Finding the epicentre...On this page, we will see that the distance of the epicentre from the seismometer location, d is proportional to the time difference Dt between the arrivals of the P and S waves. Given a seismometer trace it is therefore possible to find the distance of the epicentre from the seismometer location. From an earlier page, we saw that P waves and S waves travel at different speeds (6.0 km s-1 and 3.5 km s-1 respectively); this means that the P wave and S wave shocks will arrive at the seismology station at different times. Since velocity = distance / time, we can write equations for the P and S waves thus:
where d is the distance from the epicentre, vp and vs are the velocities of the P and S waves respectively, and tp and ts are the times of arrival (in seconds) of the P and S waves respectively at the seismology station. As ts is always going to be greater than tp, it is possible to calculate the difference between the two, Dt.
Substituting from equations (1) and (2) above gives:
So therefore:
where Dt is in s |
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