Question

why is the arrival time for the p-wave always earlier than the arrival of the s-wave the same seismograph station?

is it because the p wave always reaches before the s- wave.

Answer 1

Since this is not my area of expertise, I searched Google under the key words "<I>p s wave seismograph</I>" to get these possible sources:

http://en.wikipedia.org/wiki/Seismic_wave
http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html
http://www.geo.mtu.edu/UPSeis/waves.html

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.

I hope this helps. Thanks for asking.