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. However, increasing activity is very worrisome, especially if long-period events become dominant and episodes of harmonic tremor appear. As magma nears the surface and its pressure decreases, gases escape. This process is much like what happens when you open a bottle of soda and carbon dioxide escapes. Sulfur dioxide is one of the main components of volcanic gases, and increasing amounts of it herald the arrival of more and more magma near the surface.