Re: not the "Richter" scale anymore

Susan wrote:
That makes sense. So, rewording Susan's >argument, the radiated energy
can be only roughly approximated, so we >consider instead the strain
energy, which can be more accurately captured >by estimating the seismic
moment.

Actually it never was my argument -- I was paraphrasing only slightly
the reply that I got from Hiroo. I didn't reply to your next question
because I wasn't prepared to speak for him, but Greg's reply certainly
has the Ring of Truth...especially considering the abstract that he
posted. What I don't know is, was HK motivated to consider strain
energy because radiated energy is difficult to estimate, or because
strain energy ("work") was considered the more fundamental parameter.
I could believe the latter, precisely because, as you say, radiated
energy depends on so many variables.

And as Orowan's
condition doesn't look likely to be true, the >relation of Mw to
radiated energy is not certain either

I should know the answer to this, but I don't: hasn't this been tested?
By now we ought to have decent, independent estimates of Mw and
radiated energy for a decent number of large earthquakes.

Susan

I could be wrong, but I suspect that Hiroo was motivated primarily
by the troubling observation that the size of the very largest
earthquakes was being systematically underestimated by standard
magnitude scales. My guess is that whether Mw was intended as
a measure of strain energy or radiated energy was a secondary,
though still quite interesting, concern. Someone ought to ask him.
I will see him two weeks from today, and will try to remember to put
the question to him then, if John or Sue don't beat me to it.

Mw, or the seismic moment Mo, is often fairly well constrained,
but seismic energy is often uncertain by a factor of 5 for a single
earthquake, so I don't think there is a consensus yet on seismic
moment vs. seismic energy - even for big earthquakes.

Orowan's condition treats earthquakes as a point process. That
is, the entire earthquake is characterized in terms of its energy,
stress drop, and moment. Since a real earthquake - one of any
size at least - is no doubt quite complicated, the use of such an
assumption has never appealed to me, because I think that in a
real earthquake the stress drop can be partial over parts of the
fault, complete over other parts of the fault, and there is even
evidence of overshoot in some earthquakes. Since the average
stress drop, radiated energy, and seismic moment are the sum
total of a combination of such behaviors, I'm not sure what we
learn by studying that sum.

Greg

.

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