G. Wadge

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is < 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.

Atmospheric models, GPS and InSAR measurements of the tropospheric water vapour field over Mount Etna

[1] Dynamic models of atmospheric movement over the Mount Etna volcano are used to calculate the path delays affecting radar caused by variable water vapour in the troposphere. We compare these model results with the equivalent differential radar interferogram generated by two ERS-2 SAR images taken 35 days apart and the water vapour delay retrievals from a network of fourteen GPS stations distributed over the volcano. The atmospheric model delay field agrees wellwith thelong-wavelength spatial differences measured by InSAR and those measured by GPS. INDEX TERMS: 6924 Radio Science: Interferometry; 1243 Geodesy and Gravity: Space geodetic surveys; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; 6964 Radio Science: Radio wave propagation; 8499 Volcanology: General or miscellaneous. Citation: Wadge, G., et al., Atmospheric models, GPS and InSAR measurements of the tropospheric water vapour field over Mount Etna, Geophys. Res. Lett., 29(19), 1905, doi:10.1029/2002GL015159, 2002.

Atmospheric water vapour correction to InSAR surface motion measurements on mountains: results from a dense GPS network on Mount Etna

Abstract In order for synthetic aperture radar interferometry (InSAR) to effectively measure the pattern of surface deformations of dynamic phenomena, such as volcanoes, it is necessary to mitigate the effects of water vapour on this signal. One way to achieve this is to use a numerical model of the atmospheric conditions, calibrated using an independent measure of atmospheric water vapour, in order to determine the water vapour-based delay in InSAR. We describe an experiment in which such GPS-derived integrated water vapour (IWV) estimates were made using a dense array of continuous GPS receivers. Fourteen GPS receivers were deployed at stations on Mount Etna and continuous observations were recorded for a 10-day period in August/September 2000 and repeated over a 10-day period in October 2000, coincident with ascending and descending SAR image acquisition by the ERS-2 satellite. The results show maximum variations in IWV of 10 kg / m 2 between the days corresponding to the ERS-2 passes. The variations were not the same at all the stations and equated to variations in the zenith wet delay (ZWD) ranging from a few millimetres to about 6 cm. The differences in ZWD between the two passes would degrade the ability to measure surface deformations significantly if they were not modelled.

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

Abstract The 1995–present eruption of Soufrière Hills Volcano on Montserrat has produced over a cubic kilometre of andesitic magma, creating a series of lava domes that were successively destroyed, with much of their mass deposited in the sea. There have been five phases of lava extrusion to form these lava domes: November 1995–March 1998; November 1999–July 2003; August 2005–April 2007; July 2008–January 2009; and October 2009–February 2010. It has been one of the most intensively studied volcanoes in the world during this time, and there are long instrumental and observational datasets. From these have sprung major new insights concerning: the cyclicity of magma transport; low-frequency earthquakes associated with conduit magma flow; the dynamics of lateral blasts and Vulcanian explosions; the role that basalt–andesite magma mingling in the mid-crust has in powering the eruption; identification using seismic tomography of the uppermost magma reservoir at a depth of 5.5 > 7.5 km; and many others. Parallel to the research effort, there has been a consistent programme of quantitative risk assessment since 1997 that has both pioneered new methods and provided a solid evidential source for the civil authority to use in mitigating the risks to the people of Montserrat.

The potential of GIS modelling of gravity flows and slope instabilities

Abstract The component of gravitational acceleration parallel to the slope of the local surface partly determines the state of slope stability and the kinematics of flow under gravity on that slope. Geographical information systems based on digital elevation models offer the potential to be able to map this variable and permit the modelling of a variety of stability criteria and surface processes including landslides, rock avalanches, pyroclastic flows and lava flows. Three types of models and the basic map operations required to run them are discussed. The models are as follows: (i) sites of potential shallow slope failure (e.g. landslides), (ii) maps of flow deposition based on energy balance calculations (e.g. rock avalanches) and (iii) finite difference, initial value type simulations of dynamic flow (e.g. lava flows). The potential value of these models to hazard assessment is great but their application in specific cases must be assessed with reference to the accuracy of the digital elevation model ...

Four-phase tectonostratigraphic development of the southern Caribbean

Abstract Although the stratigraphy of northern South America and the southern Caribbean has been studied in detail in many areas, a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming. Since northern South America collided with southern North America in the Late Palaeozoic, we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern Caribbean. These phases derived from a synthesis of regional stratigraphy were: (1) Panama arc collistion with Colombia in the Late Miocene/Early Pliocene which drove a complex triangular wedge bounded by strike-slip faults northward into the Caribbean; (2) right-lateral strike-slip faulting associated with post-Eocene eastern motion of the Caribbean ocean floor into the gap between the separating Americas; (3) island arc development and collision associated with the entry of the present Caribbean ocean floor, an area of buoyant ocean crust, into the gap between the separating Americas; and (4) continental rifting and passive margin development in the Late Jurassic/Early Cretaceous associated with the separation of North and South America approximately along the Palaeozoic suture zone. Overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region.

Spaceborne radar measurements of the eruption of Soufrière Hills Volcano, Montserrat

Abstract Radar measurements from space are used to help monitor the evolution of a Peléean eruption on Soufrière Hills Volcano, Montserrat, during 1996 to April 1999. Data from four radar systems are used: ERS-1, ERS-2, Radarsat and JERS-1. We demonstrate that ratio images of backscattered radar energy collected at different times provide useful qualitative summary maps of gross topographic change (e.g. infilling of valleys with deposits) and changes in backscattering properties with time. Radar phase data using the interferometry technique can also provide valuable change detection information. Phase coherence images for ERS-1 and ERS-2 pairs with only one day separation on 25 and 26 September 1997 and 8 and 9 April 1999 allow the areal extent of some pyroclastic flows deposits emplaced during those 24-hour periods to be mapped. Generally, the radar phase can only be retrieved from those parts of the volcano where the vegetation is destroyed by pyroclastic flow deposits and local slopes are not too steep. Quantitative information on the topography of the volcano can also be extracted from the phase data, though not routinely from the new lava dome that grew during 1995-1998. By comparing the radar-measured post-eruption topography with the pre-eruption topography the thickness of the pyroclastic flow deposits (up to 85 m in some valleys on the northeastern slopes) are mapped. Radar interferometry also supplies a means of measuring surface deformation between radar images.

Chapter 2 Cyclic phenomena at the Soufrière Hills Volcano, Montserrat

Abstract Cycles of eruptive activity are generally interpreted as evidence of one or more mechanisms operating in equilibrium. Modulation of cycle characteristics thus reflects changes in the conditions affecting those mechanisms. This kind of semi-deterministic behaviour at the Soufrière Hills Volcano has occurred on multiple timescales and with a range of eruptive intensity. By documenting cyclic phenomena, it is possible to investigate the mechanisms that modulate the state of the eruption and examine conceptual models. Pattern recognition and model development allows some degree of short-term forecasting ability for volcanic activity. We report the cyclic eruptive phenomena that have occurred on Montserrat on scales of hours to centuries. We identify four dominant types of cyclicity: sub-daily variations in lava flux; sub-annual cycles in eruption intensity; multi-annual ‘on–off’ switching of lava extrusion; and multi-decadal recurrence of seismic crises. We exploit a wealth of multi-parameter datasets (including seismic, geodetic, thermal, archive and visual observations), and present the evidence and observations for each type of cyclicity, some of which are documented here for the first time. Wavelet time-series analysis is used to constrain cycle characteristics, where appropriate. We discuss the implications of these observations in understanding the eruptive mechanisms of the Soufrière Hills Volcano. Supplementary material: Details of all sub-daily cycles are listed in a supplementary table that is available at http://www.geolsoc.org.uk/SUP18700.

Evaporation of groundwater from arid playas measured by C-band SAR

Salt-bearing groundwater evaporates within arid playas and roughens the surface during the summer months. We show that it is possible to infer the rates of evaporation of groundwater using radar backscatter measurements. Using an empirically derived relationship between backscatter coefficient and surface roughness, a model of how surface roughness changes with a continuous process of halite crystal efflorescence, and an assumed value for the salinity of the regional groundwater, we calculate the volumetric rate at which the groundwater must have evaporated. The method is illustrated with data from the European Remote Sensing 1 satellite synthetic aperture radar sensor that imaged the Chott el Djerid playa in southern Tunisia from 1992 to 1993. Independent measurements and calculations indicate that the radar method overestimates the rate of evaporation. The reasons for this probably lie in the assumptions about the salt budget of the groundwater or the model of how the salt crust roughens with time.

Modeling the backscatter response due to salt crust development

The ERS-1 synthetic aperture radar (SAR) is shown to be highly sensitive to temporal variations in the mineral surface of the Chott el Djerid playa, Tunisia. Field measurements and modeling results confirm that the primary control on the backscatter response is due to salt crust development. Brine-rich moisture exerts a secondary control. The effect of temperature, salinity, and mineralogy are negligible. An integral equation model (IEM) solution with mean parameters accurately represents the observed behavior.