Reading a Seismogram, where are the S waves?

[fatcats]

Homework Statement

Here is the seismogram in question: http://postimg.org/image/iu0dozetn/

There are many parts to this question, but I only need help with this one: Measure the S wave arrival time.

Homework Equations

NA

3. Attempts at a Solution
I know that the P waves arrive at 40 seconds, but I need to find out when the S waves arrive. I thought maybe they arrived with the surface waves at 100 seconds and have blended in with them there. My other idea was that maybe the earthquake's epicentre occurred across water and because S waves cannot travel through water, they did not show up on the seismogram, but in that case, I don't think I can calculate the SP wave interval because there is no value for S and the question requires that as well.

I used the graph I have linked below to try and check my answer. If the P waves arrived at 40 seconds, I got that it was approximately 250 km, but when I tried to find the amount of time it would take for the S waves to travel the same distance, I got 65 seconds from the graph... except that on the seismogram it doesn't appear as though there are any waves at 65 seconds.

Graph: http://postimg.org/image/d794c5v4b/

Any help appreciated.

 

[Simon Bridge]

Need to see the seismogram.
You'll have a single trace ... do the P waves start and then stop and then you just have background vibrations for the rest of the chart?

ie.

... you may just have to look more carefully. If the s waves trace above were closer to the p waves or to the surface waves then you may not spot it. The s waves may just be smaller. cannot really tell without the trace. It may jst be that the earthquake occurred too far away for s waves to happen.
http://www.geo.mtu.edu/UPSeis/reading.html

 

[fatcats]

Thank you for your reply,

I uploaded the seismogram in my post. Here it is again: http://postimg.org/image/iu0dozetn/

As far as I can tell there are not three distinctions in it