Infrasonic monitoring at Stromboli volcano during the 2003 effusive eruption: insights on the explosive and degassing process of an open conduit system.

Stromboli is an open conduit volcano characterized by persistent conduit degassing and explosive dynamics. During the 2003 effusive eruption we monitored this activity with a five-element array deployed at ∼450 m from the active craters. The array allows us to track in real time changes of explosive activity in terms of source position and excess pressure release. Infrasonic monitoring located the new active vents in the same preeruptive position, revealing that the collapse of the crater terrace has not changed the geometry of the shallow feeding system. Elevation of the infrasonic source is stable at ∼750 m above sea level, coincident with the elevation of the crater terrace. This suggests a shallow position of the magma column or, if the source is embedded in the conduit, diffraction at the crater rim. At the end of the eruption, back azimuth analysis indicates that explosive activity was progressively reestablished at all of the craters following a NE toward SW direction along the collapsed trench. The array detected significant small-amplitude (∼1 Pa) infrasonic activity occurring at a rate of approximately one pulse every 2 s and generally located in only one vent at a time. This persistent activity is not related to Strombolian explosion, but we infer it is reflecting the overpressurized degassing of the magma column. Amplitude distribution of ∼7 million infrasonic signals shows that this degassing is 1 order of magnitude more energetic than the explosions. Besides, infrasonic amplitude distribution presents a decay with a double slope, indicating that explosions and degassing are driven by conduit dynamics acting at two different rates.