Seismicity directly under or very near Cascade volcanic centers is usually related to magmatic or geothermal activity. Such earthquakes are typically small, rarely larger than magnitude 4.
Such events, particularly when they also have a low-frequency characteristic and occur in increasing numbers with time indicate unrest at a volcano that may lead to an eruption. Such earthquake activity at Mount St. Helens is well documented before major eruptions in the early 1980s and in 2004-2007. However, a background level of seismicity at many of the volcanoes is normal ranging from 10s to a 100 earthquakes per year at Mount Rainier, Mount Hood, Lassen Peak and Mount St. Helens (between eruptions).
These events may be due to slow hydrothermal weakening of the volcanic edifice and/or gravitational stresses within or just below the edifice. The large, glacier-clad volcanoes also have "ice-quakes", earthquakes that have similar seismograms to volcanic earthquakes but are due to the sudden slipping (of only a few inches) of a large glacier.