How do seismologists locate the epicenter and focus of an earthquake?

Question

I know the focus of an earthquake is where the earthquake originated from, but what I could never figure out is, how to scientists find out where exactly the focus (and epicenter) are located?

Answer 1

If you'll remember, there are three types of Earthquake Waves, Primary, Secondary and Surface Waves.

Primary gets its name because it's the fastest of all the waves. Imagine three runners on a track, all running at a constant speed, but one is running faster than the others.

As they run down the track, the faster runner will get further and further ahead of the other ones as they go along. The exact same thing happens to the Earthquake waves- Plus we know the exact speeds of Primary and Secondary- so we can do math to find their distances from the starting line.

Surface waves have a more variable speed because they occur ...wait for it... at the surface. There's more up here to affect the speed of this wave. Therefore, we mainly use the first two, Primary and Secondary Waves, to determine an earthquake's epicenter.

Earthquake Centers are set up to monitor day and night for any incoming earthquake waves. The minute the primary wave hits, they begin a timer to wait for the secondary waves. When the secondary wave comes through, they stop the clock and do the figuring to get the exact distance their station is from the epicenter of the quake.

Trouble is, they know the distance now, but the earthquake could have come to their station from any direction- they don't know from where... so they draw a circle with the right radius around their center.

When a second station checks in and draws his radius circle- the two stations compare notes. They see that there's two possibilities of the earthquake's location- the two points where the circles intersect.

They need a third station to chime in to get the exact location- and this is why it’s called Triangulation- it takes THREE to work right... Once the third guy calls his data in, they'll draw the third circle.

Now where the three circles intersect, they have the precise location of the epicenter of the earthquake and the focus- the actual break and movement of the plate, is deep in the earth directly below the epicenter.