It’s probably wise not to push my luck talking about seismology too much, but how about just one more post? I’m not a seismologist – but I did take a seismology class once! And I passed it, you guys, I really did! Although, if my seismology prof saw any of these cartoon shenanigans she’d probably retroactively flunk me. Ah, well, moving swiftly onwards!
Here’s the basic idea: an earthquake happens, at some unknown location. Seismic waves are generated, and these waves travel through the earth. The waves arrive at each ocean bottom seismometer (OBS) in the network at a slightly different time. The OBSs record the signal, and then the differences in arrival time between each of them are used to back out both where and when the quake happened.
I’m not going to go into the math-y goodness here, because it’s complicated and I’d likely make loads of mistakes and it would all end in tears. But one thing that is very, very important is timing. We need to know those arrival times REALLY accurately. One really handy-dandy way to deal with timing is by using GPS time. Many GPS receivers, even pretty inexpensive ones, are equipped with a very accurate timer that puts out one pulse exactly once every second (aka, PPS!). Problem is, you don’t necessarily know which second it is… until you get a GPS fix! Then you are all set.
Oh wait. Guess where GPS doesn’t work so well? At the bottom of the ocean.
Scientists deal with that little speed bump by calibrating the time immediately before deployment and again immediately after recovery. A correction is then applied to all the times in between based on characteristics of the instrument (or some other black magic). Clock drift is just one of those things you’ve got to deal with when working with instruments that sit underwater for any length of time.
Rob posted an interesting comment, and I thought it might be good to just put it up into the body of the post:
Clock drift is a very annoying little problem that us seismo-dweebs like to brush under the rug as much as possible. The problem comes about, basically, because the crystal that is used inside the instrument to keep track of time is inherently flawed in some way. There are people developing new seismometers with “better” crystals that will mitigate this problem, but it is likely an issue that won’t be going away anytime soon.