[irised] Frequency vs. Magnitude

Tue Jan 29 13:56:33 PST 2008

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!
>>> _______________________________________________
>>> irised mailing list
>>> irised at iris.washington.edu
>>> http://www.iris.washington.edu/mailman/listinfo/irised
>>
>> ---------------------------------
>> 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!