Tuesday, March 22, 2011

When will the next 9 plus Cascadia Fault Quake Happen

This PDF is an Oregon Dept of Geology publication about Cascadia fault earthquakes.
Page 8 has a graph of the past earthquakes showing both the really really big 9.x quakes and the smaller 8.x quakes (still pretty damn big).

I was trying to figure out if we're really overdue for a Big One or not. The usual statement is that we get a big Cascadia fault quake every 250 years or so and it's been 300+ years so Look Out.

Which is true... IF you include the 9.x and 8.x magnitude quakes lumped together. That makes about 41 quakes in 10,000 years. According to the graph.

But only 19 have been the huge 9.x quakes. (By the way, 1 point increase = 32x increase in energy released. Not 10x like some of the newspapers and TV say. It's logarithmic, but not base 10).

So the HUGE quakes are more like 500 years apart (10,000/19)

Which makes it seem like we're not at all overdue for another one of those 9.x's. We should have some breathing room since it has only been 300 years.

Also, in the past 10,000 years there have never been more than two 9.x earthquakes within a thousand years of each other.

Except that's not exactly completely true.
The last 1000 years breaks that trend. There have been three 9.x quakes since 1300. Plus another 8.x just for good measure.
So the last 1000 years have been much much more active.

That's really good! (or bad).

If you believe that earthquakes relieve stress built up and that the stress is building up at more or less the same rate (10,000 years isn't much time in plate tectonics earth changing scales), then whoopee! We've relieved a huge amount of energy buildup with those 3 quakes (the 8.x hardly matters. it's just 1/32nd as much energy as each of the other 3 quakes since 1300).

On the other hand, maybe quakes happen in swarms and we're in the biggest swarm in 10,000 years. Maybe the plates are moving faster and more stress is building up this millennium? I haven't found info about the speed of these plates over the past 10,000 years.

Historically though, it looks as though we've relieved way more stress than our epoch's share. We've relieved 97 "units" (if a unit is one 8.x quake) of stress in 700 years (3x 32x quakes and one 1x quake). A normal average stress relief would be less than 44 units in 700 years.

So we've got 200 years before a 9.x hits (on average) and
we've had enough stress relief over the last 700 years to be owed another 700 years of tranquility!

Maybe some geologist will say I'm full of crap. I'd love to know if I am...

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