This is the strongest earthquake ever recorded in Japan. Friday, March 11, an earthquake of magnitude 8.9, followed by numerous and significant aftershocks hit north-east, triggering a tsunami with waves reaching ten meters, and causing hundreds of deaths and many missing. Jerome Vergne, a seismologist at the Institute de Physique du Globe de Strasbourg, decrypts this phenomenon.
Japan suffers from numerous and strong earthquakes. This country is indeed on the edge of two tectonic plates. At this point, the Pacific plate slides to the west and plunges under the North American plate, which includes part of Eurasia at a rate of almost 9 centimeters per year. This subduction does not occur continuously but jerky, each saccade is reflected by an earthquake.
And as the speed of evolution of faults is important, destructive earthquakes occur at a given location in the region, frequently, about every hundred years. In contrast, in France for example, the plates change only a few millimeters per year. The return of large earthquakes is then about 10 000 years.
But we will probably never earthquake with a magnitude similar to that of Sendai insofar as France, particularly the southeast, is located on an area of \u200b\u200bcollusion between two continents, while the strong earthquakes occur more in oceanic subduction zones, such as Japan. This is the strongest earthquake ever recorded in Japan and one of the ten strongest ever occurred in the world.
It is first very important because of its magnitude, 8.9 on the Richter scale, which implies a tremendous energy released in a few minutes, much larger than any nuclear test. Moreover, the home is fairly close to the surface, about twenty kilometers. The earthquake has shifted significant bodies of water, causing a tsunami.
Finally, it occurred at sea, but close enough to the coast, between 150 and 200 km. By cons, it is characterized by a rupture velocity rather rapidly, which should not cause a tsunami of great magnitude. Breaks slow, however, are the most dangerous. The earthquake in Sendai is very strong but conventional point of view of its mechanism.
We have no method of predicting some short-term and the occurrence of an earthquake. It may just indicate areas at risk. The north-eastern Japan was among areas with high probability because it had not experienced a major earthquake for nearly a hundred years. Moreover, last week, a series of strong earthquakes took place there: March 9, for example, a shock with a magnitude of 7.2 occurred about 40 km, followed on the same day, three other events of a magnitude greater than 6.
Japanese seismologists were intrigued. But this kind of event occurs very frequently in Japan, but they are followed by a major earthquake. Japan is the country best prepared in the world. For years, particularly since the Kobe earthquake of 1995, which killed more than 5,000 dead, all equipment meets standards para-seismic.
Many systems are designed to stop automatically, such as nuclear, gas lines or the automatic trains, and the population knows what to do with regular training. Everything is done to reduce the damage induced by earthquakes. Japan could not do more. The authorities can not remove the risk of populations at each major earthquake, those of magnitude greater than 6 occur several times a year, almost every month.
It depends on the extent of the fracture. It is currently estimated at 500 km long and thirty kilometers wide. When we have further studied, it will give us a clearer idea of \u200b\u200bthe magnitude of the aftershocks. They should last several weeks, although their frequency and magnitude will decrease.
But we are not immune to a bigger event, the "big one" that seismologists fear in an area further south, near Tokyo. This segment, the boundary of tectonic plates, has not experienced a major earthquake since 1940. The fault segment that broke in Sendai may have advanced the occurrence of an earthquake even stronger to the south.
Interview by Audrey Garric 
Japan suffers from numerous and strong earthquakes. This country is indeed on the edge of two tectonic plates. At this point, the Pacific plate slides to the west and plunges under the North American plate, which includes part of Eurasia at a rate of almost 9 centimeters per year. This subduction does not occur continuously but jerky, each saccade is reflected by an earthquake.
And as the speed of evolution of faults is important, destructive earthquakes occur at a given location in the region, frequently, about every hundred years. In contrast, in France for example, the plates change only a few millimeters per year. The return of large earthquakes is then about 10 000 years.
But we will probably never earthquake with a magnitude similar to that of Sendai insofar as France, particularly the southeast, is located on an area of \u200b\u200bcollusion between two continents, while the strong earthquakes occur more in oceanic subduction zones, such as Japan. This is the strongest earthquake ever recorded in Japan and one of the ten strongest ever occurred in the world.
It is first very important because of its magnitude, 8.9 on the Richter scale, which implies a tremendous energy released in a few minutes, much larger than any nuclear test. Moreover, the home is fairly close to the surface, about twenty kilometers. The earthquake has shifted significant bodies of water, causing a tsunami.
Finally, it occurred at sea, but close enough to the coast, between 150 and 200 km. By cons, it is characterized by a rupture velocity rather rapidly, which should not cause a tsunami of great magnitude. Breaks slow, however, are the most dangerous. The earthquake in Sendai is very strong but conventional point of view of its mechanism.
We have no method of predicting some short-term and the occurrence of an earthquake. It may just indicate areas at risk. The north-eastern Japan was among areas with high probability because it had not experienced a major earthquake for nearly a hundred years. Moreover, last week, a series of strong earthquakes took place there: March 9, for example, a shock with a magnitude of 7.2 occurred about 40 km, followed on the same day, three other events of a magnitude greater than 6.
Japanese seismologists were intrigued. But this kind of event occurs very frequently in Japan, but they are followed by a major earthquake. Japan is the country best prepared in the world. For years, particularly since the Kobe earthquake of 1995, which killed more than 5,000 dead, all equipment meets standards para-seismic.
Many systems are designed to stop automatically, such as nuclear, gas lines or the automatic trains, and the population knows what to do with regular training. Everything is done to reduce the damage induced by earthquakes. Japan could not do more. The authorities can not remove the risk of populations at each major earthquake, those of magnitude greater than 6 occur several times a year, almost every month.
It depends on the extent of the fracture. It is currently estimated at 500 km long and thirty kilometers wide. When we have further studied, it will give us a clearer idea of \u200b\u200bthe magnitude of the aftershocks. They should last several weeks, although their frequency and magnitude will decrease.
But we are not immune to a bigger event, the "big one" that seismologists fear in an area further south, near Tokyo. This segment, the boundary of tectonic plates, has not experienced a major earthquake since 1940. The fault segment that broke in Sendai may have advanced the occurrence of an earthquake even stronger to the south.
Interview by Audrey Garric
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