Answer: By looking at the damage that was caused and by using seismometers.
Explanation: A seismometer is an instrument that records the shaking of the Earth's surface caused by seismic waves. The term seismograph usually refers to the combined seismometer and recording device. Seismometers allow us to detect and measure earthquakes by converting vibrations due to seismic waves into electrical signals, which we can then display as seismograms on a computer screen. Seismologists study earthquakes and can use this data to determine where and how big a particular earthquake is.