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Earthquake Magnitudes and Physical Damage

From: andrew cooke <andrew@...>

Date: Tue, 15 Mar 2011 12:57:50 -0300

I am a little frustrated at the way earthquake magnitudes are used in general
discussion.  This is partly a "media problem", but also seems to be a cultural
issue based on the different needs of geophysical researchers and the general
public.

Below I'll give some basic information about magnitudes and then raise the
points that I find frustrating.  I don't have answers to everything, so this
is going to be a rather open-ended post.


There are two important things to understand about the "magnitudes" that you
see in newspaper reports:

First, a small change in magnitude has a large effect.  This is because the
scale is "logarithmic" - each increase of 1 means around 30x more energy is
released (so an increase of 2 is 30x30 or about a thousand times as much
energy, and so on).

Second, the magnitude measures the strength of the "geological event".  If you
think of the earthquake like a nuclear bomb going off somewhere underground,
then the magnitude is "the size of the bomb".


Both points above make a lot of sense if you are a geophysicist, but they make
much less sense to ordinary people like me.  The thing that I care about most
when I hear about an earthquake is "how much damage will that cause?".

The amount of damage caused by an earthquake depends on many things.  One, of
course, is "the size of the bomb", as I described it above.  But that is much
less important than you might think because:

 - It doesn't tell us how deep the earthquake was.  An earthquake deep under
   ground is going to cause much less damage than one close to the surface.

 - It doesn't tell us how the ground moves as a result.  Fast vibrations
   backwards and forwards are often less damaging than slow movements.  And
   movements up and down damage buildings in different ways to movements from
   side to side.  So the frequency and direction of movements on the surface
   of the earth are all important (size too, but that does tend to get bigger
   as the magnitude increases).

 - It doesn't tell us what the ground is made of (some soils can turn to
   liquid when vibrated, for example), or how well the buildings are
   constructed.

 - It doesn't tell us whether we are near the sea or how big a tsunami might
   be (although you do expect larger earthquakes to generate larger tsunamis,
   generally).

Some of these factors are so general that we can't hope to put them in a
single number.  But it seems to me that we could have a value similar to
magnitudes that is more useful - something that does give a weighted value
based on the kinds of vibrations experienced on the surface of the land.

In fact, such a number does exist.  It's called the "Mercalli magnitude", but
because it is based on what damage actually occurs it confuses the "social
factors" (how well buildings are made, for example) with the physical ones.


Now this might seem vaguely interesting, but rather pointless.  What help
would a new "magnitude" be that measures how dangerously the ground shakes?

Well, for example, it would help explain why the second, "smaller" earthquake
at Christchurch did so much more damage than then the previous one: a 6.3
magnitude earthquake in February killed 65 people, yet none were killed in the
magnitude 7 event in the preceding September.  That's because the earlier
earthquake was deeper and further away.

It would also help explain nuclear reactor design.  After all, when you're
building a reactor what you care about is exactly this.  You don't care how
much energy is released deep underground - you care about whether or not the
structure will survive.

The reactors that are having problems in Japan were designed to "withstand an
8.2 magnitude earthquake" according to some information I read online.  But by
itself that is almost meaningless.  Presumably there are a whole pile of
additional assumptions like depth and distance and the way the earth will move
that are all factored in.

You can argue that, presumably, nuclear reactor designers know how to do their
job.  But what about ordinary people who want to know how much danger they are
in.  If they are near a reactor designed for an 8.2 magnitude earthquake, and
a 9 magnitude even occurs, should they flee?  The answer would be a little
clearer with a more effective magnitude that measured risk factors we know to
influence damage.


Why isn't something like this in use?  I think the main problem is that it's
too imprecise for geophysicists.  They like the fact that "their" magnitude
has an exact meaning.  They don't want to get invovled in subjective things
like "which direction of wobbling causes damage and by how much?".

The media go to geophysicists for informed opinion, so they aren't going to do
any better.

But similar things do exist in other areas.  There are standard ways of
measuring sound, for example, that indicate "how loud" it is ("weighted sound
level").  We could just as easily have a "weighted surface vibration level"...

Andrew

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