Broadband characteristics of earthquakes recorded during a dome-building eruption at Mount St. Helens, Washington, between October 2004 and May 2005

From October 2004 to May 2005, the Center for Earthquake Research and Information of the University of Memphis
operated two to six broadband seismometers within 5 to 20
km of Mount St. Helens to help monitor recent seismic and
volcanic activity. Approximately 57,000 earthquakes identified during the 7-month deployment had a normal magnitude
distribution with a mean magnitude of 1.78 and a standard
deviation of 0.24 magnitude units. Both the mode and range
of earthquake magnitude and the rate of activity varied during
the deployment. We examined the time domain and spectral
characteristics of two classes of events seen during dome
building. These include volcano-tectonic earthquakes and
lower-frequency events. Lower-frequency events are further
classified into hybrid earthquakes, low-frequency earthquakes,
and long-duration volcanic tremor. Hybrid and low-frequency
earthquakes showed a continuum of characteristics that varied
systematically with time. A progressive loss of high-frequency
seismic energy occurred in earthquakes as magma approached
and eventually reached the surface. The spectral shape of large
and small earthquakes occurring within days of each other did
not vary with magnitude. Volcanic tremor events and lower-frequency earthquakes displayed consistent spectral peaks,
although higher frequencies were more favorably excited during tremor than earthquakes.