[irised] Frequency vs. Magnitude

[John Taber]

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
>
>
>