Two Types of Volcanic Earthquakes
Two types of earthquakes are associated with volcanic activity. The first type occurs as magma withdraws from fissures through which it had been flowing. The fissures collapse, producing what seismologists call volcano-tectonic (VT) earthquakes. They aren't associated with eruptions. The second type of quake, known as long-period earthquakes, occur as magma pushes its way through new rock. The magma and gases work their way into confined spaces and build up enough pressure to break the rock so that they can continue moving. When these occur for a sustained period, it's a sign that an eruption is about to occur.
Earthquakes During Eruptions
Both VT and long period earthquakes can happen while an earthquake is erupting, and although they can be felt for miles around the volcano, they aren't as strong as non-volcanic earthquakes. For example, the strongest quake felt around Mount St. Helens, when it erupted in 1981, had a magnitude of 5.1 on the Richter scale. That's less than 20 times as powerful as the devastating 7.7 quake that killed 356 people in Pakistan in September 2013. Although an earthquake may occur during an eruption, there is no guarantee that one will.
Pre-Eruption Earthquakes
Volcanic activity can increase in an area after it experiences a large earthquake. For example, Kilauea Volcano began erupting less than an hour after a magnitude 7.2 earthquake struck the Big Island of Hawaii in November 1975. The eruption lasted less than a day, suggesting that magma that was already near the surface and was pushed upward by the energy of the earthquake. Oxford University researchers have shown that volcanic activity in areas as far a 500 kilometers (311 miles) from the epicenter of a large earthquake can increase as much as four times for the year after the quake.
Predicting Eruptions
Seismologists use several methods to predict volcanic eruptions. Monitoring the incidence of long period earthquakes -- which have a different seismological signature than VT quakes -- is one. Collecting gases from a fumarole or vent is another, although this can be a dangerous undertaking. Increasing concentrations of carbon dioxide and sulfur dioxide are signs that the magma could be moving closer to the surface and that an eruption may occur. Around some volcanoes, such as the one at Mammoth Mountain in California, carbon dioxide discharges into the soil, and scientist can monitor the magma movements by collecting the soil and measuring its carbon dioxide content.