How are seismic waves measured?
By measuring the waves through the Earths crust.
When an earthquake occurs it's like dropping a stone in a pond, waves will ripple out from the epicenter and will decrease in force the farther away they get. Seismic waves are measure at certain stations with sensors that are set up in certain areas. For example the Pacific Ocean has a lot of these sensors due to the seismic activity in the ocean. The sensors measure the ripples in the Earths crust and the data is sent to a station where it is recorded and classified by intensity