Why does the earthquake depth change?

Finding the depth of the focus of an earthquake?

• How would I find the depth of focus using the lag time between the P and S waves which is 12 seconds. The average velocity of the P- waves is 4.8 miles/sec ad 2.75 miles/sec for the ...show more

• Answer:

  I can show you how to find the distance to the focus using the data given, but that is not the depth of focus (unless the data is from the epicenter directly above the focus). You need multiple stations to approximate the position of the focus. Given that, this is how you would use the given data to find the DISTANCE to the focus: let r1 = P-wave rate, r2 = S-wave rate t = P-wave travel time, t+12 = S-wave travel time (in sec) d = rt the waves traveled the same distance so set equations equal to each other and solve for t: r1*t = r2*(t+12) 4.8t = 2.75(t + 12) 2.05t = 33 t = 16.098 sec now plug value of t into equation(s) to find distance: (4.8 mi/sec)(16.098 sec) = 77.268 miles check other equation (2.75)(16.098 + 12) = 2.75(28.098) = 77.268 miles distance to focus is 77.268 miles, which is not the depth unless seismic station is right at epicenter. There are a near infinite number of places where the focus can be located using one station, which is why multiple seismic stations are needed.