Wednesday, August 25, 2010

The Next "Big One"

The LA Times has an interesting article about the likelihood of the next big earthquake in Southern California. The Easter quake near San Diego was not exactly small, but it was not on the San Andreas fault. That fault has not moved in more than 150 years, and they are now finding that the historical average prior to that is something like 88 years. Based on that finding, scientists are now saying that the “overdue” status of the fault is more severe and that the likelihood of the “big one” occurring soon is greater than ever.

The San Andreas fault is the line where two “tectonic plates,” sections of the Earth’s crust, are moving past each other, which is what causes the earthquakes. The assumption is that the rate of motion is constant, which is why they are saying that the next quake is overdue. But how do we know that the rate of movement is constant?

I actually don’t know very much about it, but what I do understand is that the boundary between plates is locked by friction so actual movement is episodic, occurring only during earthquakes, and that the pressure causing the movement is the thing that is constant. The pressure pushes on the boundary, increasing the stress on that boundary until the stress overcomes the friction, at which point the boundary gives way and movement occurs.

There is no way to measure the amount of pressure that being applied to that boundary, nor is there any way to measure the amount of stress that has accumulated within it, which is precisely why scientists cannot predict earthquakes. They can only assume that because it ruptures every 88 years in the past that it should be rupturing every 88 years now. But what if it is being subjected to less pressure now?

“Why would the plate(s) slow down?” you ask. Hell, I don’t know, but it seems they have done in the past. Look at Hawaii.

Hawaii was, or is being, formed by the Pacific plate passing over a “hot spot” on the Earth’s mantle. At one point the plate was moving pretty quickly, relatively speaking, and the volcanoes formed the smaller northern islands, but then it came pretty much to a halt and the hot spot volcano is now forming the very large island known as, appropriately, “The Big Island.” So, clearly, at least one plate has not always moved at a steady rate.

So maybe the “big one” is way overdue, and maybe the Earth is just laughing at us. On the other hand maybe all of you will be laughing at me when the “big one” does occur and knocks my house down because I was talking out of the wrong orifice.

1 comment:

Bartender Cabbie said...

At least with hurricanes you can track the status. If you live right near the coast and stay then "shame on you." Can't really see an EQ coming. I used to live in Memphis near downtown and was kinda worried that one day the New Madrid fault would act up and we would all end up in the MS. river.

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