EARTHQUAKE CLASSIFICATION, LOCATION, AND ERROR ANALYSIS IN A VOLCANICENVIRONMENT - IMPLICATIONS FOR THE MAGMATIC SYSTEM OF THE 1989-1990 ERUPTIONS AT REDOUBT VOLCANO, ALASKA

Determination of the precise locations of seismic events associated wi th the 1989-1990 eruptions of Redoubt Volcano posed a number of proble ms, including poorly known crustal velocities, a sparse station distri bution, and an abundance of events with emergent phase onsets. In addi tion, the high relief of the volcano could not be incorporated into th e HYPOELLIPSE earthquake location algorithm. This algorithm was modifi ed to allow hypocenters to be located above the elevation of the seism ic stations. The velocity model was calibrated on the basis of a poste ruptive seismic survey, in which four chemical explosions were recorde d by eight stations of the permanent network supplemented with 20 temp orary seismographs deployed on and around the volcanic edifice. The mo del consists of a stack of homogeneous horizontal layers; setting the top of the model at the summit allows events to be located anywhere wi thin the volcanic edifice. Detailed analysis of hypocentral errors sho ws that the long-period (LP) events constituting the vigorous 23-hour swarm that preceded the initial eruption on December 14 could have ori ginated from a point 1.4 km below the crater floor. A similar analysis of LP events in the swarm preceding the major eruption on January 2 s hows they also could have originated from a point, the location of whi ch is shifted 0.8 km northwest and 0.7 km deeper than the source of th e initial swarm. We suggest this shift in LP activity reflects a north ward jump in the pathway for magmatic gases caused by the sealing of t he initial pathway by magma extrusion during the last half of December . Volcano-tectonic (VT) earthquakes did not occur until after the init ial 23-hour-long swarm. They began slowly just below the LP source and their rate of occurrence increased after the eruption of 01:52 AST on December 15, when they shifted to depths of 6 to 10 km. After January 2 the VT activity migrated gradually northward; this migration sugges ts northward propagating withdrawal of magma from a plexus of dikes an d/or sills located in the 6 to 10 km depth range. Precise relocations of selected events prior to January 2 clearly resolve a narrow, steepl y dipping, pencil-shaped concentration of activity in the depth range of 1-7 km, which illuminates the conduit along which magma was transpo rted to the surface. A third event type, named hybrid, which blends th e characteristics of both VT and LP events, originates just below the LP source, and may reflect brittle failure along a zone intersecting a fluid-filled crack. The distribution of hybrid events is elongated 0. 2-0.4 km in an east-west direction. This distribution may offer constr aints on the orientation and size of the fluid-filled crack inferred t o be the source of the LP events.