Posts Tagged ‘Seiche’

Great Tohoku quake in 2011 caused standing waves in Norwegian fjords 30 minutes later

August 13, 2013

Seismic seiches are standing waves set up on rivers, reservoirs, ponds, and lakes when seismic waves from an earthquake pass through the area. They are in direct contrast to tsunamis which are giant sea waves created by the sudden uplift of the sea floor.”

A new paper describes how these seiches -standing waves – observed in the Norwegian fjords in 2011 – for the first time since the 1950’s – have been linked to the Great Tohoku Quake of 2011 half an hour earlier.

Stein BondevikBjørn Gjevik and Mathilde B. Sørensen, Norwegian seiches from the giant 2011 Tohoku earthquake, Geophysical Research Letters, DOI: 10.1002/grl.50639, 2013

Abstract: Seismic waves of the giant 2011 Tohoku earthquake triggered seiches in western Norwegian fjords. The seiching began a half hour after the earthquake origin time. The oscillations were noted by eyewitnesses and recorded by surveillance and cell phone cameras. The observations show maximum trough-to-peak amplitudes of 1.0–1.5 m and periods of 67–100 s. The water waves were not triggered from the arrival of the surface waves, the timing inferred for other seiches. Instead, the seiching began during the passage of horizontal waves. We reproduced the S wave trigger by means of a shallow-water wave model calibrated previously to Norwegian tides and storm surges. The simulations, which used the observed earthquake motion as forcing, show water waves with periods and amplitudes similar to those in the film clips. However, the strongest horizontal ground oscillations with shorter periods (20–30 s) did not contribute much to the formation of the seiches.

It is not the first time that such standing waves have been observed so far away. As the US Geological Service notes:

The term seismic seiche was first coined by Anders Kvale in 1955 to describe oscillation of lake levels in Norway and England caused by the Assam earthquake of August, 1950. But this was not the first time that seismic seiches had been observed. The first published mention was after the great earthquake of November 1755 at Lisbon, Portugal. An article in Scot’s Magazine in 1755 described seiches in Scotland in Loch Lomond, Loch Long, Loch Katrine and Loch Ness. They were also seen in English harbors and ponds and were originally described in the Proceedings of the Royal Society in 1755.

…….

Seismic waves from the Alaska earthquake of 28 March, 1964, were so powerful that they caused water bodies to oscillate at many places in North America. Seiches were recorded at hundreds of surface-water gaging stations – although they had rarely been reported following previous earthquakes. Indeed, four seiches were observed in Australia.

Some of the 1964 seiches were very large. Waves as high as 1.8 meters were reported on the Gulf Coast – probably because they were generated in resonance with the seismic surface waves.

In the case of the Norwegian fjords and the Great Tohoku quake, PhysOrg reports:

The scene was captured by security cameras and by people with cell phones, reported to local media, and investigated by a local newspaper. Drawing on this footage, and using a computational model and observations from a nearby seismic station, Bondevik et al. identify the cause of the waves—the powerful magnitude 9.0 Tohoku earthquake that hit off the coast of Japan half an hour earlier.

In closed or semi-enclosed bodies of water, seismic waves can trigger standing waves known as “seiches.” Seiching had not been recorded in Norway’s fjords since 1950. Scientists have traditionally thought that seiching is caused by seismic surface waves, but the authors find that the fjord seiching was initiated before the surface waves had arrived.

Using seismic observations and a model for local fjord behavior, they find that in this case the seiching was triggered by S waves, which travel through Earth’s body, and later was amplified by Love waves, which travel on Earth’s surface. There are a lot of open questions surrounding the connection between earthquakes and seiching, but the authors’ research supports the idea that not all earthquakes will cause seiching in all enclosed bodies of water. The occurrence of the Japanese earthquake?induced seiches depended on the period and orientation of the seismic waves aligning with the natural frequency and orientation of the body of water.


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