Chapter 14 Continuous and campaign-style gravimetric investigations on Montserrat 2006 to 2009

Abstract

Gravimetric time series can provide vital clues about subsurface dynamics associated with active volcanism. Here, we report on continuous and campaign-style gravimetric observations on Montserrat between 2006 and 2009. More than 240 days of continuous gravimetric records enabled us to derive a first local joint solid Earth tides and ocean loading model for Montserrat, and we report the tidal harmonics for 14 major wave groups. Compared to global predictions, the new model (MTY11) achieves an up to one order of magnitude better precision over diurnal and semi-diurnal frequencies. We anticipate that the model will help reduce the effects of tidal perturbations on other geodetic time series recorded on Montserrat. Abrupt variations in gravity accompanied Vulcanian explosions and probably reflect the response of a shallow aquifer to stress changes during pressurization and depressurization of the subvolcanic plumbing system.Campaign data enabled the quantification of mass variations during a cycle of activity including dome formation and repose. Both forward and inverse modelling of the spatio-temporal time series indicates that the source of the recorded gravity variations is situated beneath central Montserrat. Our favourite interpretation of the campaign data is that the gravity variations reflect volcano-tectonic interaction beneath the Centre Hills of Montserrat that are triggered by changes in the active magmatic system of Soufrière Hills Volcano (SHV). We also discuss our findings on subsurface mass variations in relation to annual precipitation records and active dome formation. Both continuous and discrete gravimetric observations indicate coupling between the dominant magmatic sources responsible for the ongoing eruption at SHV and shallow-seated local sources such as aquifers and fluid-saturated fault-damage zones. Our investigations demonstrate the value of including gravimetric observations over a wide frequency range for volcanic system characterization in a volcanic island arc setting.Supplementary material:Details on the inversion routine of the explorative source model GROWTH 2.0 and the resulting images from its application to time-lapse gravity data are available at http://www.geolsoc.org.uk/SUP18701.

Chapter 1 An overview of the eruption of Soufrière Hills Volcano, Montserrat from 2000 to 2010

“…This suggests that stress relaxation in the rock mass underlying the Centre Hills may have occurred following a magmatically induced strain event and this, in turn, increased the permeability of the rock mass. Gravimeter measurements made in the western Centre Hills from 2006 to 2009 detected temporal changes in mass that may correspond to changes in the water table there (Hautmann et al , 2014.…”

Geophysics From Terrestrial Time‐Variable Gravity Measurements

“…They can nevertheless be used for tidal analyses (Bonvalot et al, ; Ducarme & Somerhausen, ; Meurers, ) but are useless for phenomena with a period longer than a few days. Metal spring gravimeters are more stable and have been used for continuous monitoring in a broad range of applications such as studies of active volcanoes (Branca et al, ; Carbone et al, , ; Gottsmann et al, ; Hautmann et al, ; Jousset et al, ). However, the long‐term stability of spring gravimeters is usually not sufficient to resolve weak gravity signals, as those (typically smaller or equal to 100 nm/s 2 ) caused by hydrological systems.…”

“…Different fingerprints could be revealed a few minutes before two different vulcanian explosions in July and December 2008. The authors inferred important constraints for investigations of the effusive to explosive transition; gravity measurements were valuable to investigate the response of a shallow aquifer and fluid-saturated fault damage zones to stress changes during pressurization and depressurization of the plumbing system (Hautmann et al, 2014). Table 2 summarizes the different applications of the monitoring of time-varying gravity on volcanoes.…”

Chapter 11 Volcano geodesy at the Soufrière Hills Volcano, Montserrat: a review

“…In June 2006, a network of benchmarks was installed on Montserrat ( Fig. Details on explosion-related gravimetric and barometric signals were reported by Gottsmann et al (2011), while the new tidal model and its derivation are described in detail in this Memoir (Hautmann et al 2014). Original gravity observation sites were in the northern part of Montserrat, to facilitate easy access.…”