As magma nears the surface and the surface deforms and earthquakes are generated. The more magmatic activity there is, the more earthquakes there are. By keeping track of the number of earthquakes through time, one can get an idea of the degree of volcanic unrest. Although large earthquakes can be felt, the vast majority of smaller earthquakes are only detected with sensitive equipment. A seismometer is usually used to measure and record the seismic activity at a volcano. Constant monitoring is needed to keep volcanologists up to date.
Although there are unusual types of earthquakes peculiar to volcanoes that signify particular types of activity, we will not be distinguishing between them. The number of earthquakes within given time spans is all that we will use in the simulation. A change in the number of earthquakes will signify a change in the activity at the volcano.