نص أصلي - انجليزي - seismic signals

Before it erupts, a volcano produces many thousands of earthquakes. Some of these happen when rock inside the volcano cracks or slips along a fault. Others are caused by magma (molten rock) and gases trying to move under pressure within channels and cracks.
Select from the links below to find out about the four primary types of volcano-related earthquakes and to see the seismic signals that they produce. (The movement of a signal from one peak to the next is equal to one cycle.)