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.
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.
Any help appreciated.