We can locate Earthquakes, because we know how fast they travel. If we figure out how long it takes the Earthquake to get to at least 3 different seismometers then we can triangulate the location of the epicenter, because there is only 1 place that is the right distance from all of the stations.

Please look at the picture. here

Fantastic question! Earthquakes are precisely located by the use of seismometers—machines that detect the motion of the Earth at a specific place. There are hundreds of such seismometers around the world. When an earthquake occurs, waves propagate from its epicenter and are recorded by the detectors a certain time after the initial shake. By understanding basic geometry, scientists can triangulate the point of origin: with a single detector, we can determine a circle of points where the earthquake could have originated; with two detectors we can find exactly two points where two circles intersect; and finally with three detectors there is only a single point where all three circles intersect. This principle is very similar in GPS technology. A more complex analysis of seismology might require considering factors such as soil composition or density and geographic features such as mountains or bodies of water

The same way you know which direction sounds came from. The waves generated by earthquakes expand outward from the point of origin of the quake. By measuring the time when the waves reach several recording stations, you can triangulate where the waves came from - the origin point of the quake.