Answer: Three locations
Step-by-step explanation:
Seismologists use seismographs located all over the place. When an earthquake occurs, these seismographs record the time that the first and second waves of the earthquakes reached them.
When seismologists then take this data, they would need data from a minimum of three locations. Based on the times the first and second waves reached the seismographs, a location can be calculated based on the speed the waves were traveling at.