What is the difference between Audible "noise" versus seismic "noise"?
Why can't we report he Richter Magnitude of crowd "noise" ?
Why do many seismograms have different colored lines?
With the experiment of recording seismic shaking during a football game and the notoriety of the "Beast-Quake" we have received lots of questions that need some detailed explanations. Sorry but it must get a little technical here.
Audible "noise" versus seismic "noise".
- Many are confused (including the press) about what we are actually recording. When we say "crowd noise", as seismologist we mean "crowd generated vibrations". Ie a crowd that sits still and yells at the top of their lungs will not really show up on our instruments. A crowd that jumps up and down waving their arms will be recorded. Our feeling is that when excited a crowd does both, but we don't "hear" them but only feel the motion of the stadium generated by the crowd motions.
The Richter Magnitude of crowd "noise" (ie shaking)
- We got a lot of question about what was the "magnitude" of some signal or its "Richter Scale reading". This is hard to answer since an earthquake and crowd generated shaking are really two very different things. A comparable earthquake source lasts just a fraction of a second as rock breaks deep in the earth. As the waves spread out from the source they bounce off geologic formations and the earth's surface extending the duration of the shaking at distant points. If a crowd were all located at the same place and all jumped up together in one big jump and only one, then there might be a reasonable comparison and an realistic earthquake magnitude could be calculate (Richter magnitude is a function of the maximum shaking and the distance to the source). A Seahawk crowd is distributed over the whole stadium and does lots of jumping and arm waving but not all in sync. Also, our instruments are in the stadium; ie. the source of motion is all around the instruments at all different distances. If one were to sum up all of the shaking, say during a Beast-Quake run lasting some 10-20 seconds and somehow correct for distance and calculate total shaking energy (not sure how to do all of this), we are sure it would come out to be the equivalent of a very small natural earthquake. A Beast Quake may be big to Seahawk fans, but absolutely nothing compared to when we get even a moderate earthquake on a nearby fault. That all being said and as an exercise in seismic wave theory one of our students is designing a new Beast-Quake-Magnitude (BQMag). It should be useful to compare different crowd reactions to different events but will not be useful to compare crowd motions to real earthquakes. For more gory details on earthquake magnitude check out Paul Bodins blog on how the earthquake magnitude scale works and also the difference between magnitude and intensity.
The colored lines on seismograms
- We sometimes use different colors for lines on a seismogram only to aid the eye. Particularly when there is a sudden burst of wiggles lines may cross each other and if they are different colors it helps an analyst tell what is going on. They have no other significance.