Volcanic hybrid earthquakes that are brittle-failure events
Article first published online: 28 MAR 2007
Copyright 2007 by the American Geophysical Union.
Geophysical Research Letters
Volume 34, Issue 6, March 2007
How to Cite
2007), Volcanic hybrid earthquakes that are brittle-failure events, Geophys. Res. Lett., 34, L06308, doi:10.1029/2006GL028714., and (
- Issue published online: 28 MAR 2007
- Article first published online: 28 MAR 2007
- Manuscript Accepted: 5 FEB 2007
- Manuscript Revised: 17 JAN 2007
- Manuscript Received: 7 NOV 2006
- hybrid earthquakes;
- empirical Green's functions
 Volcanoes generate a variety of pre-eruptive low-frequency seismic signals. Hybrid earthquakes comprise a class of these signals having high-frequency onsets followed by low-frequency ringing. They are used empirically to predict eruptions, but their ambiguous physical origin limits their diagnostic use. The short-duration, near-field hybrid seismograms associated with the 2004 Mount St. Helens eruption indicate that much of the prolonged signal is due to path rather than resonating fluids. We show using seismic source spectra that the hybrids have a corner frequency/seismic moment relationship that scales consistently with brittle-failure. The unusually low frequency of these earthquakes can result from low rupture velocities combined with strong path effects due to their shallow sources. This new application of near-field instrumentation provides the first seismological evidence for brittle-failure as a major process in dome building, and suggests that hybrids should not be used as direct indicators of fluids.