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View Full Version : Do small earthquakes relieve stress on the fault line?

gurujulp
10-28-2011, 05:55 PM
This link, to the UC Berkeley student paper website, is to an article that quotes an earthquake authority as stating that the little quakes that happen every so often do NOT relieve stress on the fault and make any forthcoming earthquake possibly smaller. He says that is 'a pretty common fallacy.'

Bwah? I mean, there isn't any guarantee that little quakes will make a larger quake not happen, but I have read and seen several things from multiple sources that indicate that little quakes do indeed reduce stress, as measured by ground creep and other things, and so this was really surprising.

Can anyone in the know confirm or deny this for me? I can't find the other sources I remember, and they were mostly pop science, but I think they were valid pop science...

Just idle speculation, really, except that I work in downtown Berkeley, and would likely get swallowed up in a really big shaker, so I have a bit of self interest involved.

Thanks!

http://www.dailycal.org/2011/10/27/smaller-quakes-in-berkeley-could-signal-big-one-is-coming/
Whether or not these smaller quakes will end in the “Big One,” they will not reduce the magnitude of a large quake when it hits, Boatwright said.

“That the smaller earthquakes are releasing stress that would otherwise show up in a large earthquake is a pretty common fallacy and, for the most part, is entirely wrong,” he said.

GreasyJack
10-28-2011, 06:59 PM
You learn in Geo 101 that there's a nearly direct relationship between earthquake intensity and frequency, which was part of what inspired Charles Richter to make his logarithmic intensity scale. A magnitude 5 is 10 times more powerful than a magnitude 4, but you would expect about 10 magnitude 4 earthquakes along a given fault for every magnitude 5. So, in terms of simple mathematics, seeing more small quakes along a fault line would actually make you expect MORE powerful quakes at some point in the future. However, this relationship is only an empirical observation-- there's no clear physical reason why this is the case.

I did take an upper-division tectonics class that touched on this, but I must admit I don't really remember all that much of the seismology part of it. But my recollection was that there are a lot of special cases and nuances to the above observations that have been discovered since Richter's original work, I believe some of which included this situation in which smaller quakes, or other quakes along the same fault system, could relieve stress and make more intense quakes less likely. Or they could add stress and make them more likely. But I think both situations were presented more as curious outliers than the usual situation, and probably not relevant to the textbook-style seismology occurring in SoCal.

10-28-2011, 10:21 PM
Earthquakes happen when shit moves. It moves until the forces opposing its motion are greater than the force making it move. Friction is one of the forces that limit motion, and also the strength and size of rock formations that are in the way of such movement.

Every time shit moves, it releases pressure in one place, and increases it in another. The tectonic plates move because of convective forces from magma plumes below them, and from getting shoved around by other tectonic plates. Every time an earthquake occurs, the stability of every other fault or other place on the same fault is either increased or decreased as a result of the geometry of the motions and forces in play. It may also be true that a particular movement allows magma plumes to begin moving faster, resulting in more stress on all the faults in a plate, or it could do the opposite.

Keeping track of faults, and the movement of rock on opposite sides of those faults is pretty much an ongoing effort of a lot of people, most of them very highly educated on the subject of earthquakes, and not unsurprisingly, mostly of widely divergent opinions of the consequences of one earthquake, and the likelihood of another. Issuing public warnings of earthquake potential has far more effect on people than it has on rocks. The fact is that the effects on people are greatly increased when the warnings are incorrect. The secondary effects of incorrect warnings are much greater on geologists than on the general public. This has a dampening effect on the willingness of geologists to make such warnings.

Tris

gurujulp
10-31-2011, 10:38 AM
So it can happen both ways- make sense, depending on the varying forces involved...

As well as people can say all sorts of things based on non-scientific reasons.

Thanks, both of you!

dracoi
10-31-2011, 02:23 PM
I know that one of the newer ideas in seismology is that earthquakes sometimes move down a fault line. There are places where each subsequent big quake is 50 miles (or whatever) further along. The belief is that the previous earthquakes are transferring stress down the line.

If this is true, then you can see how little quakes may not prevent or reduce a big one. The little ones may just be transferring stress from relatively mobile parts of the fault down to the "sticking point" where the big quake will eventually come.

Still, it's interesting news to hear that little ones have no effect on the size of the big one. I had definitely been taught otherwise.