Seismic and infrasonic signals associated with an unusual collapse event at the Soufriere Hills volcano, Montserrat

In March 2004, during a period of no magma extrusion at Soufriere Hills volcano, Montserrat, an explosive event occurred with little precursory activity. Recorded broadband seismic signals ranged from an ultra-long-period signal with a dominant period of 120 s to impulsive, short-duration events containing frequencies up to 30 Hz. Synthetic displacement functions were fit to the long-period data after application of the seismometer response. These indicate a shallow collapse of the volcanic edifice occurred, initiated similar to 300 m below the surface, lasting similar to 100 s. Infrasonic tremor and pulses were also recorded in the 1 - 20 Hz range. The high-frequency seismicity and infrasound are interpreted as the subsequent collapse of a gravitationally unstable buttress of remnant dome material which impacted upon the edifice surface. This unique dataset demonstrates the benefits of deploying multiparameter stations equipped with broadband instruments.