Sometimes it is useful to look at data from lots of different stations at the same time. The screenshot shows a 'record section'. It is based on data taken from several seismic monitoring stations following a large earthquake.
On a record section, the x-axis (time) is common for all traces, and the position of each trace up the page (the y-axis) depends on the distance of that recording station from the event.
Features to note
Study the sesimogram screenshot and note the features below:
green marks show the arrival of direct P-waves
orange marks show the arrival of direct S-waves
red marks show the arrival of PP-waves (reflected at the surface)
yellow marks show the arrival of ScS-waves (reflected at the boundary with the outer core)
pink marks show the arrival of SS-waves (reflected at the surface)
Which waves arrive first?
What do you notice about the difference in arrival times as the distance from the source increases?
Why do the P-wave and S-wave arrival times form curves instead of straight lines?
Choose two stations near the bottom of the seismogram and note down the arrival times of the P-wave and of the S-wave at each. Find the difference in time between arrival at one station and at the other for the P-wave and then for the S-wave.
Find the approximate difference in distance travelled from the source to each of these stations, assuming that 1 degree is equivalent to 111 km along the surface.
Use the distance difference and the time difference values you found to calculate an approximate speed for P-waves and for S-waves.
Repeat steps 3–5 for another pair of stations further up the seismogram, and compare the values you calculated for the speed of P-waves and S-waves.
Why might there be a difference? Why else might there be a difference?
The lines for the direct P-wave arrival (in green) and the direct S-wave arrival (orange) do not continue up to the furthest stations. What might be causing this?
It is often useful to use a graph to let you read off values quickly without having to do a calculation. The graph uses the difference in arrival times of P-waves and S-waves to estimate the distance of a monitoring station from the source.
Why is the graph a curve, rather than a straight line?
Why does the x-axis stop at just beyond 100 degrees?
Use the time-travel graph and the same stations you used in the previous task to estimate their distances in degrees from the source. Compare the estimates with the values given on the y-axis.