VOLCANIC EARTHQUAKES AT MOUNT ST. HELENS: EVIDENCE FOR A STICK-SLIP SEISMIC SOURCE

Earthquakes originating on active volcanoes are often classified into “volcano-tectonic”, VT-earthquakes, with signal characteristics similar to tectonic earthquakes but obviously due to volcanically induced stresses and “low-frequency” or “long-period”, LP-earthquakes that have emergent, low-frequency signal characteristics. Models for the source of the LP events fall into two classes; one invoking fluid pressure interactions with cracks and the other invoking signal modification of a stick-slip type source possibly due to wave propagation effects. We have evidence that such earthquakes at Mount St. Helens are a special case of stick-slip motion.

The renewed volcanic activity at Mount St. Helens in the fall of 2004 has provided many examples of LP-earthquakes that can be used to gain insight into their source process. Over 600,000 earthquakes ranging in magnitude from less than 0 to over 3 have been recorded on a network that varied from 12 to 15 seismic stations on and near the volcano. Only about 1% of these earthquakes have been analyzed in detail by hand resulting in a very tight cluster of hypocenters within a few hundred meters of the active vent. Changes in seismicity rate were tracked using a “Real Time Seismic Amplitude Monitor” (RSAM) in which rectified amplitude is calculated in one and ten minute time intervals. We also used a modified event detection algorithm to more quickly determine sizes of individual earthquakes. Spectral analysis of seismograms reveals that the vast majority of earthquakes have low-frequency character. Because of the very short ray path from source to some seismic stations (200-500m) the low-frequency character cannot be due to only a path effect. Once a new dome began actively growing the earthquakes became remarkably regular with slight variations in rate possibly associated with extrusion rate. Evidence that they are the result of stick-slip motion at shallow depth on the periphery of the extruding viscous lava dome includes the formation of gouge like material on the surface of the dome with striations in the direction of motion. Very low rupture propagation velocities on fault surfaces bounding the vent with relatively constant dimensions but variable slip displacements are consistent with the frequency content and amplitude scaling of these events.