California is preparing to face not one, but two strong earthquakes in the years to come. The west coast is located on the San Andreas Stall Fault, a fracture zone more than 1,000 km long extending to Mexico. This geological border, between the Pacific Oceanic and North American continental plates, is conducive to large-scale seismic phenomena.
“Under the action of plate tectonics, the oceanic crust of the Pacific moves several centimeters per year towards the continental plate,” explains Jean-Paul Montagner, seismologist at the Paris Institute of Globe Physics. At the level of California, its movement towards the northwest causes an accumulation of tectonic forces between the plates. This causes, over time, a release of these forces resulting in a rapid and violent movement of the earth’s crust of 5 to 10 m, associated with an earthquake of at least 8 on the Richter scale, nicknamed The Big One by the United States ”.
The speed of movement of the oceanic plate being constant, the phenomenon is periodic. It occurs every 150 to 200 years. There are two main rupture zones: a northern segment, located at San Francisco, and a southern segment, near Los Angeles. “But the seismic cycle is not perfectly regular, specifies Jean Paul Montagner. We cannot therefore predict whether the phenomenon will occur tomorrow or in 50 years on each of these segments. “
Still, the last two major earthquakes date back to 1857 (an event of magnitude 8.3, at Fort Tejon, north of Los Angeles, in an uninhabited area) and 1906 (near San Francisco, of magnitude 7.8, making at least 3,000 victims). Initially, we should rather fear a rupture of the fault at the level of Los Angeles. The inhabitants of San Francisco would, in theory, have a reprieve of about forty years.
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