occur in swarms as opposed to mainshock–aftershock sequences, have smaller maximum sizes than tectonic structure earthquakes, have similar waveform patterns, increase in number before eruptions, and occur near or beneath the site of the eruption. Other types of seismic activity monitored in relation to volcanoes and their eruptions are long period seismic waves, which are caused by sudden sporadic movement of magma that had previously not been moving due to a blockage, and harmonic tremor, which indicates steady magma movement deep in the mantle. References Volcanism Seismology