La Message Board Archives


Posted by Rick (Lyle's Dad) on January 21, 2012, 4:09 pm, in reply to "Re: SISMO!"

P is the primary wave that is compression coming at you after the event. Some EQ's are below the ocean and some are not. THe P wave travels fastest in earth, like granite is the fastest. Can be faster than 5 kilometers in 1 second in rock. This is really the 'feel it' wave, not the most destructive wave. The S wave is a bit different in that is is a wave that is perpendicular to the P wave and if you are going to have side to side motion on an earthquake, and you usually do, this is the wave that causes that motion. Generally the S wave is about 10% slower than the P wave. S waves don't travel through water though because while you can get a pulse phenomenon going in water via the P wave, the S wave likes to cause separation say of the soil or the rock and water won't separate like that.

In Japan, it is like it is on a big old plate of jello and it is really wierd to get that first impulse P wave and that gets you to look up and you know you better move to a strong place b/c I guarantee you in the soft stuff in Japan the soon to arrive S wave will be side to side and last a very long time before dissipating.

For this one, I definately felt that P wave, looked right outside my window where I was sitting to see if there was a huge dump truck and in the second that I did not see a huge truck, I said "Earthquake" and by the time I stood up, my feet felt that S wave but it dissipated quickly. We have so much rock that we don't get that action like Loma Prieta in '89 in your neck of the woods. I went there that night in fact to help with structural evaluations and the Mission District was engulfed in flames as we came in on the plane. I'd say we were on the ground within 4 hours of the event from Denver on military planes out of Buckley ANG in Aurora, Colorado. That was some event I tell you what.

By posting, you confirm that you have read and agreed to be bound by the board's usage terms.


Be sure to visit