Photograph of a tiltmeter courtesy of U.S. Geological Survey. This is by no means a complete list, volcanologists will use whatever tool they can to help them understand volcanoes. Volcano Monitoring Techniques has more information about monitoring volcanoes. This photo shows the earthquake record for the onset of a flank eruption on Kilauea Volcano in November of 1979. Photograph by Robert Decker, U.S. Geological Survey.
Scientists use seismometers to determine the location and strength of earthquakes.
The seismometer is the device that actually measures the ground shaking. The signal is transferred to a seismograph which plots the data as a squiggly line called a seismogram. Depending on how far away the earthquake is from the instrument and how strong the earthquake is, the squiggly line can be jagged or barely noticeable. The Richter scale was developed so that earthquakes could be compared in an objective way. For example, a medium-sized earthquake might be really devastating if it occurs near a city while a big earthquake might be go unnoticed if it happens out in the desert.