Some volcanoes normally have continuing low-level seismic activity, but an increase can signify an eruption. The types of earthquakes that occur and where they start and end are also key signs.
Scientists monitor activity at Mt. Pinatubo.
They are related to the fracturing of brittle rock as the magma forces its way upward. These short-period earthquakes signify the growth of a magma body near the surface. They are similar to the clanging sometimes heard in your home's plumbing system. Harmonic tremor occurs when there is sustained movement of magma below the surface. Patterns of seismicity are complex and often difficult to interpret.