## How are Earthquakes Located?

We can locate earthquakes using a simple fact: an earthquake creates different seismic waves (P waves, S waves, etc.) The different waves each travel at different speeds and therefore arrive at a seismic station at different times. P waves travel the fastest, so they arrive first. S waves, which travel at about half the speed of P waves, arrive later. A seismic station close to the earthquake records P waves and S waves in quick succession. With increasing distance from the earthquake the time difference between the arrival of the P waves and the arrival of the S waves increases. This basic approach to locating quakes is illustrated using an example of an earthquake near Mexico.

#### Keypoints:

• Different waves each travel at different speeds and therefore arrive at a seismic station at different times.
• Difference in arrival times between P and S waves can be used to determine the distance between the station and an earthquake.
• By knowing how far away the quake was from three stations we can draw a circle around each station with a radius equal to its distance from the earthquake. The earthquake occurred at the point where all three circles intersect.

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Level: Novice

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