Yesterday there was an earthquake off of Vancouver Island, BC, Canada. This was a large quake on the cascadia fault. Cascadia is the section of fault between the northamerican plate and a portion of the pacific called the Juan de Fuca plate. This plate is bordered on the west side by the ridge down the middle of the pacific ocean, at this point called the Juan de Fuca ridge. This ridge is making the pacific ocean floor grow wider pushing the plate against the north american plate where it subducts at the Cascadia fault.
The cascadia fault has a history of large periodic earthquakes, evidence of these quakes is found in layers of sand and silt in swamps and salt marsh lands along the coast line. Each layer of sand being a deposit of ocean sand deposited inland by a large mega tsunami that has to be generated by a 9+ quake on the fault. So in light of the tsunami evidence this is a small quake for this fault zone.
Now we have no measurement data on the larger mega quakes along this fault so we are trying to indirectly calculate the size of quakes that could happen here. And we have no direct evidence of what the progression of events lead up to a mega quake. But using the theory that quakes happen as links in a chain where one quake in one area lets pressure off that section of fault but builds to more pressure on the unreleased sections of the fault.
This quake was at the extreme north end of the Cascadia fault. Recent quakes in this fault zone have been quake north of this one but none of any size lately south of it on this end of the zone. There have been no recent large quakes between here and the south end of the zone. The quake at both ends are releasing pressure at the ends but building it in the middle.
I have recently written that a mega quake will probably not happen on the zone for a long time, based on the recent findings that the salt marshes raise up and grow trees between each large quake and tsunami. At the moment those marshes are still to salty to grow trees. But if what is holding the fault from moving this time is not as strong as previous times then we could have a quake earlier but it wouldn’t be as strong with less pressure built up. It’s that bending of the edge of the north american plate that not only indicated the force involved but the subsidence of the raised up areas provides the snap of movement that sets of the tsunami.
So this quake is the first step on the road to the mega quake. There will be more quakes, at the north end and the south end of the Cascadia fault. With no other quakes in the fault of size 6 or higher, although there are a lot along the ridge to the west, it is not possible to even guess at the time between quakes on this fault once the quakes get started. But the cascade has now started. If the theory of earthquake storms holds for this quake the first domino of the chain has fallen. It is just a matter of time now for the next one to happen.
When that does then the question is does each earthquake follow the previous in a steady step by step down the fault until it reaches a tipping point and the rest of the fault then goes all at once? Or does each quake, because the stress loading grows with each quake, happen with less time between until finally the rest of the fault goes at once setting of a mega quake and mega tsunami. Either way the fault has started to go.
If history repeates and the layers of the sand and soil and trees in the salt marshes holds it should be at least 100 years before the fault lets go the big one. But if the fault is not held as tightly as previous mega quakes then it could be starting now and could happen any day. Either way it’s best to be prepared. If you live in the tsunami zone be sure of where it will be safe to go with a mega tsunami bearing down on you. Find a close by high place to go to. Figure out the best and fastest way to get there safely. The fault is only 80 km off shore, so if you feel a large quake you have only minutes to get to high ground.
Be safe out there and be ready, the Cascadia mega quake is coming.
- Quake off shore Vancouver island (echlinm.wordpress.com)
- Update: Earthquake hits Vancouver Island, felt in Seattle (westseattleblog.com)
- Cascadia mega quake coming? (echlinm.wordpress.com)