SPATIAL VARIATIONS IN THE FREQUENCY-MAGNITUDE DISTRIBUTION OF EARTHQUAKES AT SOUFRIERE HILLS VOLCANO, MONTSERRAT, WEST-INDIES

The frequency-magnitude distribution of earthquakes measured by the b- value is determined as a function of space beneath Soufriere Hills Vol cano, Montserrat, from data recorded between August 1, 1995 and March 31, 1996. A volume of anomalously high b-values (b > 3.0) with a 1.5 k m radius is imaged at depths of 0 and 1.5 km beneath English's Crater and Chance's Peak. This high b-value anomaly extends southwest to Gage 's Soufriere. At depths greater than 2.5 km volumes of comparatively l ow b-values (b similar to 1) are found beneath St. George's Hill, Wind y Hill, and below 2.5 km depth and to the south of English's Crater. W e speculate the depth of high b-value anomalies under volcanoes may be a function of silica content, modified by some additional factors, wi th the most siliceous having these volumes that are highly fractured o r contain high pore pressure at the shallowest depths.