West Coast Tectonics: The Cascadia Subduction Zone

The west coast of the NorthAmerica is renowned geologically for the San Andreas fault that runs through the state of Califonia and in particularly the San Francisco bay area. North of that area lies the Cascadia subduction zone (CSZ), the most tectonically active zone in Canada. This zone extends from Vancouver Island south to Northern California and is marked by the west coast mountainn ranges like the Cascade and part of the Rocky mountain range which were built up by the Farallon and Juan de Fuca plates being subducted underneath the North American plate. The most famous rrecent evidence of tectonic activity is the 1981 eruption of Mt. St. Helens of which gave direct evidence that the plates were and still are active in this region.

Most subduction zones usually show somewhat large and frequent seismic activity but this is not the case for the CSZ region. The CSZ is for the most part absent of tectonic activity and with recent records only going back 200 years there is no documentation of any large earthquakes to strike the area. There are two possible theories that may explain why the area has not seen much earthequake activity:

1) The Juan de Fuca plate and the remnant Farallon plate are quite lubricated that there is a smooth constant slide underneath the North American plate, thus there is no real significant stick-slip motion to create earthquakes.

2) The plates are locked and presently building up stress and strain because there is no major movement between the plates, and no minor earthquakes to indicate some movement.
Below is a cross section of the Cascadia Subduction Zone show ing the movements of the North American and Pacific plates.

Here the plates are locked and building up stress.

The plates then give way releasing a huge amount of kinetic energy in the form of shock waves or more commonly refered to as seismic waves. This is what is known as an earthquake.

After the quake the basin that was created fills with sediment and the stress again continued to build.

Scientific measurements show that thw Juan de Fuca plate is moving inland at a rate of about 4mm per year (quite slow for a subduction zone). Over a 500 year period that strain release would account for a 20 metre displacement and over 900 years a 45 metere displacement. This translates to a tremendous amount of locked potential energy that when released can cause a very large scale quake in the order of maginitude of of about 8.5 to 9.5! So with these measurments this evidence implies that indeed the plates are locked and sometime in the near future the plates will give way resulting in a large quake.

Using sedimentaty features and microfossils scientists are able to determine how large and how frequent the large scale jolts occur.

Potential Risk.

Stratigraphy and how it is used.

Microfossils and how they are used.

How big and how long for the next one?

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