A magnitude 7 earthquake struck on Sunday, July 10 northeast of Japan, which had already been devastated by an earthquake and a devastating tsunami on March 11.
Why the Earth is shaking she still in Japan?
To simplify, we can say that Japan is made to be seismic. This huge island chain is located right on the border of four tectonic plates (see diagram below): the Pacific plate (light pink), the North American plate (purple), the Eurasian plate (in green) and the Philippine plate (in pink).
The Pacific plate, which moves rapidly to the west (more than 8 cm per year), sinks under the other: this is called a subduction. Subduction zones result in the largest earthquakes that are, but also by intense volcanism. The Japanese archipelago is located elsewhere on the Pacific Ring of Fire, a huge arc of more than 450 volcanoes that made all around the Pacific, from Patagonia to Alaska and Kamchatka to New Zealand. If, in Japan, is best known Mount Fuji, inactive for three centuries, we must not forget that dozens of other volcanoes exist in the country, as Shinmoedake whose recent eruption, in January, has been dramatic as shown in the video below:
What were the biggest earthquakes in Japanese history?
If one takes into account the magnitude, the earthquake of Friday, March 11 is the largest in the history of Japan with a magnitude of 8.9. For comparison, the magnitude of the deadly earthquake in Kobe on 16 January 1995 was "only" 6.9. Until now, the earthquake was the most intense, according to the USGS website, that of June 15, 1896, with a magnitude of 8.5. This caused a massive tsunami (there is talk of a wave 25 meters high) that devastated the port of Sanriku and killed 22,000 people. To date, the most deadly earthquake ever recorded in Japan is the 1st September 1923 who, according to sources, a magnitude ranging from 7.9 to 8.3. Nearly 700,000 homes were destroyed in the area of Tokyo and Yokohama and about 143 000 people lost their lives in this natural disaster.
How does the Richter scale?
From the measurement of seismic waves recorded by seismographs, we calculate the magnitude of the earthquake, that is to say, its amplitude and the energy was released during the phenomenon. The Richter scale is logarithmic so-called, not linear, meaning that a difference of one point corresponds to an amplitude 10 times and an energy release about thirty times more! Thus, an earthquake of magnitude 6 is it equivalent to the explosion of the atomic bomb on Hiroshima (15 kilotons of TNT) and an earthquake of magnitude 7 passes to 474 kilotons of TNT). An earthquake with a magnitude of 8.9, like today, is equivalent to the explosion of 336 million tons of TNT.
In theory, the scale is open but never recorded an earthquake with a magnitude greater than 9.5, which is the 1960 earthquake in Chile, which caused a massive tsunami killed more than 2,000 people. It is generally believed that an earthquake is felt from a magnitude of 3 or 4 (tens of thousands of events per year on average). From 6, the earthquake is considered strong. Earthquakes of magnitude 8 or greater are generally infrequent but devastating (one per year). Thus, in the first decade of the century, 13 of these natural disasters have they been identified.
What is the danger of an earthquake?
Simply put, "the most dangerous earthquakes are those closest to where people live and not necessarily those with the largest magnitude, said Nikolai Shapiro, director of research at CNRS and head of the team seismology at the Institute of earth Physics in Paris. Therefore, an earthquake occurs on the fault of a big city will do more damage, as was the case during the earthquake in Kobe in 1995. Its magnitude was 6.9, two points less than today's earthquake, which is huge, but it has produced more deaths because of flaws in the city was activated. "
Why do we often fires after an earthquake?
When the great Japanese earthquake of 1923, many houses were destroyed by fire. This reflects the fact that the earthquake took place at noon, when many fires were lit in the kitchen to prepare lunch. Similarly, the great earthquake of 1906 in San Francisco was marked by severe fires, mainly attributed to the rupture of gas pipelines. There are the shorts to the list of causes of fire outbreaks.
Is there a link between the earthquake and that of New Zealand from 22 February or the eruption of Shinmoedake?
"There is no scientific reason to talk about a possible link with the earthquake in New Zealand or the volcanic eruption. However, there has been a clear precursor of the earthquake two days ago, March 9th. There was an earthquake of magnitude 7.2 in exactly the same area, which was followed by several aftershocks 9 and March 10. A tsunami warning was even launched. Today we know that this was the precursor of a larger earthquake, the beginning of the destabilization of the fault. But we can not generalize because if often earthquakes of 7.2, they are rarely followed by earthquakes of 8.9 ... "
South Korea may she something?
[Question of Internet] The tsunami wave propagated like a falling stone in water, which is in western Japan (including Korea) can not be touched because the archipelago Japanese is actually a shield. Here is a model of the tsunami's progress:
Obviously, Hawaii, the wave was higher than 3 feet, slightly less than one meter.
How is a tsunami?
In an undersea earthquake, the floor abruptly deforms the ocean, which has the effect of a wave train (see diagram below). This is not the sea that moves, but just this wave, much like the "ola" in a football stadium: the spectators are just getting up and sitting down one after the other, they are not being run around the stadium.
Offshore, the wave is not necessarily a great height. Although the tsunami travels fast, its wavelength is so large that the wave is often overlooked boats that are on it. It is said also that the word "tsunami" (which means, in Japanese, harbor wave) was invented by fishermen at sea, had not noticed anything and they were returning to port devastated. When it comes close to land where the seabed rises, the wave is slowed and compressed, which makes it mechanically gain height, much like a piece of paper which brings the edges. For a short time, the sea withdrew from the beaches, as drawn back by the wave and surf, with a power such that nothing can stop, if not the landforms. For this reason, when a tsunami warning, it is recommended that residents of higher ground.
How far does this go to the tsunami on March 11?
In the open ocean, the tsunami wave is moving very quickly because speed is of the order of several hundred kilometers per hour. Both the terrestrial relief is able to stop quickly enough because the coast acts as a natural barrier, as its alleviation will be slower in the open ocean. According to Nikolai Shapiro, "the tsunami of today will be recorded by tide gauges around the Pacific Ocean, perhaps even to Chile."
The wave will therefore necessarily French Polynesia, but the question is whether its amplitude is a few centimeters to a few feet or more ... "A series of waves is likely to affect all of French Polynesia, with risks of wave height different from place to place," warned the High Commission of the Republic in Papeete.
