PSN-L Email List Message

Subject: Re: What is an earthquake?
From: Brett Nordgren brett3nt@.............
Date: Sat, 19 May 2012 12:14:58 -0400


Thomas,

No problem.

First thing I looked at was what amount of energy would go into the 
earth in the pumping process.  Using an extreme case of 1.1E7 liters 
at 100MPa pressure gives 1.1E6 Joules.  More typical values, I 
suspect, are at least 10x less.  If all that energy, 1.1 MegaJoules, 
could somehow be released as a quake, it would still be significantly 
below 'magnitude' 1.0, more typically much below.  For quakes larger 
than that, it implies that the energy was already there, apparently 
stored as strain in the rocks and that the quakes are probably just 
being triggered.

For one discussion of mid-continent Intraplate Triggered Quakes see 
BSSA October 2003 v. 93 no. 5 p. 2212-2221  in which the author 
presents evidence that the big New Madrid quakes and the Charleston 
quake may have been triggers for a number of quakes in the Midwest on 
previously unknown faults.  My suspicion is that there often are 
stresses on unknown, dormant faults which in the normal course of 
things might never rupture, but which can and do when they are helped along.

I'm not arguing that these are naturally occurring quakes, just that 
their shake energy comes from the same source as ordinary ones.  They 
would almost certainly not occur naturally, at least not for a very 
long time, if they weren't triggered by pumping water into them.

To really understand what's happening would probably require a good 
number of short-period instruments planted nearby.

Brett

At 09:35 AM 5/19/2012, you wrote:
>>
>Brett,
>  Your opinion is logical. But....at least two "quakes" listed as 
> occuring in Indiana this month are not on faults. The Timpson quake 
> was described as "poor" in quality. Who/what is studying such 
> earthquakes? What can be gained from the data where much of the 
> wave phases are missing or distorted?  Of the last ten quakes 
> listed (at the present) for the middle states on Memphis website, 
> only three are(more likely) naturally occurring quakes. I am not 
> wanting to appear belligerent sounding.  I am raising the question 
> not in an argumentative way but trying to understand the scientific value.
>__________________________________________________________


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