SEISMICITY AT MOUNT ST-HELENS, 1987-1992 - EVIDENCE FOR REPRESSURIZATION OF AN ACTIVE MAGMATIC SYSTEM

Several changes have occurred in seismicity associated with Mount St. Helens since it last erupted in October 1986. Perhaps the most significant is the recurrence of earthquakes deeper than 3 km, previously observed only in the months following the cataclysmic eruption of May 18, 1980, and briefly in March 1982. Events located below 6.5 km define a circular aseismic zone, similar in location to one observed after May 18, 1980. Focal mechanisms are computed for two depth ranges, 4-6.5 km (Intermediate) and 6.5-10 km (Deep), using both polarity and amplitude ratio data. Mechanisms for Intermediate events are mostly strike-slip, and many P and T axes point in directions approximately 80-degrees offset from the regional stress field, indicating that earthquakes in this zone are caused by magmatic as well as tectonic forces. Mechanisms for Deep events are also mostly strike-slip, and P and T axes form a pattern suggestive of a pressure increase within the aseismic zone. The observed Deep patterns are compared with theoretical stress fields generated by a numerical model of a pressurized hole in an elastic plate. Results from modeling support the pressure increase hypothesis for 1987-1992 seismicity, in contrast to a pressure drop for post-May 18, 1980, earthquakes. This apparent repressurization is proposed to be a result of the sealing of the shallow conduit system.