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Mass variations in response to magmatic stress changes at Soufrière Hills Volcano, Montserrat (W.I.): Insights from 4-D gravity data
Earth and Planetary Science Letters 290 (2010) 83–89
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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Mass variations in response to magmatic stress changes at Soufrière Hills Volcano, Montserrat (W.I.): Insights from 4-D gravity data Stefanie Hautmann a,⁎, Joachim Gottsmann a, Antonio G. Camacho b, Nicolas Fournier c, I. Selwyn Sacks d,1, R. Stephen J. Sparks a a
Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Instituto de Astronomía y Geodesia (CSIC-UCM), Facultad CC Matemáticas, Universidad Complutense Madrid, 28040 Madrid, Spain Wairakei Research Centre, GNS Science, 114 Karetoto Rd, State Highway 1, Wairakei, Taupo 3377, New Zealand d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 52541 Broad Branch Road NW, Washington, DC 20015-1305, USA b c
a r t i c l e
i n f o
Article history: Received 9 January 2009 Received in revised form 3 December 2009 Accepted 4 December 2009 Available online 3 January 2010 Editor: R.D. van der Hilst Keywords: dynamic gravity Soufrière Hills Volcano fault zone fluid migration volcano–tectonic interaction
a b s t r a c t The eruption of Soufrière Hills Volcano (Montserrat, West Indies) has been ongoing for more than a decade, yet routine monitoring of the activity did not include gravity surveillance for most of the time. In June/July 2006, we installed a new elevation-controlled microgravity network, which we re-occupied in January/ February 2007 and August/September 2008. Residual gravity changes of up to 74 µGal between the surveys allow us to infer net mass and/or density changes beneath the central part of the island. Data inversion for causative source parameters indicates mass changes along NW–SE elongated structures beneath the Centre Hills at a minimum depth of 700 m. We suggest the observed gravity variations to be related to poroelastic dynamics involving groundwater migration and/or fracture opening/closing along a hitherto unrecognized fault zone. The perturbations appear to be triggered by changes in the stress field of the shallow plumbing system of Soufrière Hills Volcano. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Soufrière Hills Volcano (SHV), Montserrat, West Indies is an andesitic volcano that started its current eruptive phase in 1995. Since then, volcanic activity has been unsteady, dominated by an apparently cyclic pattern of explosive eruptions and dome growth/collapse. While geodetic, seismic and geochemical monitoring has been routinely conducted since the eruption began, gravimetric surveillance has not been included for most of this activity (Aspinall et al., 2002). Gravity-height time series, however, can provide vital information about subsurface mass and/or density changes in active volcanic areas (e.g., Rymer, 1994; Jousset et al., 2000; Battaglia et al., 2008) and are uniquely suited to discriminate between magmatic and hydrothermal processes (Gottsmann et al., 2006a, 2007; Battaglia et al., 2003, 2006). In June 2006, we began a joint gravimetric and ground deformation study along an island-wide network of benchmarks in Montserrat. The microgravity surveys were repeated in order
⁎ Corresponding author. Tel.: +44 117 9545244; fax: +44 117 9153385. E-mail addresses: [email protected] (S. Hautmann), [email protected] (J. Gottsmann), [email protected] (A.G. Camacho), [email protected] (N. Fournier), [email protected] (I.S. Sacks), [email protected] (R.S.J. Sparks). 1 Fax: +1 202 4788821. 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.12.004
to assess spatio-temporal variations in the gravity field on the order of months to years. In the present paper, we evaluate recorded changes in gravity in order to quantify mass redistributions beneath Montserrat related to volcanic activity at SHV. 2. Data collection and reduction 2.1. Setup and sampling In June/July 2006, we established an elevation-controlled microgravity network on Montserrat. A total of 10 benchmarks were distributed to provide coverage on the accessible part of the island (Fig. 1a). The average distance between adjacent benchmarks is 2– 3 km. Benchmark BASE located in the northeast of the network, is the base station to which all other measurements are referenced. The southernmost benchmarks, those closest to SHV, are located at St. Georges Hill (STGH; 3 km from the active vent), the Montserrat Volcano Observatory and Garibaldi Hill (MVO2 and GRBD; both 6 km from the active vent). The choice of location for the benchmarks with respect to the active vent was based on a combination of safety considerations and an effort to minimize noise in the gravimetric readings due to eruptive activity. The first re-occupation of the network was about 7 months after its installation, in January/February 2007. A third survey was conducted about 19 months later, in August/
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Fig. 1. Maps showing the gravity network on Montserrat (a) and the recorded gravity changes from the time periods June/July 2006–Jan/Feb 2007 (7 months; b) and Jan/Feb 2007–Aug/ Sept 2008 (19 months; c). Coordinates in a are TM. Gravity changes in b–c are contoured at 5 μGal. Onshore regions, in which the gravity field is not constrained, are shaded in grey.
September 2008. Each campaign lasted about three weeks, during which individual benchmarks were re-occupied up to 10 times. Note, that the northernmost benchmark, SLVH, was destroyed by construction work between the 2007 and 2008 surveys. Gravity measurements during all surveys were carried out via optical reading using a LaCoste&Romberg gravimeter (G-667), while benchmark position and elevation were simultaneously controlled by Leica 500, Trimble 5700 and Topcon HiPer Pro GNSS receivers using static occupations. The GNSS receivers were operating for about 1 h at 0.5 Hz recording frequency at each benchmark.
For most of the stations, we report an uncertainty over multiple readings at individual benchmarks of ±1–10 μGal. The only remarkable exception was found during the third campaign, when gravity data recorded at Jack Boy Hill (JBYH) dropped by 56 μGal over the course of the three weeks survey. Gravity changes between the second and third surveys at JBYH (−159 μGal) were also highly inconsistent with variations recorded at all other sites (between 0 and 74 μGal). We hence assume that the station is affected by local anthropogenic disturbances (possibly water pipes from a visitor centre 200 m uphill from JBYH), and exclude the data collected at Jack Boy Hill from further analysis.
2.2. Correction for instrumental drift and tides
2.3. Accounting for free-air effect
After accounting for instrument drift, raw gravity data were reduced for contributions from Solid Earth tides from theoretical predictions using the QuickTide software (http://www.microglacoste. com/quicktide.htm). In an arc island setting, significant effects from ocean loading are expected and need to be accounted for. However, global ocean loading models such as SCHW80, GOT′00 and FES2004 (Schwiderski, 1980; Ray, 1999; Letellier, 2004), where found to only partially remove ocean loading effects as recorded by a continuouslyrecording gravity meter, which we operated at the Montserrat Volcano Observatory (MVO2) and St. Peters (STPT) during our first campaign. Residual loading effects were significant enough to compromise data if adopting generic observation routines and we therefore devised and followed a specific measurement protocol for the microgravity work. We re-occupied the base station every 2 h and interpolated the resulting differences every minute on the readings from other sites taken in between the base station readings. As a test for the appropriateness of the procedure we compared the extrapolated values to those obtained from reducing times series for the observed ocean loading variations and found that both procedures yielded the same precision of residual gravity changes. This protocol hence enabled us to account for ocean loading effects during later campaigns, during which we did not have continuous gravimetric records available. The resultant frequent re-occupation of the reference benchmark was a time-intensive survey strategy, and we had to compromise on the number of network benchmarks, in order to meet the targeted precision of the gravity data.
Vertical ground deformation triggered by subsurface magmatic processes affects the gravity field and, hence, needs to be accounted for in the processing of gravity data (Walsh, 1975). We performed a GNSS survey simultaneously with each gravity campaign, in order to identify vertical displacements at individual benchmarks for gravity data reduction. Our GNSS data, however, did not reveal any significant ground deformation relative to the base. Vertical benchmark displacements were found to be smaller than ±0.02 m (corresponding to a freeair effect of ±5 µGal) for the first observation interval and within ±0.03 m (corresponding to a free-air effect of ±9 µGal) for the second monitoring period. The deformation data uncertainties, however, are on the same order as the variations. We therefore treat the vertical deformation as noise for the purpose of gravity data reduction for freeair effects. We thus propagated a ±5 µGal uncertainty (for the second observation period ±9 µGal) for the free-air effect in our evaluation of uncertainty in the gravity data. The reported residual gravity variations hence represent dedrifted and de-tided values with corresponding errors accounting for both measurement errors and potential free-air effects within the precision of the GNSS measurements. 3. Results Temporal residual gravity variations recorded over a network of benchmarks must be evaluated in terms of mass and/or density changes in the subsurface. Overall, the residual gravity changes are
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not accompanied by statistically significant ground deformation. As such, assuming an isotropic linearly elastic relationship between subsurface pressurization and ground deformation (e.g., Mogi, 1958), the gravity residuals must be regarded to reflect only mass changes. However, significant gravity changes without surface deformation have been reported at other volcanoes (Tiampo et al., 2004; Gottsmann et al., 2006b) and thus alternative mechanical scenarios of coupling between subsurface stressing, density changes and rock deformation are worth exploring. Our data allow us to define two observation periods, which we evaluate individually in the following. 3.1. June/July 2006–Jan/Feb 2007 (7 months) Residual gravity changes between the first and second surveys document a maximum gravity decrease of − 34 µGal around St. Peters (STPT), northwestern Montserrat, while the southern stations Garibaldi (GRBD) and St. Georges (STGH) show a slight increase in gravity of up to 11 µGal (Fig. 1b). Analysing both negative and positive residuals would require multiple sources (e.g., two sources of positive gravity change north and south the gravity minimum) in order to interpret the recorded changes. However, such a scenario appears mathematically flawed, since the problem becomes under-determined and, given the coherent distribution of the residual gravity changes also physically and geologically unreasonable. In further calculations, we therefore apply an offset value to the data from the first observation interval with dg1off = −11 μGal. To assess the statistical significance of the changes we ran a paired two-sample t-test (Brandt, 1998) on the data using a routine of resampling the normal-distributed gravity residuals. Performing 1000 bootstrap tests we found that the residual gravity variations for all benchmarks are significant at a 95% confidence level. The spatial distribution of the gravity changes indicates that the causative source is elongated in a NW–SE orientation. A Gaussian integration of the net mass loss gives 1.0 × 1010 kg to 3.6 × 1010 kg at a 95% confidence level. 3.2. Jan/Feb 2007–Aug/Sept 2008 (19 months) In contrast to observations from the first monitoring interval, the residual gravity changes between the survey campaigns in 2007 and 2008 reveal a gravity increase at nearly all stations, with a maximum of 74 μGal at MVO2. A paired two-sample t-test on the data, showed all gravity variations to be significant at a 95% confidence level. The 2007–2008 gravity change is centered 1.5 km south of the focus of the 2006–2007 change (Fig. 1c). The shape of the causative source is as well elongated in a NW–SE direction. The Gaussian integration of the net mass gain gives 2.3 × 1010 kg to 5.4 × 1010 kg at a 95% confidence level.
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[accounting for the density difference between solid rock and aqueous fluids]. Results are given in Fig. 2 and Table 1. Note, that inferred source depths should be regarded as a first approximation, since the full wavelength of the signal is unknown, due to the limited size of the island, and hence, the survey area. Considering a horizontal cylindrical source geometry, we find a best fit of model to data for a source depth of 1700 m (first observation interval) and 2100 m (second observation interval). The calculated source radius depends on the density contrast, with a smaller density contrast yielding a larger source radius (35–50 m radius for 400 kg m− 3 density contrast) and vice versa (18–25 m radius for 1700 kg m− 3 density contrast). Assuming the source parameters inferred from the inversions together with a source length of 5000 m gives a mass change of −8.2 × 109 kg for the first observation period and 1.7 × 1010 kg for the second observation interval. Employing a model based on a semi-infinite horizontal sill as source geometry, we find that the residual gravity changes from both observation intervals are best explained with a source located at 700 m depth and a width of approximately 4000 m. The anomaly created by a sill-like body depends on both the density contrast and the thickness of the source, but not on its width or depth of burial. Thus, for a density contrast of 400 kg m− 3, we find that a sill thickness of 3 m best explains the data from the first observation interval, while data from the second monitoring period fits a model with a sill thickness of 5 m. Assuming a density difference of 1700 kg m− 3, the modelled source thickness is either 0.7 m (first interval) or 1.2 m (second interval). For a source length of 5000 m, the inversions give a mass change of − 2.4 × 1010 kg (first interval) and 4.1 × 1010 kg (second interval). Applying an infinite dyke model, the best fit to the recorded data is given for a source that extends between 1200 m and 2300 m depth. The thickness of the dyke is controlled by the density difference between source and medium. A density contrast of 400 kg m− 3 gives a dyke thickness of 17 m (first interval) and 29 m (second interval), while a density contrast of 1700 kg m− 3 results in a dyke thickness of 4 m (first interval) and 7 m (second interval). Assuming a source length of 5000 m, the results correspond to a mass change of −3.1 × 1010 kg and 5.4 × 1010 kg.
4. Data inversion and results In order to evaluate the characteristics of the source that caused the observed microgravity changes, we employed three inversion models considering different source geometries: horizontal cylinder, horizontal sill, and horizontal dyke. All sources were assumed to be infinite in length. We refer to the recent work of Battaglia et al. (2008) for details on gravity modelling using finite sources. In the absence of resolvable ground deformation possible coupling between source density changes, mechanical stressing and rock deformation are ignored and source bodies are treated as porous media. The location and orientation of modelled sources are deduced from the spatial distribution of the recorded gravity changes, with the orientation fixed to NW–SE and the horizontal source locations fixed to N 54200, E 77550 (first interval) and N 51700, E 76600 (second interval), respectively. In all models we apply density contrasts of 400 kg m− 3 [accounting for the density difference between solid rock (2700 kg m− 3; Melnik and Sparks, 2002) and magma (2300 kg m− 3; Rivers and Carmichael, 1987)] and 1700 kg m− 3
Fig. 2. Results of gravity inversions for different finite source geometries, including a horizontal cylinder (black), a horizontal sill (red) and a horizontal dyke (blue). Inversion results are shown with data from each observation period. The recorded gravity data are plotted with distance to the source centres on a projection vertical to the source directions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Source characteristics inferred from inversions of gravity data. Fixed source parameters
First observation interval (2006–2007)
Second observation interval (2007–2008)
Horizontal source location
N 54200, E 77550
N 51700, E 76600
Source orientation
NW–SE
Model inferred source parametersa
Cylinder
Sill
Dyke
Cylinder
Sill
Dyke
Source depthb [m] Lateral extensionb [m] Source radius/thickness [m] For 400 kg m− 3 For 1700 kg m− 3 Mass changec [kg]
1700
700 4000
1200 to 2100
2100
700 4000
1400 to 2300
35 18 − 8.2 × 109
3 0.7 − 2.4 × 1010
17 4 − 3.1 × 1010
50 25 1.7 × 1010
5 1.2 4.1 × 1010
29 7 5.4 × 1010
a b c
NW–SE
Fit residuals are within ± 3 μGal of the recorded data for all model approaches. Ranges of variability are set as 0–10,000. For a source length of 5000 m in NW–SE direction.
Inversions of our recorded microgravity data indicate that the observed mass/density changes occurred at shallow depths (minimum 700 m) beneath central Montserrat, along elongated structures that are directed towards NW–SE. While the set of inversions we performed on our data differs in the source geometry used, the fit quality of model to data is similar for all model approaches (fit residuals are within ±3 µGal of the recorded data). We can therefore not favour one model over another based on the quality of the fit. The mass changes inferred from the different inversion models all fit the integration-derived mass changes (1.0–5.4 × 1010 kg) to within one order of magnitude, with a slightly higher overestimate of the resulting mass changes as inferred from the dyke-source model (3.1–5.4 × 1010 kg), as opposed to a slightly lower estimate of the resulting mass changes yielded from the cylinder-source model (0.8– 1.7 × 1010 kg). Thus, for further interpretation of the geometry of the source, a better handle on the nature of the observed mass loss is required. In the following section, we evaluate possible source mechanisms of the observed gravity signal.
cumulative shortfall of rainfall yields a water level drop of 5.4 m, while 11.7 m of cumulative rainfall leads to a water table rise of 8.8 m. Seasonality on Montserrat is characterized by a dry season with an average cumulative precipitation of 0.7 m between January and June and a wet season with an average cumulative rainfall of 1.2 m between July and December (average precipitation inferred from monthly records of rainfall between 1999 and 2009; Montserrat Water Authority, pers. comm.). In particular, the amount of cumulative rainfall in 2006 was slightly above average as it increased from 1 m to 1.6 m between the period January–June 2006 and the period July 2006–January 2007 (with the latter one as the first gravity monitoring interval). In contrast, the total precipitation in the time between February 2007 and August 2008 (i.e., the second gravity monitoring interval) was markedly below average with 2.3 m. This shows not only that the changes in rainfall are by one order of magnitude smaller than required in order to trigger gravity signals in the recorded scale, but also that the observed gravity variations are inverse to the signal that would be related to possible water level oscillations due to changes in precipitation. Thus, our results demonstrate that groundwater table changes due to seasonal variations neither contribute to nor explain the recorded gravity signals.
5.1. Groundwater level variations due to seasonal changes in precipitation
5.2. Subsurface magmatic processes
It is well known that residual gravity changes can be dominated by groundwater level fluctuations related to seasonal changes in the cumulative rainfall (e.g., Jachens and Roberts, 1985). Because there are no direct measurements of water table changes on Montserrat over the time span of our study, we use the method of cumulative rainfall departure (CRD; Xu and van Tonder, 2001) to calculate the amount of cumulative rainfall/shortfall of rain that would trigger gravity signals comparable to those recorded during our survey campaigns. For an infinite horizontal layer of water, the water table effect on gravity is 42 μGal/m × φ (e.g., Llubes et al., 2004), where φ is the effective open porosity of the host rock. We are aware that modelling the water table as an infinite slab is based on a simplified viewing and that this method consequently holds limitations, nonetheless, this approach offers a first-order approximation to calculate a possible influence of precipitation on our gravity data. Assuming φ = 0.2 (Moreau-Fournier, 2008), a drop in groundwater level by 5.4 m would induce an observed gravity change of −45 μGal (first observation period), while a rise in the aquifer head by 8.8 m would trigger a 74 μGal gravity change (second observation interval). Groundwater level changes can be calculated via
Assigning the observed gravity changes to magma emplacement/ drainage in northwest Montserrat (resulting in a density difference of 400 kg m− 3) would require a magma volume of approximately 7.5 × 107 m3 to account for the observed gravity changes. Using a simple Mogi approach for surface deformation modelling (Mogi, 1958) we found that magma emplacement/drainage of 7.5 × 107 m3 at about 4000 m depth would trigger a vertical ground deformation larger than 1 m above the source, and decays to 0.4 m at a radial distance of 4000 m from the vertical projection of the source. Deformation in that scale would have been detected during the GNSS surveys. Also, magma migration of this magnitude over the time span of a few months would likely be associated with significant seismicity, which was not recorded. Magma drainage out of a shallow reservoir resulting in a mass decrease and creation of void space also appears to be an unlikely scenario, as it would require a magma withdrawal of 8.7 × 106 m3 to fit the recorded signal of the first observation interval. In order to also account for the different locations of maximum mass changes in the first and second observation interval, one would need to argue that an intrusion body even larger than 8.7 × 106 m3 is located beneath the Centre Hills and magma withdrawal out of and new magma injection into separate magmatic pockets resulted in the observed gravity signal. This, however, is highly implausible, as the Centre Hills is an extinct volcano with the last known activity at 400 ka (Harford et al., 2002). In contrast, mass and/or density changes in the magmatic system beneath the active SHV would result in a gravity signal different from that observed, with maximum gravity changes at benchmarks located
5. Discussion
Δh = ðr = SÞ × ðCRDÞ
ð1Þ
where r is a percentage of the CRD, which results in recharge from rainfall, and S is the aquifer storativity (specific yield). The recharge r on Montserrat is about 15% of the CRD (Montserrat Water Authority, pers. comm.). Hence, assuming 20% aquifer storativity, 7.2 m of
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closest to the volcano (e.g., at STGH). We therefore exclude magmatic activity as the causative source for the measured gravity changes. 5.3. Effects related to changes in volcanic activity at SHV (groundwater flow dynamics and/or volcano–tectonic interaction) We conducted our gravity surveys during different phases of volcanic activity at SHV. The first campaign (June/July 2006) was carried out during a phase of rapid dome growth (up to 10 m3 s− 1, representing peak activity at SHV), while during the second survey (Jan/Feb 2007), volcanic activity switched from dome growth to dome growth stagnation, marking the begin of a new period of volcanic quiescence that continued through the time of our third data collection (Aug/Sept 2008). It has been recognized from other active volcanoes, such as Kilauea (Hurwitz and Johnston, 2003) and Mt. Usu (Shibata et al., 2008) that changes in volcanic activity can cause changes in the groundwater level in the vicinity of the volcano, with water level fluctuations of up to 7 m reported from Mt. Usu. These groundwater level variations are on the same scale of what we calculated for water table changes to fit the recorded gravity data. A decrease in the groundwater level during the time of enhanced magmatic activity could be associated with a higher rate of evaporation through the vent, triggered by hot material ascending the feeder system. This process evokes a disequilibrium in the level of the aquifer head, leading to a drainage of water from the northern part of the island towards the south. The recorded gravity changes would correspond to a migration of 2–4 × 107 m3 of water, which corresponds to 2% of the pore volume of SHV (assuming a rock porosity 0.2 of a total volume of 7.5 × 109 m3), and hence is of a reasonable order of magnitude. The spatial distribution of the gravity changes, however, hints towards non-isotropic structural properties of the rocks in the north, with discontinuities that are oriented NW–SE. Tamagawa and Pollard (2008) found increased hydrocarbon fluid penetration as a result of enhancements in fracture permeability created by perturbed stress fields around active faults. We thus suggest that the signals from the
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microgravity surveys can be explained by a pre-existing fault damage zone beneath the Centre Hills, which, together with the Belham Valley fault to the south (Fig. 3), provides a trace for stress-induced fluid migration. An alternative or additional mechanism to explain the recorded gravity signal, is the dilation or medium rarefaction (associated with the negative gravity change of the first monitoring interval) and closure or medium compression (associated with the positive gravity change of the second monitoring interval) of fractures due to stress cycling (stress relaxation and stress increase). Increased microfracturing and rarefaction at Etna Volcano (Carbone et al., 2003, 2009) lead to a mass decrease on the same order of magnitude as that inferred from our data. Observations from Etna Volcano showed that rarefaction occurred in a pre-existing zone of structural weakness. Hence, a sequence of rarefaction and compression in a fault zone in the Centre Hills explains the recorded gravity changes on Montserrat. We consequently infer that the shape of the gravity source is either oblate, when considering a model that invokes groundwater flow, or dyke-like, when accounting for a model of fracture dilation/closure. A combination of both mechanisms [with a dyke-like geometry at depth (as to vertical fractures) that is overlaid by an oblate shaped water lens] is another possible scenario to explain the recorded data. The assumption of a previously unrecognized NW–SE trending zone of structural weakness (i.e., fault zone) that is located at shallow depths beneath the Centre Hills of Montserrat seems also warranted in the context with other geologic observations. The proposed fault zone follows the inner volcanic arc and is likely part of the en echelon fault zone, which is formed by the Redonda, Bouillante-Montserrat and Les Saintes fault systems (Fig. 3; Feuillet et al., 2001, 2002). Surface expressions of a fault beneath the Centre Hills are the Soldier Ghaut, a steep and prominent valley in the north of Montserrat, and an offset of the shelf-edge NW of the island, similar to the offset caused by the Belham Valley Fault to the south. Young faulted sediments have been recognized to the NW of Montserrat in seismic streamer data (Kenedi et al., 2008). Additional field evidence is given by a total number of 23 small-scaled faults and hydrothermal veins that we mapped in road cuts
Fig. 3. (Left) Sketch of the central and part of the northern segments of the Lesser Antilles island arc, including NW–SE directed major tectonic features of the inner volcanic arc, known as the Redonda, Bouillante-Montserrat, and Les Saintes fault systems [modified from Wadge (1986) and Feuillet et al. (2002)]. (Right) Map of Montserrat and its surrounding seafloor. The solid line indicates the location of the Belham Valley fault, dashed lines correspond to the proposed Centre Hills fault as well as parts of the Belham Valley fault that are not fully known. Both faults are interpreted to be part of an en echelon fault zone that follows the inner volcanic arc.
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west of the Centre Hills. The mapped structures have a dip angle between 60° and 90° and a strike direction of NW–SE to NNW–SSE. 6. Conclusion and outlook Over a period of 2 years, we recorded gravity changes along NW– SE elongated structures beneath the Centre Hills, Montserrat. We presented two possible scenarios, which both explain, separately or in combination, our recorded data: Assuming poroelastic mechanical behaviour our models invoke the existence of a previously unrecognized fault zone beneath central Montserrat that is highly influenced by changes in stress distribution associated with volcanic activity at SHV. The fault zone either provides a trace for flow dynamics in the aquifer of the island or responds to a changed stress field though variations in the rate of microfracturing. Note that there is a measure of uncertainty in the inferred location of faults, since the location of the maximum gravity change strongly depends on the time interval chosen for the analyses and the gravity network geometry. For a better resolution of structural discontinuities in the subsurface more frequent observations (to counter aliasing effects) are required along an extended network with a finer mesh in the centre of the island. Future surveys should also include additional gravity stations in the outer north and south of Montserrat in order to put better constraints on the full wavelength of the signal. Another focus of future studies should be to investigate the lack of gravity changes in the records that are primarily related to changes in volcanic activity at SHV. A possible explanation might be that the small aperture of the gravity network limits detection of gravity changes that are triggered by a deep source. During the time of our surveys, the magma chamber in the intermediate crust underwent source volume changes; the wavelength of the resulting gravity signal is likely to be too long to identify the signal in our data. Another aspect to be studied is that dome growth corresponding to a mass of approx. 2.6 × 1011 kg in between the first and the second surveys should result in a negative gravity change radial around SHV with −34 µGal at STGH and −5 µGal at MVO2 and GRBD. While the signal would be within the measurement error and hence, not detectable at the stations MVO2 and GRBD, it should be resolvable at STGH. We can only surmise that dome growth was accompanied by a density increase at 1000± 500 m depth (i.e., the dyke to conduit transition zone of the magmatic feeder system; Hautmann et al. 2009) yielding a compensation of the gravity changes due to magma extrusion at the surface. Possible mechanisms to trigger density increase at depth could be degassing and crystallization of magma. The present gravity network, however, includes on the basis of safety only a single benchmark site (STGH) that is located in the immediate vicinity of the volcano. For better constraints on magma dynamics in the upper magmatic system of SHV, the gravity network should ideally be extended to cover areas around the active vent, which, however, requires access to the current exclusion zone. Acknowledgments SH acknowledges funding by the Bavarian Research Foundation (BFS DPA-53/05) and the “Arthur von Gwinner-Stiftung” (Max-PlanckSociety). JG was supported by a Royal Society University Research Fellowship and NERC grant NE/E007961/1. Research of AGC was supported as part of the projects CGL2005-05500-C02 and CGL200806426-C02 (Spanish Ministry of Science and Innovation). We are grateful to H. Rymer for provision of L&R gravimeter D-41, to the MVO staff for logistical support, to C. Gans and A. Geyer for field assistance and the Montserrat Water Authority for provision of rainfall data. The manuscript greatly benefited from careful reviews by D. Carbone and M. Poland.
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Gottsmann, J., Camacho, A., Fernandez, J., Tiampo, K.F., 2006a. Spatio-temporal variations in vertical gravity gradients at the Campi Flegrei caldera (Italy): a case for source multiplicity during unrest? Geophys. J. Int. 167, 1089–1096. Gottsmann, J., Wooller, L.K., Marti, J., Fernandez, J., Camacho, A.G., Gonzalez, P., Garcia, A., Rymer, H., 2006b. New evidence for the reactivation of Teide Volcano. Geophys. Res. Lett. 33. doi:10.1029/2006GL027523. Gottsmann, J., Carniel, R., Coppo, N., Wooller, L., Hautmann, S., Rymer, H., 2007. Oscillations in hydrothermal systems as a source of periodic unrest at caldera volcanoes: multiparameter insights from Nisyros, Greece. Geophys. Res. Lett. 33. doi:10.1029/2007GL029594. Harford, C.L., Pringle, M.S., Sparks, R.S.J., Young, S.R., 2002. The volcanic evolution of Montserrat using 40Ar/39Ar geochronology. In: Druitt, T.H., Kokelaar, B.P. (Eds.), The Eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999: Geological Society London, Memoir, vol. 21, pp. 93–114. Hautmann, S., Gottsmann, J., Sparks, R.S.J., Costa, A., Melnik, O., Voight, B., 2009. Modelling ground deformation caused by oscillating overpressure in a dyke conduit at Soufrière Hills Volcano, Montserrat. Tectonophysics 471, 87–95. Hurwitz, S., Johnston, M.J.S., 2003. Groundwater level changes in a deep well in response to a magma intrusion event on Kilauea Volcano, Hawaiʻi. Geophys. Res. Lett. 30. doi:10.1029/2003GL018676. Jachens, R.C., Roberts, C.W., 1985. Temporal and areal gravity investigations at Long Valley Caldera, California. J. Geophys. Res. 90, 11,210–11,218. Jousset, P., Dwipa, S., Beauducel, F., Duquesnoy, T., Diament, M., 2000. Temporal gravity at Merapi during the 1993–1995 crisis: an insight into the dynamical behaviour of volcanoes. J. Volcanol. Geotherm. Res. 100, 289–320. Kenedi, C.L., Sparks, R.S.J., Dean, S., Hammond, J., Malin, P.E., Minshull, T., Paulatto, M., Peirce, C., Ryan, G., Shalev, E., Voight, B., 2008. Volcano–Tectonic History of the Island of Montserrat, West Indies, From Seismic Reflection Profiles [abs.]. EOS Transactions AGU Fall Meeting Supplement, p. V53C-06. Letellier, T., 2004. Etude des ondes de marée sur les plateux continentaux [Ph.D. thesis]. Université de Toulouse III, Ecole Doctorale des Sciences de l'Univers, de l'Environnement et de l'Espace, 237 pp. Llubes, M., Florsch, N., Hinderer, J., Longuevergne, L., Amalvict, M., 2004. Local hydrology, the Global Geodynamics Project and CHAMP/GRACE perspective: some case studies. J. Geodyn. 38, 355–374. Melnik, O., Sparks, R.S.J., 2002. Dynamics of magma ascent and lava extrusion at Soufrière Hills Volcano, Montserrat. In: Druitt, T.H., Kokelaar, B.P. (Eds.), The Eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999: Geological Society London, Memoir, vol. 21, pp. 153–171. Mogi, K., 1958. Relations between the eruptions of various volcanoes and the deformations of the ground surfaces around them. Bull. Earthq. Res. Inst. Tokyo 36, 99–134. Moreau-Fournier, M., 2008. Investigation of Groundwater Storage in St. Vincent's Soufrière Crater (West Indies) [Master thesis]. St. Augustine, The University of the West Indies, Department of Chemical Engineering, 113 pp. Ray, R.D., 1999. A Global Ocean Tide Model From TOPEX/POSEIDON Altimetry: GOT99.2. NASA Technical Memorandum, vol. 209,478. Rivers, M., Carmichael, I., 1987. Ultrasonic studies of silicate melts. J. Geophys. Res. 92, 9247–9270. Rymer, H., 1994. Microgravity change as a precursor to volcanic activity. J. Volcanol. Geotherm. Res. 61, 311–328. Schwiderski, E.W., 1980. On charting global ocean tides. Rev. Geophys. Space Phys. 18, 243–268. Shibata, T., Akita, F., Hirose, W., Ikeda, R., 2008. Hydrological and geochemical change related to volcanic activity of Usu volcano, Japan. J. Volcanol. Geotherm. Res. 173, 113–121.
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Wadge, G., 1986. The dykes and structural setting of the volcanic front in the Lesser Antilles island arc. Bull. Volcanol. 48, 349–372. Walsh, J.B., 1975. An analysis of local changes in the gravity due to deformation. Pure Appl. Geophys. 113, 97–106. Xu, Y., van Tonder, 2001. Estimation of recharge using a revised CRD method. Water SA 27, 341–343.
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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Mass variations in response to magmatic stress changes at Soufrière Hills Volcano, Montserrat (W.I.): Insights from 4-D gravity data Stefanie Hautmann a,⁎, Joachim Gottsmann a, Antonio G. Camacho b, Nicolas Fournier c, I. Selwyn Sacks d,1, R. Stephen J. Sparks a a
Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Instituto de Astronomía y Geodesia (CSIC-UCM), Facultad CC Matemáticas, Universidad Complutense Madrid, 28040 Madrid, Spain Wairakei Research Centre, GNS Science, 114 Karetoto Rd, State Highway 1, Wairakei, Taupo 3377, New Zealand d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 52541 Broad Branch Road NW, Washington, DC 20015-1305, USA b c
a r t i c l e
i n f o
Article history: Received 9 January 2009 Received in revised form 3 December 2009 Accepted 4 December 2009 Available online 3 January 2010 Editor: R.D. van der Hilst Keywords: dynamic gravity Soufrière Hills Volcano fault zone fluid migration volcano–tectonic interaction
a b s t r a c t The eruption of Soufrière Hills Volcano (Montserrat, West Indies) has been ongoing for more than a decade, yet routine monitoring of the activity did not include gravity surveillance for most of the time. In June/July 2006, we installed a new elevation-controlled microgravity network, which we re-occupied in January/ February 2007 and August/September 2008. Residual gravity changes of up to 74 µGal between the surveys allow us to infer net mass and/or density changes beneath the central part of the island. Data inversion for causative source parameters indicates mass changes along NW–SE elongated structures beneath the Centre Hills at a minimum depth of 700 m. We suggest the observed gravity variations to be related to poroelastic dynamics involving groundwater migration and/or fracture opening/closing along a hitherto unrecognized fault zone. The perturbations appear to be triggered by changes in the stress field of the shallow plumbing system of Soufrière Hills Volcano. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Soufrière Hills Volcano (SHV), Montserrat, West Indies is an andesitic volcano that started its current eruptive phase in 1995. Since then, volcanic activity has been unsteady, dominated by an apparently cyclic pattern of explosive eruptions and dome growth/collapse. While geodetic, seismic and geochemical monitoring has been routinely conducted since the eruption began, gravimetric surveillance has not been included for most of this activity (Aspinall et al., 2002). Gravity-height time series, however, can provide vital information about subsurface mass and/or density changes in active volcanic areas (e.g., Rymer, 1994; Jousset et al., 2000; Battaglia et al., 2008) and are uniquely suited to discriminate between magmatic and hydrothermal processes (Gottsmann et al., 2006a, 2007; Battaglia et al., 2003, 2006). In June 2006, we began a joint gravimetric and ground deformation study along an island-wide network of benchmarks in Montserrat. The microgravity surveys were repeated in order
⁎ Corresponding author. Tel.: +44 117 9545244; fax: +44 117 9153385. E-mail addresses: [email protected] (S. Hautmann), [email protected] (J. Gottsmann), [email protected] (A.G. Camacho), [email protected] (N. Fournier), [email protected] (I.S. Sacks), [email protected] (R.S.J. Sparks). 1 Fax: +1 202 4788821. 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.12.004
to assess spatio-temporal variations in the gravity field on the order of months to years. In the present paper, we evaluate recorded changes in gravity in order to quantify mass redistributions beneath Montserrat related to volcanic activity at SHV. 2. Data collection and reduction 2.1. Setup and sampling In June/July 2006, we established an elevation-controlled microgravity network on Montserrat. A total of 10 benchmarks were distributed to provide coverage on the accessible part of the island (Fig. 1a). The average distance between adjacent benchmarks is 2– 3 km. Benchmark BASE located in the northeast of the network, is the base station to which all other measurements are referenced. The southernmost benchmarks, those closest to SHV, are located at St. Georges Hill (STGH; 3 km from the active vent), the Montserrat Volcano Observatory and Garibaldi Hill (MVO2 and GRBD; both 6 km from the active vent). The choice of location for the benchmarks with respect to the active vent was based on a combination of safety considerations and an effort to minimize noise in the gravimetric readings due to eruptive activity. The first re-occupation of the network was about 7 months after its installation, in January/February 2007. A third survey was conducted about 19 months later, in August/
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Fig. 1. Maps showing the gravity network on Montserrat (a) and the recorded gravity changes from the time periods June/July 2006–Jan/Feb 2007 (7 months; b) and Jan/Feb 2007–Aug/ Sept 2008 (19 months; c). Coordinates in a are TM. Gravity changes in b–c are contoured at 5 μGal. Onshore regions, in which the gravity field is not constrained, are shaded in grey.
September 2008. Each campaign lasted about three weeks, during which individual benchmarks were re-occupied up to 10 times. Note, that the northernmost benchmark, SLVH, was destroyed by construction work between the 2007 and 2008 surveys. Gravity measurements during all surveys were carried out via optical reading using a LaCoste&Romberg gravimeter (G-667), while benchmark position and elevation were simultaneously controlled by Leica 500, Trimble 5700 and Topcon HiPer Pro GNSS receivers using static occupations. The GNSS receivers were operating for about 1 h at 0.5 Hz recording frequency at each benchmark.
For most of the stations, we report an uncertainty over multiple readings at individual benchmarks of ±1–10 μGal. The only remarkable exception was found during the third campaign, when gravity data recorded at Jack Boy Hill (JBYH) dropped by 56 μGal over the course of the three weeks survey. Gravity changes between the second and third surveys at JBYH (−159 μGal) were also highly inconsistent with variations recorded at all other sites (between 0 and 74 μGal). We hence assume that the station is affected by local anthropogenic disturbances (possibly water pipes from a visitor centre 200 m uphill from JBYH), and exclude the data collected at Jack Boy Hill from further analysis.
2.2. Correction for instrumental drift and tides
2.3. Accounting for free-air effect
After accounting for instrument drift, raw gravity data were reduced for contributions from Solid Earth tides from theoretical predictions using the QuickTide software (http://www.microglacoste. com/quicktide.htm). In an arc island setting, significant effects from ocean loading are expected and need to be accounted for. However, global ocean loading models such as SCHW80, GOT′00 and FES2004 (Schwiderski, 1980; Ray, 1999; Letellier, 2004), where found to only partially remove ocean loading effects as recorded by a continuouslyrecording gravity meter, which we operated at the Montserrat Volcano Observatory (MVO2) and St. Peters (STPT) during our first campaign. Residual loading effects were significant enough to compromise data if adopting generic observation routines and we therefore devised and followed a specific measurement protocol for the microgravity work. We re-occupied the base station every 2 h and interpolated the resulting differences every minute on the readings from other sites taken in between the base station readings. As a test for the appropriateness of the procedure we compared the extrapolated values to those obtained from reducing times series for the observed ocean loading variations and found that both procedures yielded the same precision of residual gravity changes. This protocol hence enabled us to account for ocean loading effects during later campaigns, during which we did not have continuous gravimetric records available. The resultant frequent re-occupation of the reference benchmark was a time-intensive survey strategy, and we had to compromise on the number of network benchmarks, in order to meet the targeted precision of the gravity data.
Vertical ground deformation triggered by subsurface magmatic processes affects the gravity field and, hence, needs to be accounted for in the processing of gravity data (Walsh, 1975). We performed a GNSS survey simultaneously with each gravity campaign, in order to identify vertical displacements at individual benchmarks for gravity data reduction. Our GNSS data, however, did not reveal any significant ground deformation relative to the base. Vertical benchmark displacements were found to be smaller than ±0.02 m (corresponding to a freeair effect of ±5 µGal) for the first observation interval and within ±0.03 m (corresponding to a free-air effect of ±9 µGal) for the second monitoring period. The deformation data uncertainties, however, are on the same order as the variations. We therefore treat the vertical deformation as noise for the purpose of gravity data reduction for freeair effects. We thus propagated a ±5 µGal uncertainty (for the second observation period ±9 µGal) for the free-air effect in our evaluation of uncertainty in the gravity data. The reported residual gravity variations hence represent dedrifted and de-tided values with corresponding errors accounting for both measurement errors and potential free-air effects within the precision of the GNSS measurements. 3. Results Temporal residual gravity variations recorded over a network of benchmarks must be evaluated in terms of mass and/or density changes in the subsurface. Overall, the residual gravity changes are
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not accompanied by statistically significant ground deformation. As such, assuming an isotropic linearly elastic relationship between subsurface pressurization and ground deformation (e.g., Mogi, 1958), the gravity residuals must be regarded to reflect only mass changes. However, significant gravity changes without surface deformation have been reported at other volcanoes (Tiampo et al., 2004; Gottsmann et al., 2006b) and thus alternative mechanical scenarios of coupling between subsurface stressing, density changes and rock deformation are worth exploring. Our data allow us to define two observation periods, which we evaluate individually in the following. 3.1. June/July 2006–Jan/Feb 2007 (7 months) Residual gravity changes between the first and second surveys document a maximum gravity decrease of − 34 µGal around St. Peters (STPT), northwestern Montserrat, while the southern stations Garibaldi (GRBD) and St. Georges (STGH) show a slight increase in gravity of up to 11 µGal (Fig. 1b). Analysing both negative and positive residuals would require multiple sources (e.g., two sources of positive gravity change north and south the gravity minimum) in order to interpret the recorded changes. However, such a scenario appears mathematically flawed, since the problem becomes under-determined and, given the coherent distribution of the residual gravity changes also physically and geologically unreasonable. In further calculations, we therefore apply an offset value to the data from the first observation interval with dg1off = −11 μGal. To assess the statistical significance of the changes we ran a paired two-sample t-test (Brandt, 1998) on the data using a routine of resampling the normal-distributed gravity residuals. Performing 1000 bootstrap tests we found that the residual gravity variations for all benchmarks are significant at a 95% confidence level. The spatial distribution of the gravity changes indicates that the causative source is elongated in a NW–SE orientation. A Gaussian integration of the net mass loss gives 1.0 × 1010 kg to 3.6 × 1010 kg at a 95% confidence level. 3.2. Jan/Feb 2007–Aug/Sept 2008 (19 months) In contrast to observations from the first monitoring interval, the residual gravity changes between the survey campaigns in 2007 and 2008 reveal a gravity increase at nearly all stations, with a maximum of 74 μGal at MVO2. A paired two-sample t-test on the data, showed all gravity variations to be significant at a 95% confidence level. The 2007–2008 gravity change is centered 1.5 km south of the focus of the 2006–2007 change (Fig. 1c). The shape of the causative source is as well elongated in a NW–SE direction. The Gaussian integration of the net mass gain gives 2.3 × 1010 kg to 5.4 × 1010 kg at a 95% confidence level.
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[accounting for the density difference between solid rock and aqueous fluids]. Results are given in Fig. 2 and Table 1. Note, that inferred source depths should be regarded as a first approximation, since the full wavelength of the signal is unknown, due to the limited size of the island, and hence, the survey area. Considering a horizontal cylindrical source geometry, we find a best fit of model to data for a source depth of 1700 m (first observation interval) and 2100 m (second observation interval). The calculated source radius depends on the density contrast, with a smaller density contrast yielding a larger source radius (35–50 m radius for 400 kg m− 3 density contrast) and vice versa (18–25 m radius for 1700 kg m− 3 density contrast). Assuming the source parameters inferred from the inversions together with a source length of 5000 m gives a mass change of −8.2 × 109 kg for the first observation period and 1.7 × 1010 kg for the second observation interval. Employing a model based on a semi-infinite horizontal sill as source geometry, we find that the residual gravity changes from both observation intervals are best explained with a source located at 700 m depth and a width of approximately 4000 m. The anomaly created by a sill-like body depends on both the density contrast and the thickness of the source, but not on its width or depth of burial. Thus, for a density contrast of 400 kg m− 3, we find that a sill thickness of 3 m best explains the data from the first observation interval, while data from the second monitoring period fits a model with a sill thickness of 5 m. Assuming a density difference of 1700 kg m− 3, the modelled source thickness is either 0.7 m (first interval) or 1.2 m (second interval). For a source length of 5000 m, the inversions give a mass change of − 2.4 × 1010 kg (first interval) and 4.1 × 1010 kg (second interval). Applying an infinite dyke model, the best fit to the recorded data is given for a source that extends between 1200 m and 2300 m depth. The thickness of the dyke is controlled by the density difference between source and medium. A density contrast of 400 kg m− 3 gives a dyke thickness of 17 m (first interval) and 29 m (second interval), while a density contrast of 1700 kg m− 3 results in a dyke thickness of 4 m (first interval) and 7 m (second interval). Assuming a source length of 5000 m, the results correspond to a mass change of −3.1 × 1010 kg and 5.4 × 1010 kg.
4. Data inversion and results In order to evaluate the characteristics of the source that caused the observed microgravity changes, we employed three inversion models considering different source geometries: horizontal cylinder, horizontal sill, and horizontal dyke. All sources were assumed to be infinite in length. We refer to the recent work of Battaglia et al. (2008) for details on gravity modelling using finite sources. In the absence of resolvable ground deformation possible coupling between source density changes, mechanical stressing and rock deformation are ignored and source bodies are treated as porous media. The location and orientation of modelled sources are deduced from the spatial distribution of the recorded gravity changes, with the orientation fixed to NW–SE and the horizontal source locations fixed to N 54200, E 77550 (first interval) and N 51700, E 76600 (second interval), respectively. In all models we apply density contrasts of 400 kg m− 3 [accounting for the density difference between solid rock (2700 kg m− 3; Melnik and Sparks, 2002) and magma (2300 kg m− 3; Rivers and Carmichael, 1987)] and 1700 kg m− 3
Fig. 2. Results of gravity inversions for different finite source geometries, including a horizontal cylinder (black), a horizontal sill (red) and a horizontal dyke (blue). Inversion results are shown with data from each observation period. The recorded gravity data are plotted with distance to the source centres on a projection vertical to the source directions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Source characteristics inferred from inversions of gravity data. Fixed source parameters
First observation interval (2006–2007)
Second observation interval (2007–2008)
Horizontal source location
N 54200, E 77550
N 51700, E 76600
Source orientation
NW–SE
Model inferred source parametersa
Cylinder
Sill
Dyke
Cylinder
Sill
Dyke
Source depthb [m] Lateral extensionb [m] Source radius/thickness [m] For 400 kg m− 3 For 1700 kg m− 3 Mass changec [kg]
1700
700 4000
1200 to 2100
2100
700 4000
1400 to 2300
35 18 − 8.2 × 109
3 0.7 − 2.4 × 1010
17 4 − 3.1 × 1010
50 25 1.7 × 1010
5 1.2 4.1 × 1010
29 7 5.4 × 1010
a b c
NW–SE
Fit residuals are within ± 3 μGal of the recorded data for all model approaches. Ranges of variability are set as 0–10,000. For a source length of 5000 m in NW–SE direction.
Inversions of our recorded microgravity data indicate that the observed mass/density changes occurred at shallow depths (minimum 700 m) beneath central Montserrat, along elongated structures that are directed towards NW–SE. While the set of inversions we performed on our data differs in the source geometry used, the fit quality of model to data is similar for all model approaches (fit residuals are within ±3 µGal of the recorded data). We can therefore not favour one model over another based on the quality of the fit. The mass changes inferred from the different inversion models all fit the integration-derived mass changes (1.0–5.4 × 1010 kg) to within one order of magnitude, with a slightly higher overestimate of the resulting mass changes as inferred from the dyke-source model (3.1–5.4 × 1010 kg), as opposed to a slightly lower estimate of the resulting mass changes yielded from the cylinder-source model (0.8– 1.7 × 1010 kg). Thus, for further interpretation of the geometry of the source, a better handle on the nature of the observed mass loss is required. In the following section, we evaluate possible source mechanisms of the observed gravity signal.
cumulative shortfall of rainfall yields a water level drop of 5.4 m, while 11.7 m of cumulative rainfall leads to a water table rise of 8.8 m. Seasonality on Montserrat is characterized by a dry season with an average cumulative precipitation of 0.7 m between January and June and a wet season with an average cumulative rainfall of 1.2 m between July and December (average precipitation inferred from monthly records of rainfall between 1999 and 2009; Montserrat Water Authority, pers. comm.). In particular, the amount of cumulative rainfall in 2006 was slightly above average as it increased from 1 m to 1.6 m between the period January–June 2006 and the period July 2006–January 2007 (with the latter one as the first gravity monitoring interval). In contrast, the total precipitation in the time between February 2007 and August 2008 (i.e., the second gravity monitoring interval) was markedly below average with 2.3 m. This shows not only that the changes in rainfall are by one order of magnitude smaller than required in order to trigger gravity signals in the recorded scale, but also that the observed gravity variations are inverse to the signal that would be related to possible water level oscillations due to changes in precipitation. Thus, our results demonstrate that groundwater table changes due to seasonal variations neither contribute to nor explain the recorded gravity signals.
5.1. Groundwater level variations due to seasonal changes in precipitation
5.2. Subsurface magmatic processes
It is well known that residual gravity changes can be dominated by groundwater level fluctuations related to seasonal changes in the cumulative rainfall (e.g., Jachens and Roberts, 1985). Because there are no direct measurements of water table changes on Montserrat over the time span of our study, we use the method of cumulative rainfall departure (CRD; Xu and van Tonder, 2001) to calculate the amount of cumulative rainfall/shortfall of rain that would trigger gravity signals comparable to those recorded during our survey campaigns. For an infinite horizontal layer of water, the water table effect on gravity is 42 μGal/m × φ (e.g., Llubes et al., 2004), where φ is the effective open porosity of the host rock. We are aware that modelling the water table as an infinite slab is based on a simplified viewing and that this method consequently holds limitations, nonetheless, this approach offers a first-order approximation to calculate a possible influence of precipitation on our gravity data. Assuming φ = 0.2 (Moreau-Fournier, 2008), a drop in groundwater level by 5.4 m would induce an observed gravity change of −45 μGal (first observation period), while a rise in the aquifer head by 8.8 m would trigger a 74 μGal gravity change (second observation interval). Groundwater level changes can be calculated via
Assigning the observed gravity changes to magma emplacement/ drainage in northwest Montserrat (resulting in a density difference of 400 kg m− 3) would require a magma volume of approximately 7.5 × 107 m3 to account for the observed gravity changes. Using a simple Mogi approach for surface deformation modelling (Mogi, 1958) we found that magma emplacement/drainage of 7.5 × 107 m3 at about 4000 m depth would trigger a vertical ground deformation larger than 1 m above the source, and decays to 0.4 m at a radial distance of 4000 m from the vertical projection of the source. Deformation in that scale would have been detected during the GNSS surveys. Also, magma migration of this magnitude over the time span of a few months would likely be associated with significant seismicity, which was not recorded. Magma drainage out of a shallow reservoir resulting in a mass decrease and creation of void space also appears to be an unlikely scenario, as it would require a magma withdrawal of 8.7 × 106 m3 to fit the recorded signal of the first observation interval. In order to also account for the different locations of maximum mass changes in the first and second observation interval, one would need to argue that an intrusion body even larger than 8.7 × 106 m3 is located beneath the Centre Hills and magma withdrawal out of and new magma injection into separate magmatic pockets resulted in the observed gravity signal. This, however, is highly implausible, as the Centre Hills is an extinct volcano with the last known activity at 400 ka (Harford et al., 2002). In contrast, mass and/or density changes in the magmatic system beneath the active SHV would result in a gravity signal different from that observed, with maximum gravity changes at benchmarks located
5. Discussion
Δh = ðr = SÞ × ðCRDÞ
ð1Þ
where r is a percentage of the CRD, which results in recharge from rainfall, and S is the aquifer storativity (specific yield). The recharge r on Montserrat is about 15% of the CRD (Montserrat Water Authority, pers. comm.). Hence, assuming 20% aquifer storativity, 7.2 m of
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closest to the volcano (e.g., at STGH). We therefore exclude magmatic activity as the causative source for the measured gravity changes. 5.3. Effects related to changes in volcanic activity at SHV (groundwater flow dynamics and/or volcano–tectonic interaction) We conducted our gravity surveys during different phases of volcanic activity at SHV. The first campaign (June/July 2006) was carried out during a phase of rapid dome growth (up to 10 m3 s− 1, representing peak activity at SHV), while during the second survey (Jan/Feb 2007), volcanic activity switched from dome growth to dome growth stagnation, marking the begin of a new period of volcanic quiescence that continued through the time of our third data collection (Aug/Sept 2008). It has been recognized from other active volcanoes, such as Kilauea (Hurwitz and Johnston, 2003) and Mt. Usu (Shibata et al., 2008) that changes in volcanic activity can cause changes in the groundwater level in the vicinity of the volcano, with water level fluctuations of up to 7 m reported from Mt. Usu. These groundwater level variations are on the same scale of what we calculated for water table changes to fit the recorded gravity data. A decrease in the groundwater level during the time of enhanced magmatic activity could be associated with a higher rate of evaporation through the vent, triggered by hot material ascending the feeder system. This process evokes a disequilibrium in the level of the aquifer head, leading to a drainage of water from the northern part of the island towards the south. The recorded gravity changes would correspond to a migration of 2–4 × 107 m3 of water, which corresponds to 2% of the pore volume of SHV (assuming a rock porosity 0.2 of a total volume of 7.5 × 109 m3), and hence is of a reasonable order of magnitude. The spatial distribution of the gravity changes, however, hints towards non-isotropic structural properties of the rocks in the north, with discontinuities that are oriented NW–SE. Tamagawa and Pollard (2008) found increased hydrocarbon fluid penetration as a result of enhancements in fracture permeability created by perturbed stress fields around active faults. We thus suggest that the signals from the
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microgravity surveys can be explained by a pre-existing fault damage zone beneath the Centre Hills, which, together with the Belham Valley fault to the south (Fig. 3), provides a trace for stress-induced fluid migration. An alternative or additional mechanism to explain the recorded gravity signal, is the dilation or medium rarefaction (associated with the negative gravity change of the first monitoring interval) and closure or medium compression (associated with the positive gravity change of the second monitoring interval) of fractures due to stress cycling (stress relaxation and stress increase). Increased microfracturing and rarefaction at Etna Volcano (Carbone et al., 2003, 2009) lead to a mass decrease on the same order of magnitude as that inferred from our data. Observations from Etna Volcano showed that rarefaction occurred in a pre-existing zone of structural weakness. Hence, a sequence of rarefaction and compression in a fault zone in the Centre Hills explains the recorded gravity changes on Montserrat. We consequently infer that the shape of the gravity source is either oblate, when considering a model that invokes groundwater flow, or dyke-like, when accounting for a model of fracture dilation/closure. A combination of both mechanisms [with a dyke-like geometry at depth (as to vertical fractures) that is overlaid by an oblate shaped water lens] is another possible scenario to explain the recorded data. The assumption of a previously unrecognized NW–SE trending zone of structural weakness (i.e., fault zone) that is located at shallow depths beneath the Centre Hills of Montserrat seems also warranted in the context with other geologic observations. The proposed fault zone follows the inner volcanic arc and is likely part of the en echelon fault zone, which is formed by the Redonda, Bouillante-Montserrat and Les Saintes fault systems (Fig. 3; Feuillet et al., 2001, 2002). Surface expressions of a fault beneath the Centre Hills are the Soldier Ghaut, a steep and prominent valley in the north of Montserrat, and an offset of the shelf-edge NW of the island, similar to the offset caused by the Belham Valley Fault to the south. Young faulted sediments have been recognized to the NW of Montserrat in seismic streamer data (Kenedi et al., 2008). Additional field evidence is given by a total number of 23 small-scaled faults and hydrothermal veins that we mapped in road cuts
Fig. 3. (Left) Sketch of the central and part of the northern segments of the Lesser Antilles island arc, including NW–SE directed major tectonic features of the inner volcanic arc, known as the Redonda, Bouillante-Montserrat, and Les Saintes fault systems [modified from Wadge (1986) and Feuillet et al. (2002)]. (Right) Map of Montserrat and its surrounding seafloor. The solid line indicates the location of the Belham Valley fault, dashed lines correspond to the proposed Centre Hills fault as well as parts of the Belham Valley fault that are not fully known. Both faults are interpreted to be part of an en echelon fault zone that follows the inner volcanic arc.
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west of the Centre Hills. The mapped structures have a dip angle between 60° and 90° and a strike direction of NW–SE to NNW–SSE. 6. Conclusion and outlook Over a period of 2 years, we recorded gravity changes along NW– SE elongated structures beneath the Centre Hills, Montserrat. We presented two possible scenarios, which both explain, separately or in combination, our recorded data: Assuming poroelastic mechanical behaviour our models invoke the existence of a previously unrecognized fault zone beneath central Montserrat that is highly influenced by changes in stress distribution associated with volcanic activity at SHV. The fault zone either provides a trace for flow dynamics in the aquifer of the island or responds to a changed stress field though variations in the rate of microfracturing. Note that there is a measure of uncertainty in the inferred location of faults, since the location of the maximum gravity change strongly depends on the time interval chosen for the analyses and the gravity network geometry. For a better resolution of structural discontinuities in the subsurface more frequent observations (to counter aliasing effects) are required along an extended network with a finer mesh in the centre of the island. Future surveys should also include additional gravity stations in the outer north and south of Montserrat in order to put better constraints on the full wavelength of the signal. Another focus of future studies should be to investigate the lack of gravity changes in the records that are primarily related to changes in volcanic activity at SHV. A possible explanation might be that the small aperture of the gravity network limits detection of gravity changes that are triggered by a deep source. During the time of our surveys, the magma chamber in the intermediate crust underwent source volume changes; the wavelength of the resulting gravity signal is likely to be too long to identify the signal in our data. Another aspect to be studied is that dome growth corresponding to a mass of approx. 2.6 × 1011 kg in between the first and the second surveys should result in a negative gravity change radial around SHV with −34 µGal at STGH and −5 µGal at MVO2 and GRBD. While the signal would be within the measurement error and hence, not detectable at the stations MVO2 and GRBD, it should be resolvable at STGH. We can only surmise that dome growth was accompanied by a density increase at 1000± 500 m depth (i.e., the dyke to conduit transition zone of the magmatic feeder system; Hautmann et al. 2009) yielding a compensation of the gravity changes due to magma extrusion at the surface. Possible mechanisms to trigger density increase at depth could be degassing and crystallization of magma. The present gravity network, however, includes on the basis of safety only a single benchmark site (STGH) that is located in the immediate vicinity of the volcano. For better constraints on magma dynamics in the upper magmatic system of SHV, the gravity network should ideally be extended to cover areas around the active vent, which, however, requires access to the current exclusion zone. Acknowledgments SH acknowledges funding by the Bavarian Research Foundation (BFS DPA-53/05) and the “Arthur von Gwinner-Stiftung” (Max-PlanckSociety). JG was supported by a Royal Society University Research Fellowship and NERC grant NE/E007961/1. Research of AGC was supported as part of the projects CGL2005-05500-C02 and CGL200806426-C02 (Spanish Ministry of Science and Innovation). We are grateful to H. Rymer for provision of L&R gravimeter D-41, to the MVO staff for logistical support, to C. Gans and A. Geyer for field assistance and the Montserrat Water Authority for provision of rainfall data. The manuscript greatly benefited from careful reviews by D. Carbone and M. Poland.
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