Saturday, November 15, 2008

Interpreting seismograms with travel time curves

The travel time curve below looks much more complicated than the ones seen in introductory text books. We'll need to spend a couple of blogs explaining that. But notice the repetition of the letters P and S. These are the two physically different types of body waves that you're probably familiar with, the push-pull P-waves and the shear S-waves. The third different physical type of waves are the surface waves, often designated with the letter L. The way that the P's and S's are combined (PP, PcS, PKP, etc.) indicate the path that these waves take through the body of the Earth. All of this is plotted against the great circle angular distance between where the earthquake occurred to where it is observed. I'll show you how to calculate those angles later, but for now, accept that the distance from the earthquake in Pakistan to the seismograph at Lamont in New York is 100°. Note that for this distance, the travel time for the P wave is 14 minutes, and the travel time for the PP curve is 18 minutes.


Now look at the Lamont seismogram below. It looks to me like the first real small blip of energy that we can detect with the naked eye is at 11:51. I'm not talking abut the larger waves that arrive later, but that first small sinusoidal pulse of energy. This blip of energy is 18 minutes after the quake occurred, and at this distance from the quake, would seem to the the so-called PP phase. What is PP (no scatological humor, please)? Stay tuned.

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