Thread: Frequency vs. Magnitude

Thanks Michael and John -

Yes, I think the frequency was probably what changed. The amplitude
varied slightly, too, as the shaking got faster, but I think it
wasn't the primary variable.

The program is focused more as a general overview of earthquakes -
where they occur, when, what magnitude is (just enough to talk
about how we measure quakes against one another). The building is
supposed to be a fun, hands-on wrap-up. Many classes treat this
either as a good summary/reminder of their earlier curriculum or as
an intro to an upcoming curriculum.
I guess I was assuming (and implying, though not explicitly stating
to the students) that the "buildings" were at the epicenter of the
earthquake, and the quake was in some quake-prone area such as So.
California. At this point in the program, the "magnitude" was
really meant to convey stronger shaking to wrap up what I'd
reviewed/introduced about quakes. I don't go into too much about
waves, as I'm generally dealing with younger students. I'm only
wedded to the current activity as it is already created, but the
table is failing, and with the new version (frequency variable), I
was confused about the frequency part and its relationship to
magnitude.

For simplicity's sake, were I to get the new table, am I correct in
saying that as frequency increases, magnitude increases (all else
being constant), so that as we turn up the frequency, we're
approximating an increase in magnitude? (as I don't talk about
frequency, period, etc. - just magnitude).

Thanks for all your help. Hope this isn't too off-topic!
-Katie


John Taber wrote:

Katie,

As Michael says, the key question is what concepts were you
presenting with your old system? Did the amplitude of the
shaking appear to vary? If you were primarily talking about
building damage then varying the frequency may be enough, as
building damage depends more on the local acceleration of the
ground than it does on the magnitude of the earthquake.

If you have someone one on your staff who can build you a new
shake table you might consider John Lahr's version or any of the
other ones he lists on his web page: http://jclahr.com/science/
earth_science/shake/. John may want to comment, but it looks like
most of the low cost, motor-driven shake tables vary the frequency
more so than the amplitude.

John

On Jan 28, 2008, at 2:59 PM, Michael Hubenthal wrote:

Hi Katie,

I haven't seen a reply to your post yet so I will take a crack at
a response.

In short, my guess is that the previous machine took some
artistic liberties and it sounds like they weren't discussed in
the manuals. There is a relationship between Period (1/
frequency) and Magnitude however there are other factors
involved. For example the equation to determine magnitude using
body waves is Mb = log(A/T) + s. Where (A) is the amplitude of
the P-wave train, the first arriving body wave, (T) is the period
of the displacement, and (s) is a correction term. Larry Braile
suggest that for the AS-1 program s=0.01*D + 5.9 where D is
distance to the event.

The problem with describing the changes in shaking only in terms
of magnitude only is that it doesn't account for other factors
such as the distance to the epicenter, regional geology etc.
However, the folks at Pitsco probably just said, lets assume that
all of these other factors are held as constants and put a
magnitude sticker on there (or they may not have even thought
about it that much).

I guess in thinking about your activity, what is the content that
you want to convey to them?

Best Wishes
Michael








On Jan 22, 2008, at 12:15 PM, Katie Stofer wrote:

From the Maryland Science Center ...

I have a more general question about quakes that I hope one of
the experts can help me with. We have been using a machine to
simulate earthquakes that ostensibly varies magnitude - Pitsco's
Epicenter Earthquake Simulator. They no longer make this and now
sell instead the EQ Tremor Table, which, instead of magnitude,
varies cycles per second (frequency), from 0-30, "in the range
of P waves." So, is there a relationship between frequency and
magnitude (the latter being the one that school groups visiting
the science center are more familiar with)? Was our original
machine really varying magnitude (being built, as far as I can
tell, on some sort of belt sander with a variable voltage
control), or just frequency as far as anyone can tell?

Any ideas/insight are appreciated before I rewrite our entire
program due to a broken simulator!

Thanks,
Katie Stofer
-- -- Kathryn Stofer TerraLink Exhibit Manager Maryland Science
Center 601 Light Street Baltimore, Maryland 21230 410/545-5976
410/545-5974 fax BODYWORLDS - Coming to MSC Feb. 2 - Sept. 1, 2008!
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---------------------------------
Michael Hubenthal
Education Specialist
IRIS Consortium
607-777-4612
www.IRIS.edu
hubenth<at>iris.edu

--
--
Kathryn Stofer
TerraLink Exhibit Manager
Maryland Science Center
601 Light Street
Baltimore, Maryland 21230
410/545-5976
410/545-5974 fax
BODYWORLDS - Coming to MSC Feb. 2 - Sept. 1, 2008!


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