The Japanese earthquake has caused waves in Norway

Aurlandsford — one of those where the waves were observed. (Photo by Grisha Levit.)

July 2, 2013. An earthquake biggest load assumes a region located near the epicenter, and sometimes a push, you can register and at a great distance. But seismic waves emitted by an earthquake, in fact diverge around the world several times and bend around the globe, turning the planet into a large bell ringing. The stronger the earthquake, the louder he "calls." A earthquake magnitude 9.0, which occurred off the coast of Japan March 11, 2011, was a very, very powerful.

Seismometers are sensitive enough to detect seismic waves of distant earthquakes even on their second or third turn of a planet, and satellites can observe concomitant atmospheric disturbances. However, the ultra-precise measurements are needed not always. For instance, station monitor groundwater levels in the U.S. state of Virginia noted rise of the water table by passing waves from the Japanese earthquake …

See also: Effects of the earthquake and tsunami in Japan

That backfired … even in the Norwegian fjords (GoogleMap)! Several people noticed the strange waves quiet morning. Water arrived and just as suddenly went somewhere around half a meter per minute for several hours, and it started after about half an hour after the earthquake in Japan.

Long, slow waves in fluids of this type are called seichesAnd they occur for several reasons in the closed (or partially closed) reservoirs. Usually they cause the wind is forcing the water to one edge of a lake or bay, but they can cause and earthquake ground shaking. Then seiche like fluctuations of the liquid surface in the tank. At this time, the tank became fjords — steep, but wide valley carved by glaciers and later flooded to raise the seas.

A group of Norwegian researchers tried to find out how this could happen. To do this, the scientists used a computer model developed to simulate the tides and storm surges. They entered data on a shake, filed a nearby seismometer, and compared the simulation results with video and eyewitness seiche. Model coincides with the observations fairly well, allowing to take up the study of the causes of the phenomenon.

Seismic waves diverge from earthquakes place in two ways: in the radial direction through the thickness of the ground or to one side, sliding on the surface. First faster then the second lead to more dropped, especially in areas located far from the epicenter. That surface seiche waves cause.

In the case of the fjords, however, were the cause of the so-called S-waves passing through the earth. Vibration of the S-wave is perpendicular to its direction of movement. If you squeeze the end of the rope tied to the wall, and gently yank his wrist, sending a wave to the opposite end, you produce S-wave. The model showed that it is horizontal vibration led to fluctuations in the water in the fjord, and similar vibrations of surface waves them slightly increased. By the way, it was clear from observations, for seiche emerged after the S-wave arrived, but before a few minutes later they came for surface waves.

If the S-waves produce less vibration, why they have caused seiche in this case? All the matter in the oscillation frequency. S-wave varied at a lower frequency in synchronism with the amount of time required on it to pass through the fjord, i.e. essentially resonate.

Seishi associated with earthquakes, are not new in this area — they were seen in 1755, 1920 and 1950. But why do some earthquakes are produced, and the other not? It is not just the magnitude — for example, the earthquake in Chile in 2010 the power of 8.8 had no effect on the Norwegian water. Again, the answer given simulation.

The difference is related to the properties of seismic waves and the destination of the earthquake. Low-frequency, transverse component of the Chilean earthquake was weaker. Moreover, and this is obvious from Chile seismic waves come to the other side of Norway than Japanese. Importantly, the seismic waves from Japan, as it happens, were almost along the fjords, ie transverse vibration was focused just across the fjord. Seismic waves from Chile just lined up in such a way as to cause fluctuations of water in the fjords.

Topics March morning vibration in the right direction and at the right frequency has been noticed by travelers random before the terrible news came from Japan …

The study is published in the journal Geophysical Research Letters.

Based on: Ars Technica
Source: Kompyulenta

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