Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano: volcano-tectonic earthquakes and long period earthquakes.
Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma (molton rock) are called volcano-tectonic earthquakes (Chouet, 1993). These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't indicate that the volcano will be erupting but can occur at anytime.
The second category of volcanic earthquakes are long period earthquakes which are produced by the injection of magma into surrounding rock. These earthquakes are a result of pressure changes during the unsteady transport of the magma. When magma injection is sustained a lot of earthquakes are produced (Chouet, 1993). This type of activity indicates that a volcano is about to erupt. Scientists use seismographs to record the signal from these earthquakes. This signal is known as volcanic tremor.
People living near an erupting volcano are very aware of volcanic earthquakes. Their houses will shake and windows rattle from the numerous earthquakes that occur each day before and during a volcanic eruption. Residents in Pompeii felt earthquakes daily before Vesuvius erupted in A.D. 79 but continued to go about their daily routines (Francis, 1993). When Mount Pinatubo in the Philipines erupted in 1991, nerves were rattled as much as windows by volcanic earthquakes.
Earthquakes exhibiting volcanic tremor warn of an impending eruption so that people can be evacuated to areas of safety. The volcanic tremor signal has been used successfully to predict the 1980 eruptions Mount St. Helens and the 1991 eruption of Pinatubo. Volcano-tectonic earthquakes can cause damage to manmade structures and landsliding. To prevent damage from being done, structures should be built according to earthquake standards, building foundations should be constructed on firm ground and not unconsolidated material which may amplify earthquake intensity, and buildings should be constructed on stable slopes in areas of low hazard potential.
Text by C.M. Riley, Photo by M.T. Dolan