[time-nuts] OT: NZ Christchurch member

[Brooke Clarke]

Hi:

The Richter method of analyzing earthquakes is based on what can be 
learned from a simple seismometer.  For example by looking at the time 
difference between the P wave and the main shaking you can determine how 
far away the epicenter is located.  The Richter number depends on the 
peak of the main shaking.  But the energy depends on the integral of the 
magnitude of the shaking over the time it lasts.  The damage is 
proportional to the total energy not it's peak.  The public is used to 
hearing the Richter number even though it does not really describe the 
damage level.

For example I had just left work when the Loma Prieta quake happened and 
was standing in front of the building watching the wall sized windows 
oil canning.  With each cycle the window displacement was getting 
larger.  If the quake had lasted about 30 second longer all the windows 
would have exploded, either with glass going into the building or coming 
at those of us standing in in front.  We started to get on the ground to 
get some protection, but then the quake stopped.

http://www.prc68.com/I/Seismometer.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com


William H. Fite wrote:
> For meteorologists and geologists, the Richter scale has a carefully defined
> meaning and is used only for purposes where that definition fits.
>
> This per a friend of mine who does seismic stuff for NOAA:
> Him:  The Richter number means something very specific to us and something
> quite different to the media.  Actually, the Richter doesn't have a great
> deal of analytical value to us.  You can say, this is a Category Four
> hurricane but that really tells you very little about what is going on in
> the storm.  Richter is like that.
> Me:  You're saying that the Richter is a poor predictor of surface
> disruption?
> Him:  Well, obviously a 9 will be expected to do much more damage than a 6
> but it is at best a very rough indicator.  The location of the epicenter and
> a dozen other factors play into it.
> Me:  So how do you assess the damage potential?
> Him:  Lots of people think we still rely mainly on the old
> pendulum-and-stylus seismographs from the 1930s.  Actually, we take a great
> many measurements in addition to seismometry.  But when it comes to
> assessing the damage, we go outside and look, just like the TV stations do.
>
> And his final comment:  By the way, did you know that when the shuttle
> launches we capture that on virtually every strain guage seismometer in the
> country?
>
> I found that interesting.
>
>
>
> On Thu, Feb 24, 2011 at 10:56 PM, jimlux<jimlux at earthlink.net>  wrote:
>
>    
>> On 2/24/11 5:23 PM, Bob Bownes wrote:
>>
>>      
>>> What is the conversion factor for Richter to dBm? :)
>>>
>>> Bob
>>> As a guy with degrees in geology and EE. I really should know this...:)
>>>
>>>
>>>
>>>        
>> Especially since both are log scales..
>>
>> The problem is that Richter is log magnitude displacement on a particular
>> kind of seismometer (which is sort of a low pass filter) and dBm is log
>> power.  However, there should be some sort of scale factor that converts it.
>>
>> I think it's energy goes as amplitude^1.5.  there's also a scale factor for
>> how far the seismograph is from the epicenter.
>>
>>
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