Magamtic earthquakes are caused by the movement of magma within the volcano. They are often shallow and can damage property and set off landslides and rockfalls. They also indicate that the volcano is active and may be preparing for and eruption.
Long period events have been used to successfully predict volcanic eruptions Mount Redoubt,Alaska;Galeras,Colombia and Popocatapetl,Mexico, where thousands of people were safely evacuated from the largest eruption for a thousand years.
One of the leaders in eruption prediction is Bernard Chouet who has pioneered a technique of looking at long period earthquakes (harmonic tremors)beneath a volcano which are the result of gas being pressurised within the volcanoes plumbing. His theory was put to the test when the Mexican authorities evacuated 10s of 1000s of people away from Popocatepetl based on the prediction that there woud be an eruption in 2 days time. He was right, the eruption happened and noone was hurt. After the disaster at Armero caused by Nevado del Ruiz where warning signs had been disregarded this was a great success.
Mt St Helens
A magamatic earthquake was responsible for setting off the landslide which dislodged the bulge on the northern slopes of MtSt Helens which in turn released the pressure on the magma elow and caused a massive eruption.