Could the Seismic Activity in Central and South America Affect the Cascadia Fault Line?

In just 48 hours, South America has been rocked by volcanic explosions and a series of earthquakes.

The news has been full of the 8.2 magnitude quake that struck Chile, but since then they have suffered 27 more quakes of magnitude 5+, three of which were more than 6+ and a 7.6 not long ago at 0553 GMT. On top of this there have been literally dozens of minor quakes.

Panama has had two 6+ quakes some hours apart and numerous smaller shocks. El Salvador, Argentina and Mexico have both experienced 4.2+ tremblors in the last 24 hours and Puerto Rico is currently experiencing smaller quakes of a magnitude between 2 and 3.

Ubina volcano in Peru, quiet for four decades, is throwing up ash which has forced the evacuation of several villages around the base of the mountain.

Ubina is not alone. Reventador in Peru is also erupting as are volcanoes in Mexico (Colima) and Guatemala (Santa Maria, Fuego and Pacaya). Another dozen are showing minor activity and a further 16 are marked as showing signs of unrest. You can see the full list here.

The jury is out on whether one part of the Ring of Fire can affect another area of a seismically active belt that runs around the Pacific rim and with so much activity south of the Cascadia Fault I hope they are right.

The Cascadia Fault is a subduction zone, an area where one of the tectonic plates is forced underneath the plate it collides into. In the case of Cascadia, the Juan de Fuca Plate is moving under the North American Plate. These plates have been locked together since 1700, 313 years. A particular feature of the Cascadia Fault is that it doesn’t produce small quakes, it remains still and silent until the pressure gets too much and one of the plates slips, giving rise to a mega-thrust earthquake of massive force. These quakes can be compared to the Indonesian quake of 2004 and the Japanese quake of 2011.

The Cascadia fault is long, very long, just over 800 miles (1300km) in length. Based on the findings of the scientists, the tsunami caused by the 1700 event moved inland for more than 60 miles, wiping out everything in its path. Of course in 1700 Seattle wasn’t there, neither was Vancouver, San Francisco, Portland or any of the other cities and metropolitan areas currently occupied by millions of people.

The Cascadia, courtesy of its length, will give rise to a long quake if the whole fault ‘unzips’ at the same time, as it’s believed to have done in 1700. The first P waves will travel the length of the fracture in a minute or two. The S waves that follow, the ones that cause the real damage, are slower and will cause shaking and movement for about five minutes though their speed can vary depending on the rock they are traveling through.

Any buildings not built to withstand earthquakes will collapse. Depending on the duration and magnitude of the quake it cannot be assumed that even ‘hardened’ buildings will remain standing. Many areas have sedimentary basins under urban sprawl and this amplifies the waves and the damage they cause.

Within approximately 20-30 minutes the tsunami will roll in. Unlike normal waves which have a few yards between them the tsunami waves can have hundreds of miles between them. The entire wall of water displaced when the plates slip hurtles outwards in all directions from the epicenter of the quake. As the waves approach the continental shelf the water at the back of the wave starts to catch up with the water at the front of the wave, which has slowed down as it moves up the incline that marks the start of the continental land mass. Its this that allows the water to build up and give the tsunami its characteristic ‘wall of water’ appearance.

The water will just keep on coming, flowing forward taking almost everything in its path with it. It may or may not be followed by more waves, there is no way to know if it will be a solo wave or a series until it happens. There is also no way to know in advance how deep the water will be. The tsunami caused by an earthquake in Lituya Bay Alaska in 1958 reached 1720 feet. (source)

Surviving an earthquake and tsunami is very challenging, but recognizing that should the Cascadia rupture, a tsunami is almost certain to follow gives you a firm footing to plan from. Don’t wait and see. Subduction earthquakes come into the mega-thrust category, and there are rarely, very, very rarely foreshocks. These faults lie silent for decades, sometimes for centuries before they rip. The forces released are unimaginable as it has been building, unreleased since the last time it ripped, in the case of the Cascadia 1700. 313 years worth of tension that will distort and bend the sea bed. The displacement of water in the ocean above the rupture will send a tsunami out in all directions.

You will have 20-30 minutes to get to higher ground. Knowledge about your location, the terrain, and the distance to higher ground will all impact your survival. All the drills in the world won’t help you if you are faced with a 50ft wall of water.

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