Seismic investigations of subsurface volcanic structures and processes at Mount Spurr, Alaska and Soufriere Hills Volcano, Montserrat, West Indies

Abstract

Seismological techniques are used to infer the subsurface structures and volcanic processes at two recently active volcanoes: Mount Spurr, Alaska, and Soufriere Hills Volcano, Montserrat, West Indies. The three-dimensional P-wave velocity structure of Mount Spurr is determined to depths of 10 km by tomographic inversion of 3,754 P-wave arrival times from local earthquakes. Results show a prominent low-velocity zone beneath the southeast flank of Crater Peak extending from the surface to 3--4 km below sea level, spatially coincident with an active geothermal system. Beneath Crater Peak an approximately 3-km-wide zone of relatively low velocities correlates with a near vertical band of seismicity, suggestive of a magma conduit. No large low-velocity zone indicative of a magma chamber occurs within the upper 10 km of the crust. In the three years bracketing the 1992 eruptions of Mount Spurr's Crater Peak vent, approximately 2,500 located events were classified as Volcano-Tectonic (VT) earthquakes, Long-Period (LP) events, or Hybrid events. An unusual mix of VT, LP, and hybrid events at 20 to 40 km depth began coincident with the onset of unrest and peaked shortly after eruptive activity ended. The classified seismic events are combined with geophysical and geological data to develop a simplified model of the magmatic plumbing system of Mount Spurr. The major components of this model are a deep magma source zone at 20--40 km depth, a smaller storage zone at about 10 km depth, and a pipe-like conduit that extends to the surface. The frequency-magnitude distribution of earthquakes measured by the b-value is determined as a function of space beneath Soufriere Hills Volcano, from data recorded between August 1, 1995 and March 31, 1996. A volume of high b-values (b > 3.0) with a 1.5 Ian radius is imaged between 0 and 1.5 Ian beneath English's Crater and Chance's Peak. This anomaly extends southwest to Gage's Soufriere. At depths greater than 2.5 km, volumes of comparatively low b-values ( b <math> <f> ~</f> </math> 1) are found beneath St. George's Hill, Windy Hill, and below 2.5 kin to the south of English's Crater.