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 Volcano, 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 km 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 low b‐values (b∼1) are found beneath St. George's Hill, Windy Hill, and below 2.5 km depth and to the south of English's Crater. We speculate the depth of high b‐value anomalies under volcanoes may be a function of silica content, modified by some additional factors, with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths.