SOUFRIERE HILLS ERUPTION, MONTSERRAT, 1995-1997 - VOLCANIC EARTHQUAKELOCATIONS AND FAULT PLANE SOLUTIONS

A total of 9242 seismic events, recorded since the start of the erupti on on Montserrat in July 1995, have been uniformly relocated with stat ion travel-time corrections. Early seismicity was generally diffuse un der southern Montserrat, and mostly restricted to depths less than 7 l an. However, a NE-SW alignment of epicentres beneath the NE flank of t he volcano emerged in one swarm of volcano-tectonic earthquakes (VTs), and later nests of VT hypocentres developed beneath the volcano and a t a separated location, under St. George's Hill. The overall spatial d istribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentr ated below the crater and, after dome building started in Fall 1995, V Ts diminished and repetitive swarms of 'hybrid' seismic events became predominant. By late-1996, as magma effusion rates escalated, most sei smic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth - the diminutio n of shear failure earthquakes suggests the pathway for magma discharg e had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linea r VT activity, due to interaction with stresses in the overriding lith ospheric plate, may encourage normal fault growth and promote sector w eaknesses in the volcano.