Recording the Shaking Ground
  How many seismographs are there?  

There are thousands of permanent seismographs operating in the world today. The Global Seismographic Network (GSN) is just one of many networks of seismic recording stations. The GSN includes 128 permanent stations located in 80 countries spanning all the continents. Every earthquake is recorded on many seismographs, thus allowing scientists to determine precisely where the event occurred and its magnitude.

Since seismic waves travel through the Earth and along its surface, GSN stations that are closest to the earthquake's epicenter will record ground motion first, but eventually the waves will reach even the most remote stations.