Saturday, November 8, 2008

Interpreting the Pakistan earthquake seismogram

I'll try to work through this one step at a time, in order to allow our many readers to catch up.

Let's consider when waves from the recent Pakistan earthquake should have arrived at the Lamont-Doherty seismograph, and compare that to when they actually did.

The NEIC information says that this quake (the second of two similarly sized events) occurred at Oct. 29 at 11:32:43 UTC (coordinated universal time).

Let's solve this today by using the theoretical P-wave arrival times tabulated and graphed by the NEIC. According to the table, the P (diffracted wave) should arrive in New York at 11:46:30, 14 minutes after the earthquake occurred.

Look at the seismogram below. There seems to be a small blip of energy at about 11:51, 18minutes and 20 seconds after the earthquake occurred.

Is this the wave arrival referred to above? Why is this blip so small compared to what follows? Can we get a more detailed look at this seismogram? How is this approach related to the use of travel-time curves? And what is Pdiffracted anyway?

You'll have to come back for that.

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