A total of 40, 000 kilometers are the subduction zones of the earth (blue lines). They are all a danger zone for Megabeben. (c) Science
In the last five decades, there have been five Earthquakes of magnitude 9 on Earth? the highest category ever seen on Earth. That could have been a random accumulation of such events, the geophysicist Robert McCaffrey has calculated. According to his results, the average value should be three megabits per century. But this good news has a catch: According to McCaffrey, such quakes can occur in all so-called subduction zones. At this plate boundary type, an oceanic plate slides under a continent or under another oceanic plate. Due to the well-known earthquake history and certain physical considerations, geoscientists had previously assumed that megabounds of magnitude 9 can not occur in principle in some subduction zones. The surprise over the 2004 Sumatra Andaman earthquake, which reached 9.2 magnitude and caused the devastating tsunami in the Indian Ocean, was huge among geoscientists: according to popular models, there should not have been a mega-heir there.

McCaffrey now consulted statistical considerations. Since there have been seismographs for only a hundred years and historical legends go back only a few centuries, it is not possible to conclude from the well-known earthquake history that some sections of subduction zones are spared by megabs. Because of the long return times, the statistical data base is insufficient for such conclusions. Since subduction zones are generally located in the ocean, tsunamis must be expected at all such plate boundaries.

Geophysicists have been trying for some time to make at least statistical forecasts of earthquake recurrence times at certain plate boundaries. The underlying calculations are relatively simple: In a magnitude 9 quake, the earth's crust shifts an area of ​​several hundred kilometers of a plate boundary by 20 meters at a stroke. Depending on how fast the plate moves? between two and ten centimeters a year? It takes between 200 and 1, 000 years to accumulate enough tension for a mega life. If the voltage in between is reduced by smaller quakes, the intervals between two megabounds can be even longer.

Based on these considerations, McCaffrey comes to the conclusion that unusually many megabounds have occurred in recent decades. The strongest ever quake occurred in Chile in 1960 (magnitude 9.5), there were other magnitude 9 quakes in the Aleutian Islands in 1946, in Kamchatka in 1952, in Alaska in 1964 and in Sumatra in 2004. display

Robert McCaffrey (GNS Science, Lower Hutt, New Zealand) et al .: Geology Vol. 36, p. 263 Ute Kehse

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