Seismic hybrid swarm precursory to a major lava dome collapse: 9–12 July 2003, Soufriere Hills Volcano, Montserrat

Abstract

A swarm of 9500 hybrid earthquakes preceded the 12-13 July 2003 dome collapse at Soufriere Hills Volcano, Montserrat. Most events had nearly identical waveforms and cross-correlation was applied to measure inter-event periods as well as phase arrival times to determine accurate relative location. Hypocenter depths were shallow (< 3 km), and relative locations were con ned to a radius of < 150 m. This small source volume is consistent with the observed waveform similarity. Changes in inter-event periods and energy release, measured from the seismic records, showed that the volcano evolved through several energetic states, possibly linked to cyclic magma movement. Shorter inter-event periods were linked to higher energy release rates and possibly re ect increased pressurization during periods of low extrusion rates.

Seismic source displacement by coda wave interferometry at Soufrière Hills Volcano, Montserrat, WI

“…The duration of individual swarms could range from few to several hours, and they were recorded in episodes lasting days to months. Within swarms, waveforms could be nearly identical, consistent with the hypothesis of relatively small source volumes (Ottemoller, 2008). Green and Neuberg (2006) recognized the existence of groups, or families, of similar LF earthquakes, and suggested a relation between the relative timing of these families and cyclic tilt observed at SHV.…”

Quiescent‐explosive transitions during dome‐forming volcanic eruptions: Using seismicity to probe the volcanic processes leading to the 29 July 2008 vulcanian explosion of Soufrière Hills Volcano, Montserrat

“…The codas of the waveforms remain correlated, suggesting that the same resonator (magma conduit/column) is being activated [ Neuberg et al , ]. This is similar to the results of Ottemöller [], who noted that a high degree of correlation between earthquakes in a hybrid swarm preceding the 2003 dome collapse event was dominated by correlation of the low‐frequency coda of the earthquakes, rather than the high‐frequency onset.…”

“…Repeating earthquakes are found during most significant seismic swarms at volcanoes [ West , ], and their presence alone is not indicative of eruptive processes. Repeating LF waveforms have been observed in many different types of volcanic settings, including dome‐forming eruptions at Redoubt, Alaska; Unzen, Japan; Mount St Helens, Washington; Bezymianny, Kamchatka; and SHV [ Power et al , ; Lamb et al , ; Thelen et al , ; West , ; Green and Neuberg , ; Ottemöller , ; De Angelis , ]; at persistently restless volcanoes such as Telica, Nicaragua, and Shishaldin, Alaska [ Rodgers et al , , ; Petersen , ]; and at basaltic systems such as Kilauea, Hawaii [ Battaglia et al , ]. Repeating families (multiplets) are often used to improve locations [ Battaglia et al , ], determine depth of explosive activity [ West , ; Thelen et al , ], constrain source mechanisms [ Green and Neuberg , ], and assess the stability of seismic sources [ Rodgers et al , ] and the stability of stress fields [ West , ].…”

Lava dome collapse detected using passive seismic interferometry

“…Interestingly, the NCFs do not appear to be sensitive to any changes in volcanic behaviour before the collapse. Intense seismic activity starting on 9 June [ Ottemöller , 2008] preceded the dome collapse, suggest an increase in magma pressure at shallow depths below the volcano. However, there is no clear evidence of any change in the NCFs in the few days prior to the collapse.…”

“…The subsequent collapse of the lava dome, beginning on 12 July, is the largest recorded in historical times, with approximately 210 million m 3 of material removed in 18 hours [ Herd et al , 2005]. A swarm of over 9500 hybrid earthquakes preceded the dome collapse [ Ottemöller , 2008], most of which had nearly identical waveforms, suggesting a highly repeatable source.…”