Do small earthquakes prevent large earthquakes?

[kaleidoscope]

Is it true that a series of small magnitude earthquakes help prevent a large magnitude eartquake? Aren't earthquakes basically independent events from a statistical point of view?

 

[H2Bro]

Yes it is true, usually. Smaller earthquakes let out some of the pressure built up along a fault line.

The amount of slippage depends on the force integrated since the time of the last slippage. If a fault sticks for a very long time, for whatever reason, the amount of energy released will be greater as energy released is proportional to distanceXmass of the moved plate.

 

[Drakkith]

My knowledge of plate tectonics and related areas isn't really that great, but I would think that a series of small earthquakes could prevent a larger one if it allows the stress between the two plates to be released gradually instead of all at once.

 

[H2Bro]

Interestingly, when there is slippage between points A and B on a fault, additional stress builds up along the fault just beyond points A and B. You can imagine this as just inside points A and B there has been some movement, but just outside no movement, and this difference in movement in a solid body produces shear strain.

I think this is the usual cause of afterschocks, which are smaller earthquakes following a larger one.

 

[berkeman]

Yes it is true, usually. Smaller earthquakes let out some of the pressure built up along a fault line.

The amount of slippage depends on the force integrated since the time of the last slippage. If a fault sticks for a very long time, for whatever reason, the amount of energy released will be greater as energy released is proportional to distanceXmass of the moved plate.

I think we are going to need to see some peer-reviewed research posted here, before we can make statements like this. I used to believe that what you say is true, but then I read in a credible paper that it is not true. I'll see if I can find that paper, but if you have mainstream research showing that minor quakes help to prevent larger ones on the same fault, I'd like to read it.

(I live on the Hayward Fault in Northern California, BTW...)

 

[H2Bro]

Unfortunately what I learned in Grade 11 Geology did not come with provided peer reviewed articles, so I can't give you direct source on that.

It's possible what they teach in school is wrong.

 

[berkeman]

I think I found where I learned that it is false -- at the USGS website:

http://earthquake.usgs.gov/learn/topics/megaqk_facts_fantasy.php

You can prevent large earthquakes by making lots of small ones, or by "lubricating" the fault with water

FICTION: Seismologists have observed that for every magnitude 6 earthquake there are about 10 of magnitude 5, 100 of magnitude 4, 1,000 of magnitude 3, and so forth as the events get smaller and smaller. This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event. It would take 32 magnitude 5's, 1000 magnitude 4's, and 32,000 magnitude 3's to equal the energy of one magnitude 6 event. So, even though we always record many more small events than large ones, there are far too few to eliminate the need for the occasional large earthquake. As for "lubricating" faults with water or some other substance, if anything, this would have the opposite effect. Injecting high- pressure fluids deep into the ground is known to be able to trigger earthquakes—to cause them to occur sooner than would have been the case without the injection. This would be a dangerous pursuit in any populated area, as one might trigger a damaging earthquake.

 

[H2Bro]

What I'm seeing from this site:

http://seismo.berkeley.edu/outreach/faq.html#smalleqs

Indicates that preceding earthquakes do relieve pressure along a fault, however most cases there are an insufficient number of them to significantly postpone the larger quake that follows.

Just goes to show that if intuition were at all accurate, we wouldn't need science.

 

[davenn]

Interestingly, when there is slippage between points A and B on a fault, additional stress builds up along the fault just beyond points A and B. You can imagine this as just inside points A and B there has been some movement, but just outside no movement, and this difference in movement in a solid body produces shear strain.

I think this is the usual cause of afterschocks, which are smaller earthquakes following a larger one.

be a little wary of what you are posting ;)

The aftershock sequence is use to define the rupture zone of a quake
The areas outside that zone where stress has been transferred to is where you can look to the probability of future events that will occur usually well before the "return period' of the zone that has just ruptured.

Aftershocks are basically the readjusting of stresses within the rupture zone. You need to realize that fault systems are rarely a simple single plane that slips. and that the system is most likely made up of multiple large and small faults. All of these intertwined faults can produce aftershocks as the stresses are released within the area.

Dave

 

[manojr]

If you haven't already read this, six Italian scientists have been convicted of involuntary manslaughter because they were "criminally mistaken" in "falsely reassuring" people that major earthquake is unlikely after series of some low-level tremors.

 

[berkeman]

If you haven't already read this, six Italian scientists have been convicted of involuntary manslaughter because they were "criminally mistaken" in "falsely reassuring" people that major earthquake is unlikely after series of some low-level tremors.

There is a long thread about this in the General Discussion forum. Please keep discussions about this subject in that thread:

https://www.physicsforums.com/showthread.php?t=646127

Thank you.