## PSN-L Email List Message

Subject: Re: Time of Quake??
From: ChrisAtUpw@.......
Date: Sat, 21 Apr 2007 17:21:29 EDT

```In a message dated 2007/04/21, tchannel@.............. writes:

> Hi Chris, A question about P travel time?  I read somewhere it varies, from
> __km to __km per second.
> Does it vary depending on the material, type of earth, that it is moving
> through?
> I have calculated, from recordings, a range of 10,000 to 30,000 miles per
> hour. That's a big range.

Hi Ted,

I suggest that you download and print out a copy of the Seismograph
Training manual from http://psn.quake.net/info/analysis.pdf and also a travel
time graph from
http://neic.usgs.gov/neis/travel_times/
The velocity of P waves in the mantle increase with depth from 8.2 to
13.5 km / sec.

P and S wave velocities depend on the depth and on the rock type.
There are large variations in the surface rocks, but the incidence angle is often
quite steep, so uncertainties may be relatively small. The tables / graphs of
velocities with angular distance are averaged velocities. You may notice
variations if say you are west of the Rockies and the quake is to the east, or vice
versa. You also have the problem that a plane wave often follows an upwardly
curving path through the earth - NOT a straight line! This is due to the
variation of the velocity with depth and to the curve of the earth.

Regards,

Chris Chapman
In a me=
ssage dated 2007/04/21, tchannel@.............. writes:

Hi Chris, A question about P tr=
avel time?  I read somewhere it varies, from __km to __km per second.<=
BR>
Does it vary depending on the material, type of earth, that it is moving th=
rough?
I have calculated, from recordings, a range of 10,000 to 30,000 miles per h=
our. That's a big range.

Hi Ted,

ut a copy of the Seismograph Training manual from http://psn.quake.net/info/=
analysis.pdf and also a travel time graph from
http://neic.usgs.gov/ne=
is/travel_times/
The velocity of P waves in the mant=
le increase with depth from 8.2 to 13.5 km / sec.

P and S wave velocities depend on the d=
epth and on the rock type. There are large variations in the surface rocks,=20=
but the incidence angle is often quite steep, so uncertainties may be relati=
vely small. The tables / graphs of velocities with angular distance are aver=
aged velocities. You may notice variations if say you are west of the Rockie=
s and the quake is to the east, or vice versa. You also have the problem tha=
t a plane wave often follows an upwardly curving path through the earth - NO=
T a straight line! This is due to the variation of the velocity with depth a=
nd to the curve of the earth.

Regards,

Chris Chapman
```