Yealm Estuary. At St Michael's Mount there were reports of 'hair standing on end' before the wave struck.


Many reports of the event concluded that the tsunami was created by a submarine landslide on the continental slope off of the South West, 200 miles south-west of Newlyn. However, this does not fit the observations says the BSG.

It says: "Our conclusion is that the tsunami was not geological in origin. It was probably caused by a meteorological effect, such as a squall over the ocean developed during summer storms; these events are called meteotsunami. These types of waves are common all over the world and are known by their local names, such as: rissaga (Spain), milghuba (Malta), marrobbio (Italy), and abiki (Japan).

In explaining the unlikely occurnace of an earthquake or submarine landslide, the BGS said: "From the available data the tsunami wave progressed and decreased in height from east to west, indicating a source in the eastern English Channel: this rules out a source to the west.

Thus a submarine landslide on the south-west continental slope is discounted; a source here would have resulted in a tsunami decreasing in height the opposite direction, from west to east. In addition, if a submarine landslide was located here there would have been a tsunami wave at other locations, such as in southern Ireland and France - none has been reported.

"Most likely geological sources in the eastern channel are considered to be either an earthquake or a submarine landslide. No earthquakes were reported at this time. It is also unlikely that a submarine landslide took place in the eastern channel. Submarine landslides need unstable sediment to form; additionally they need a trigger, most commonly an earthquake. In addition, a submarine landslide would have created a tsunami focused on the nearest coastline, and the area of inundation would have been limited - less than 200 miles."

"Hair Raising Event"

One aspect of the June 27 event that is considered novel, and may bear on the answer, is the 'hair standing on end' before the tsunami that was reported from St Michael's Mount, that suggests an atmospheric source, as it is known that this phenomena can take place in association with lightning. According to the BGS: "We know that there were storms in the south-east of England on 27 June. At Wimbledon, play was restricted to the covered Centre Court on Monday afternoon, and there was thunder and lightning.".

"Monitoring Stations"

Meanwhile, a network of coastal tidal and wave monitoring stations maintained by Southampton, UK-based EMU Limited recorded the progress of the waves caused by the minor tsunami.

EMU Limited's Principal MetOcean Scientist, Mr Robin Newman, initially thought there was a malfunction with the oceanographic instruments, installed by EMU for the Southeast and Southwest Regional Coastal Monitoring Programmes, due to the unusual data patterns recorded by the Etrometa Step Gauges and Rosemount WaveRex Radars.

"There was a significant amount of variation in the observed data against what would be expected so I checked at multiple sites and they were all consistent with some sort of movement from east to west," Mr Newman said. "We subsequently realised we had recorded what appears to be a minor tsunami."

"While 0.8 metres waves may not sound like much, it could have caused flooding events if combined with high tides," he said. "Tsunamis are not expected to affect the UK so this is a very rare event. But from this week's experience, clearly they can, but also clearly we can now say we have an operational system which can detect these unusual events."

EMU Limited will monitor the continuing wave movements as these may be a form of standing-wave oscillation, or seiching, in the Channel.

HISTORY

Meteotsunamis are not unusual in the UK . along the south coast of England.

On 18 August 1892 tsunamis hit the Yealm (Devon) and Fowey estuaries (Cornwall), following thunderstorms in the region. The Times reported the event in the River Yealm as well as stating that 'there was a rapid rise in the River Fowey as a great tidal wave, but this immediately subsided'.

A large tsunami-like wave struck the Kent and Sussex coasts, busy with tourists, and drowned two people, at Folkestone (Kent) and Brighton (Sussex) on 20 July 1929. At Brighton and Worthing the wave was accompanied by sudden downpours of rain and high winds, but at Folkestone and Hastings, where one person drowned at each, the weather was clear and the unexpected wave was estimated to be between 3.5 m and 6 m high, respectively. Later analysis of the event suggested the wave was caused by a squall-line travelling up the English Channel, coincident with thunderstorms, and so may be referred to as a meteotsunami.