Given a seismogram trace it is possible to detect the distance from an earthquake using the difference in arrival time between P and S waves. The P wave travels faster than the S wave so arrives first.

Here is a map of SE Asia. Imagine that you are a seismologist working at Phnom Penh.  At the top is an imaginary seismometer trace taken shortly after 14.00 GMT, 7th December 1998.

1. Place the mouse at the start of the P wave disturbance on the trace and make a record (results data sheet) of the time when the P wave first arrived.
2. Repeat for the S wave.
3. Calculate the time, Dt, between the P and S waves arriving at Phnom Penh.
4. From Dt, determine the distance, d, from Phnom Penh to the earthquake epicentre. (Equation here...)
5. Click and drag the mouse from Phnom Penh across the map until the radius of the circle is approximately your measured value of  d.

The earthquake occured somewhere on the circle that you have drawn. 

What is the approximate distance of Phnom Penh from the epicentre of the earthquake?