How do you locate an epicenter of an earthquake?

Scientists called geologists use seismometers to calculate the exact location of the epicentre of an earthquake. If the speed of the waves are known from the quake, the time can be recorded that they reach multiple places. By using speed x time = distance , they can calculate the exact location of the epicentre as long as they have 3 or more recording stations. it works on the same principle as GPS
  

Fossil found in strata reveal that unused body part decrease in size