[irised] Frequency vs. Magnitude

John Taber taber at iris.edu
Thu Jan 31 06:44:00 PST 2008 

Katie,

For the purposes of a simple demonstration for younger students, and  
if you aren't using it to demonstrate the effects of frequency (ie  
the varying resonance of buildings of different heights) and don't  
talk about increasing the frequency, then I guess you could say the  
magnitude is increasing.    If a teacher asks a question, you could  
say that what you really are doing is increasing the acceleration and  
thus the likelihood of damaging the building.  We do in our building  
design and destruction exercise where we place an accelerometer on  
the structure.

John

On Jan 29, 2008, at 4:56 PM, Katie Stofer wrote:

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

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