What is a Cascadia mega-quake
Cascadia: Mega quake near Seattle?
There are several seismically active zones in the USA along which researchers expect a very strong earthquake. One of these zones extends from southern Canada to northern California. In the middle are the metropolises of Vancouver, Seattle and Portland.
The Cascadia subduction zoneWe are talking about the Cascadia subduction zone, along which the Pacific tectonic plate descends below that of the North American continent. Along the Cascadia Fault Zone, which extends more than 1,000 km off the coast of North America, the Pacific Plate has broken into several small plates. The largest of the plates is the Yuan de Fuca plate, which dominates the fault zone. In the north is the smaller Explorer Platte and in the south the Gorda Platte. The boundary between the Yuan de Fuca Plate and the Pacific Plate is marked by an oceanic ridge. Jacket material rises in a convection cell and pushes the plates apart. This creates a divergent plate boundary. A series of volcanoes formed behind the subduction zone on the North American plate: the Cascaden Range. There the magma is erupted, which is formed by the melting of the subducted Yuan de Fuca plate in the earth's mantle. The melt rises and formed volcanoes such as Mt. Rainier and Mt. St. Helens. The southern limit of the fault zone is of particular interest. There the Gorda plate meets the Pacific plate and that of the North American continent. At this triple point, the Mendocino fracture zone arises, which meets the Cascadia fault zone perpendicularly. This is also where the famous San Andreas Fault begins, which is the most famous earthquake zone in North America.
Wedged Yuan de Fuca plateThe Yuan de Fuca Plate, plunging under the North American continent, reaches a depth of 60 km under the metropolitan area of Seattle. The worrying thing about this situation is that not only the Juan de Fuca plate is shifting, but also the foreland of Seattel: it is shifting towards the west at a speed of 4 cm per year. The submerged Yuan de Fuca plate is wedged with the continental plate and upsets the land. This arches upwards and is stretched like a spring. At some point the wedged rock will break and the tensions will discharge in a tremendous earthquake. Earth tremors with a magnitude of 9 are possible. The North American continental plate shoots up several meters. What is particularly fatal is that the plate boundary runs in the ocean. This will trigger a huge tsunami that will spread both west and east. So after 4 hours it would hit the Hawaiian archipelago and after 8 hours it would visit the coast of Japan.
The probability that a major earthquake will occur within the next 40 years is 90%. Geologists also found that the underground in the Seattle area amplified earthquake waves. Because under the city there is a basin that is deeply filled with loose sediments. Such sedimentary basins act as resonance bodies, which rock the earthquake waves.
Earthquake in the 18th centurySeismologists have found that, on average, one very powerful earthquake manifests itself along the Cascadia Fault Zone every 240 years. But the last major earthquake in this region was more than 300 years ago. It occurred in January 1700 and had an estimated magnitude of 9.0. At that time the area was sparsely populated and the number of victims was limited. Today an earthquake of this magnitude would be fatal.
Unlike the cities along the San Andreas Fault, the metropolises in the north of the USA are less well prepared for a strong earthquake. Protection and evacuation measures are now being rehearsed in schools. Seismologists warn, however, that these measures would be useless in the event of a magnitude 9 earthquake. Engineers examine the stability of the building and come to different conclusions. Some predict that the multitude of large buildings in the region would be destroyed in a mega-quake. Others believe that older brick buildings in particular would collapse.
Preparation for the mega quake near SeattleBut how do you counter the threat of a mega-quake of this magnitude? Can you even prepare for it? It certainly makes sense to train for an emergency and to stock up as a precaution. In the event of survival, these would certainly be useful. But if the prophecies of the scientists were to come true, the deaths would be very high. If you want to be on the safe side, you should avoid the region. Status 2018. Photo © USGS
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