Find distance when you know the lag time:
oppose you know the lag time between the arrival of the P and S waves at a location. You can
then use the graph to find out how far the place is from the epicenter. Use the following
instructions:
• Take a sheet of paper and hold it against the vertical axis (y axis) of the graph.
• On the edge of the paper, make two marks - one at zero, and the other
at the amount of lag time. (Ex: if the lag time is 30 seconds, there
would be one mark at zero and one mark at 30 seconds)
Slide the paper to the right, keeping the paper straight up-and-down as
you move it.
Find a place on the graph where the distance between the two curves
(P wave and S wave) is the same distance as the marks on your paper.
Follow that point straight down to the x-axis to find the distance to the
epicenter for the specific lag time.
4. At your location, you record a lag time of 180 seconds between the P and S waves. How far are
you from the epicenter?
a. When did the P wave arrive?
b. When did the S wave arrive?
5. The geologists at Penn State recorded a lag time of 140 seconds between the P and S waves for
an earthquake. How far is Penn State from the epicenter?