A seismometer is an instrument that measures ground vibrations caused by a variety of processes. A seismic network, typically 6-8 seismometers within 20 km (13 mi) of a volcano, is required for basic earthquake location capabilities. However, for very high-threat volcanoes where high-accuracy is critical for tracking subtle changes in earthquake location, networks should have double this number of seismometers within 20 km (13 mi) of the volcano.
Seismic data from monitoring networks is viewed and processed in real time to aid in tracking and understanding processes happening beneath the ground surface. Using automated analysis, volcano seismologists can often determine the near real-time changing character of a volcano's earthquake activity and thus the changing eruptive behavior. By combining this information with data acquired from other disciplines, VHP scientists have a better understanding of the volcanic plumbing system and can better determine the processes that may lead to an eruption.