Re: "fast" earthquakes more destructive?


"rick++" <rick303@xxxxxxxxxxx> wrote in message
news:1188484353.894803.179230@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Any interesting claim in Science
http://www.sciencemag.org/cgi/content/summary/317/5840/905
that fast earthquakes, which slide faster than 2km per second,
are more destructive than slower quakes.

I was under the impression it was duration, which scales up with
magnitude,
was the most important destructive factor. A five minute M9 shake
versus a 30 second M7 shake.


Rick, upon re-reading your post, I realize that you actually brought up a
more interesting seeming contradiction than I first realized.

First, the Sciencemag URL you provided requires a subscription, but the gist
of the Science article can be found at
http://www.sciam.com/article.cfm?chanID=sa003&articleID=85763F1C-E7F2-99DF-3B120C750BF0C4A4&ref=rss

As you state, duration scales up with magnitude, at least typically. So, for
any given rupture length (and all other factors being equal), a
"super-shear" quake (as the article termed them), with a rupture speed
approximately twice that of a normal quake, would have half the duration of
the normal quake. You infer that the magnitude, then, would be lower in the
super-shear quake due to its shorter duration. I think this might be where
the seeming contradiction to which you refer arises. Though magnitude does
typically increase with duration, it is not BECAUSE of duration. Duration is
not one of the parameters. I know you know this, Rick - I'm just trying to
lay out the problem logically.

Ultimately, all earthquake magnitude scales attempt to measure the energy
released in a quake. There is no reason to believe that a 100-km long fault
segment rupturing at high speed would release less energy than the same
segment rupturing at half that speed. Moment magnitudes, for instance are
derived from 3 factors only, none of them concerned with speed of rupture.
Those factors are the shear modulus of the rocks involved, area of rupture
(length X width), and the average displacement.

For our hypothetical 100-km super-shear rupture those factors would all
remain equal. But, how could a super-shear, then be MORE destructive? As you
point out, duration is an important factor in destructiveness. This is
because the force, or acceleration and amplitude, of the shaking tends to
max out at a fairly low magnitude, leaving only duration to vary. The
article seems to suggest that in super-shear quakes the maximum acceleration
exceeds that value found in normal quakes, and does so sufficiently to more
than offset the reduced duration.

So, all other factors being equal (rock modulus, rupture area, total
displacement, site conditions), the super-shear rupture will last about half
as long, and be more destructive in spite of its short duration due to the
higher "intensity" of the resulting motions.

The Scientific American version of Rick's article mentions a few other
interesting aspects of super-shear ruptures. As of now, the phenomenon only
is observed on strike-slip faults. It is thought that super-shear rupture
can only occur along fairly straight sections of a fault, stopping at major
bends. The article suggests that the 1906 San Francisco quake was more
damaging than its magnitude would fully account for, and that super-shear
may be responsible for the difference.

I am an amateur, and would welcome either corrections or approval from
professionals regarding the above!

Mike Williams
Arroyo Grande, CA USA


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