Benefits and Limitations of the Growth Inversion Approach in Volcano Gravimetry Demonstrated on the Revisited 2004–2005 Tenerife Unrest (original) (raw)
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On the possibilities of modern gravimetric methods in interpreting gravity changes in volcanic areas
Temporal gravity changes observed at the Central Volcanic Complex (CVC) of Tenerife (Canary Islands) between May 2004 and July 2005, accompanying the unrest and suspected reawakening of the Teide volcano, are reinterpreted here. Our objective is to seek multiple sources generating the observed gravity signal. Our interpretation is based on the decomposition of the gravity signal into shallow and deep fields, and subsequent inversion of each field by means of 3D line segments. The found shallow (near-surface) segments are interpreted as hydrothermal fluids. The detected short deep segment at the depth of about 6 km is interpreted as magma injection that subsequently triggered the migration of the hydrothermal fluids. This deep segment correlates very well with the NW seismogenic zone of the detected VT event swarms accompanying the unrest.
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This study used spatiotemporal land gravity data to investigate the 2021 eruption that occurred in the Cumbre Vieja volcano (La Palma, Canary Islands). First, we produced a density model by inverting the local gravity field using data collected in July 2005 and July 2021. This model revealed a low-density body beneath the western flank of the volcano that explains a highly fractured and altered structure related to the active hydrothermal system. Then, we retrieved changes in gravity and GNSS vertical displacements from repeated measurements made in a local network before (July 2021) and after (January 2022) the eruption. After correcting the vertical surface displacements, the gravity changes produced by mass variation during the eruptive process were used to build a forward model of the magmatic feeding system consisting of dikes and sills based on an initial model defined by the paths of the earthquake hypocenters preceding the eruption. Our study provides a final model of the ma...
The Interpretation of Gravity Changes and Crustal Deformation in Active Volcanic Areas
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The 3-D gravity inversion package GROWTH2.0 and its application to Tenerife Island, Spain
We present the gravity inversion software GROWTH2.0 and its application to recently obtained gravity data from the volcanic island of Tenerife (Canary Islands, Spain) to inform on its subsurface density structure. GROWTH2.0 is an inversion tool which enables the user to obtain, in a nearly automatic and nonsubjective mode, a 3D model of the subsurface density anomalies based on observed gravity anomaly data. The package is composed of three parts: (a) GRID3D to generate a 3D partition of the subsurface volume into parallelepiped elements, (b) GROWTH to perform the inversion routine and to obtain a 3D anomalous density model, and (c) VIEW for visual representation of the input data, the inversion model, and modeling residuals. The current version of the tool has been developed from an earlier code (Camacho et al., 2002) and now incorporates several novelties: (1) a Graphical User Interface (GUI), (2) an optional automated routine for determination of parameter l, which controls the balance between model fitness and smoothness, (3) optional determination of values for minimum density contrast, (4) a robust handling of outlier data, and (5) improved automated data reduction for terrain effects based on anticorrelation with topographic data. The new capabilities and applicability of GROWTH2.0 for 3-D gravity inversion are demonstrated by a case example using new gravity data from the volcanic island of Tenerife. In a nearly automatic approach, the software provides a 3-D model informing on the location and shape of the main structural building blocks of the island. Our model results allow us to shed light on the low-density structure of the islands dominant Pico Viejo–Pico Teide (PV–PT) volcanic complex and the identification of an intrusive structure (the east bulge volcano) embedded in Teide’s east flank. A low-density body located at around 5.8 km depth beneath PT’s summit may represent a current magma or hybrid reservoir.
Recent Developments and Trends in Volcano Gravimetry
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The aim of this chapter is to take a look at some developments and new trends in volcano gravimetry. First, we will review the objectives of the research work within this subfield of geophysics, discuss the data and methods it uses, and outline the outputs it strives for. Then, we will turn our attention to three areas where innovative approaches possibly can forward this field of study. The first has to do with the coupling between vertical deformations of the topographic surface (elevation changes) and the observed gravity changes or, in other words, with the removal of the deformation-induced gravimetric signal from the observed gravity changes to obtain the net gravity changes caused by volcanic signals. The second and third areas regard the inversion of the observed gravity changes and deal with two recently or newly developed inversion approaches that both are characterized by the ability to produce a suite of diverse solutions that can be analyzed and discriminated based on additional independent constraints stemming from other earth science disciplines or from the cognition of the interpreter. With this in mind, the final goal is a better understanding of the mechanisms and processes of volcanic unrest or reawakening of a volcano and forecasting the threat of consequent activity and impacts.
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Geophysical Journal International, 2001
The application of a gravity inversion method enables us to obtain a 3-D density contrast model of the upper crustal anomalies of the volcanic island of Lanzarote (Canary Islands). For this, we use a network of 296 gravity stations distributed over the whole island, and a digital terrain model of about 45 000 terrestrial and oceanic data to determine the corresponding terrain correction. A density value of 2480 kg m x3 is chosen for this correction by means of a new approach. The resulting Bouguer anomaly is analysed by means of a least-squares prediction which gives us a mean level of uncorrelated observational noise of about 1.2 mgal. This anomaly is considered in order to obtain independent information about the inner anomalous mass density distribution by means of a 3-D gravity inversion based on a systematic exploration on a prismatic partition of the subsoil volume, and adopting a priori values of the density contrast (positive and negative) to determine the geometry of the anomalous bodies. The problem of non-uniqueness of the solution is avoided by using a minimization mix condition on the weighted residuals and the weighted whole anomalous mass. The structural solution is finally presented by means of horizontal sections and vertical profiles.
Chapter 4 Recent Developments and Trends in Volcano Gravimetry
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The aim of this chapter is to take a look at some developments and new trends in volcano gravimetry. First, we will review the objectives of the research work within this subfield of geophysics, discuss the data and methods it uses, and outline the outputs it strives for. Then, we will turn our attention to three areas where innovative approaches possibly can forward this field of study. The first has to do with the coupling between vertical deformations of the topographic surface (elevation changes) and the observed gravity changes or, in other words, with the removal of the deformation-induced gravimetric signal from the observed gravity changes to obtain the net gravity changes caused by volcanic signals. The second and third areas regard the inversion of the observed gravity changes and deal with two recently or newly developed inversion approaches that both are characterized by the ability to produce a suite of diverse solutions that can be analyzed and discriminated based on a...
Journal of Volcanology and Geothermal Research, 2014
We present results from the inversion of gravity changes observed at the central volcanic complex (CVC) of Tenerife, Canary Islands, between May 2004 and July 2005. Marking a period of elevated activity and a reawakening of the volcanic system, the data depict spatial and temporal variations in the sub-surface processes that defined this period of unrest at the Pico Viejo (PV) -Pico Teide (PT) complex, after the last volcanic eruption on Tenerife in 1909. An initial non-linear inversion, based on 3D line segments approximation, yielded three line segments at depths between 1 km a.s.l. and 2 km b.s.l. Our interpretation of the initial inversion results is that the line segments represent apparent composite sources, a superposition of deep and shallow seated sources. We therefore decomposed the gravity changes into shallow and deep parts (fields) using a procedure based on triple harmonic continuation. The shallow and deep fields could then be inverted separately, using the same inversion methodology. The deep field constrains two connected line segments at the depth of about 6 km b.s.l., in the center of the NW seismogenic zone of VT events swarm of the seismic unrest, that we interpret as magma input. The inversion of the shallow field images three weak line segments that are all situated at very shallow, near-surface depths. We interpret the weak segments as hydrothermal sources potentially excited by the deeper magma injection. Our results indicate no significant input into the shallow phonolitic plumbing system of the PV-PT complex, but rather a deeper-seated rejuvenation of the mafic feeder reservoir. The emerging picture from our analysis is that the 2004/5 unrest on Tenerife was of a hybrid nature due to the combination of a deep magma injection (failed eruption?) coupled with fluid migration to shallow depths. The identified causative link between deep and shallow unrest sources indicates the presence of permeable pathways for shallow fluid migration at the CVC.