The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but scientists cant measure it with a tape because it is so deep in the ground. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A small wiggly line that does not wiggle a lot means a small earthquake, and a big wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip and then they can determine if the earthquake is going to be big or small.