Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.
Abstract. The interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task. Locations are subject to uncertainties, which will not be the same for every earthquake. The data should therefore not be interpreted simply by inspection, which is difficult in the case of three-dimensional data anyway. Instead, we propose using the location uncertainties as a guide in processing the data. Earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained, which can then be interpreted in terms of some simplified structure such as faults. The aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data. The method is applied to three synthetic sets of data. These illustrate the potential and limitations of the method. Application to a real earthquake data set from Rabaul Caldera in Papua New Guinea gives an image of the caldera ring fault that suggests departures from the simple ringfault structure previously assumed. Sensitivity analysis on the Rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it.
Abstract. A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel-time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 kin. However, a NE-SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano-tectonic earthquakes (VTs), and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution 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 concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of 'hybrid' seismic events became predominant. By late-1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth -the diminution of shear failure earthquakes suggests the pathway for magma discharge 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 linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.
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