Soufrière Hills Eruption, Montserrat, 1995–1997: Volcanic earthquake locations and fault plane solutions

Abstract

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.

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

“…Phase equilibria and melt inclusions indicate that the magma at depth contained 4-5 wt.% H 2 O in the rhyolitic melt phase (Barclay et al 1998). Temperature and water pressure conditions are estimated at 830-860°C and about 120-140 MPa , consistent with the seismic estimate to top of magma reservoir at >5 km (Aspinall et al 1998).…”

Montserrat geothermal system: A 3D conceptual model

“…Fault intersections produce zones of enhanced permeability and also produce crustal weaknesses that can act as preferential magma pathways. The area beneath SGH has been the site of periodic seismic swarms that have been recorded in 1933-1937, 1966-1967, and 1992-1996; there was also a preinstrumental seismic crisis 1897-1898 [Aspinall et al, 1998] that was also associated with this area. These seismic crises are consistent with repeated magmatic injection episodes occurring beneath SGH as well as SHV.…”

“…[7] EQ swarms with hypocenters beneath SGH occurred between 1995 and 1997 [Aspinall et al, 1998] in the vicinity of the geothermal system. A study of all located EQs detected by the Montserrat Volcano Observatory network showed that virtually all local seismicity has been located beneath the SHV since 1997.…”

“…In this study, we analyze the spatial relationships between the available geophysical data sets including MT [Ryan et al, 2009;Poux and Brophy, 2012], seismic tomography [Shalev et al, 2010], and earthquake (EQ) hypocenters [Aspinall et al, 1998] to determine which conceptual model is best supported by the data. Furthermore, the geophysical data are used to constrain the location of drilling targets in the geothermal system to provide both geothermal power and obtain further information about the structure of the geothermal system.…”

Seismic characterization of pyroclastic flow activity at Soufrière Hills Volcano, Montserrat, 8 January 2007

“…The ongoing eruption began on 18 July 1995 Aspinall et al, 1998) following a three-year period of precursor seismic activity. The eruption has been characterized by the growth of an andesite lava dome with associated pyroclastic flows (PF), vulcanian explosions and debris flows.…”