Researchers led by Michigan Technological University Assistant Professor Gregory Waite hope their findings into the causes of the earthquakes that accompany volcanic eruptions will help scientists better assess the hazard of a violent explosion at Mount St. Helens and similar volcanoes.

When a volcano such as Mount St Helens erupts, it can cause a series of shallow, repetitive earthquakes at intervals so regular they've been called "drumbeat earthquakes," the scientists said. Until now, its been generally believed such earthquakes are caused by jerky movements of a solid plug of molten rock traveling up from the volcano's core, a process known as the stick-slip model.

But modeling of seismic data collected by Waite and colleagues dispute that explanation. "The regularity and similarity of the shallow earthquakes seem consistent with a stick-slip model," said Waite. "But this is not typical of a stick-slip event. Rather, it suggests a source with a net volume change, such as a resonating fluid-filled crack."

Waite and co-authors Bernard Chouet and Phillip Dawson detail their findings in the Journal of Geophysical Research.