Characteristics and causes of shallow seismicity in andesite volcanoes

Recent observations from Soufriere Hills volcano in Montserrat reveal a wide variety of seismic signals. We focus here on characteristics of long-period events, and their role in the pressurization of the volcanic plumbing system. These events can occur in swarms and merge into tremor, an observation that sheds further light on the generation and composition of harmonic tremor. A two-dimensional finite-difference method has been employed to model major features of low-frequency seismic signatures and compare them with the observations. A depth-dependent seismic velocity model for a fluid-filled conduit is used that accounts for the varying gas content in the magma. We analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in excess pressure due to unloading and degassing events. Potential trigger mechanisms are suggested and discussed.