Three-dimensional resistivity tomography of Vulcan's forge, Vulcano Island, southern Italy

International audience9,525 DC resistivity measurements were taken along 9 profiles crossing the volcanic edifice of La Fossa di Vulcano (the forge of God Vulcan in ancient Roman mythology), Vulcano Island (Italy) using a total of 958 electrode locations. This unique data set has been inverted in 3D by minimizing the L2 norm of the data misfit using a Gauss-Newton approach. The true 3D inversion was performed using parallel processing on an unstructured tetrahedral mesh containing 75,549 finite-element nodes and 398,208 elements to accurately model the topography of the volcanic edifice. The 3D tomogram shows a very conductive body (>0.1 S/m) comprised inside the Pietre Cotte crater with conductive volumes that are consistent with the position of temperature and CO2 anomalies at the ground surface. This conductive body is interpreted as the main hydrothermal body. It is overlaid by a resistive and cold cap in the bottom of the crater. The position of the conductive body is consistent with the deformation source responsible for the observed 1990-1996 deflation of the volcano associated with a decrease of hydrothermal activity

Structure of an active volcano associated with a resurgent block inferred from thermal mapping: The Yasur–Yenkahe volcanic complex (Vanuatu)

International audienceSubsurface thermal measurements provide a valuable tool to map hydrothermal-fluid release zones in active volcanic areas. On the Yasur–Yenkahe volcanic complex (Tanna Island, Vanuatu archipelago), fumaroles and hot springs abound, signs of upraising heat fluxes associated to a well-developed hydrothermal activity. Combination of high resolution mapping of ground thermal anomalies with geomorphological analysis allows the characterization of the structural relationships between the active Yasur volcano and the Yenkahe resurgent block.A complex system of heat release and hydrothermal fluid circulation below the Yasur–Yenkahe complex is evidenced. Circulation, though propagating vertically as a whole, is funneled by stratification. Thus, the main thermal fluid released is almost exclusively concentrated along structural limits that break the seals induced by the stratified nature of the ground. Three types of medium/high temperature anomalies have been evidenced: (1) broad hydrothermalized areas linked with planar stratification that favor lateral spreading, (2) linear segments that represent active faults, and (3) arcuate segments related to paleo-crater rims. The limit between the Yasur volcano and the Yenkahe resurgent block is characterized by an active fault system accommodating both the rapid uplift of the Yenkahe block and the overloading induced by the volcano weight. In such a setting, faults converge below the cone of Yasur, which acts as a focus for the faults. Evidence of such structures, sometimes hidden in the landscape but detected by thermal measurements, is critical for risk assessment of flank landslides

Blast waves from violent explosive activity at Yasur Volcano, Vanuatu

Infrasonic and seismic waveforms were collected during violent strombolian activity at Yasur Volcano (Vanuatu). Averaging similar to 3000 seismic events showed stable waveforms, evidencing a low-frequency (0.1-0.3Hz) signal preceding similar to 5-6s the explosion. Infrasonic waveforms were mostly asymmetric with a sharp compressive (5-106Pa) onset, followed by a small long-lasting rarefaction phase. Regardless of the pressure amplitude, the ratio between the positive and negative phases was constant. These waveform characteristics closely resembled blast waves. Infrared imagery showed an apparent cold spherical front similar to 20 m thick, which moved between 342 and 405m/s before the explosive hot gas/fragments cloud. We interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front. We suggest that violent strombolian activity at Yasur was driven by supersonic dynamics with gas expanding at 1.1 Mach number inside the conduit

Small-scale spatial variability of soil CO2 flux: Implication for monitoring strategy

In recent decades, soil CO2 flux measurements have been often used in both volcanic and seismically active areas to investigate the interconnections between temporal and spatial anomalies in degassing and telluric activities. In this study, we focus on a narrow degassing area of the Piton de la Fournaise volcano, that has been chosen for its proximity and link with the frequently active volcanic area. Our aim is to constrain the degassing in this narrow area and identify the potential processes involved in both spatial and temporal soil CO2 variations in order to provide an enhanced monitoring strategy for soil CO2 flux. We performed a geophysical survey (self-potential measurements: SP; electrical resistivity tomography: ERT) to provide a high-resolution description of the subsurface. We identified one main SP negative anomaly dividing the area in two zones. Based on these results, we set ten control points, from the site of the main SP negative anomaly up to 230 m away, where soil CO2 fluxes were weekly measured during one year of intense eruptive activity at Piton de la Fournaise. Our findings show that lateral and vertical soil heterogeneities and structures exert a strong control on the degassing pattern. We find that temporal soil CO2 flux series at control points close to the main SP negative anomaly better record variations linked to the volcanic activity. We also show that the synchronicity between the increase of soil CO2 flux and deep seismicity can be best explained by a pulsed process pushing out the CO2 already stored and fractionated in the system. Importantly, our findings show that low soil CO2 fluxes and low carbon isotopic signature are able to track variations of volcanic activity in the same way as high fluxes and high carbon isotopic signature do. This result gives important insights in terms of monitoring strategy of volcanic and seismotectonic areas in geodynamics contexts characterized by difficult environmental operational conditions as commonly met in tropical areaPublished13-264A. Oceanografia e climaJCR Journa

Estudio del sistema hidrotermal y estructural del volcán Ticsani mediante el método de potencial espontáneo

Este trabajo presenta una investigación llevada a cabo en el volcán Ticsani ubicado en el segmento norte de la Zona Volcánica de los Andes Centrales (70 ° 36'O, 16 ° 44'S, 5408 m). El objetivo de este trabajo es estudiar la estructura del volcán Ticsani, por el intermedio del método de Potencial Espontaneo (SP). Se realizaron 5850 medidas en todo el complejo volcánico del Ticsani. En base a estos datos, se ha identificado 2 zonas de mayor interés: (1) una transición entre las anomalías de la zona Sur y Norte, relacionada con una estructural importante de colapso lateral del complejo Norte del volcán hacia el oeste y (2) una zona de subida preferencial de fluidos hidrotermales a lo largo de una estructural de forma cuasi elíptica, relacionable a una posible caldera en el área de la cumbre. La originalidad de este estudio ha sido de evidenciar que no existe una anomalía positiva en la cumbre del volcán Ticsani (como identificado de forma “clásica” en otros volcanes del Perú; como El Misti o el Ubinas), pero tener una anomalía positiva que está bordeando la parte de la cumbre. Este resultado ha permitido mostrando que el complejo de domos que constituye el volcán Ticsani, tiene una estructura interna que drena los fluidos hidrotermales de forma más compleja que otros estratovolcanes como El Misti o el Ubinas. La localización de estas anomalías positivas de SP podrán ser utilizadas en el futuro para colocar instrumentación de monitoreo del sistema hidrotermal del volcán Ticsani

Evidences of volcanic unrest on high-temperature fumaroles by satellite thermal monitoring: The case of Santa Ana volcano, El Salvador

International audienceOn October 1st, 2005, Santa Ana volcano (El Salvador) underwent a VEI 3 phreatomagmatic eruption after approximately one century of rest. Casualties and damages to some of the local infrastructures and surrounding plantations were followed by the evacuation of the nearby communities. The analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) infrared data reveals that the main explosion was preceded by a one-year-long thermal unrest, associated to the development of a fumaroles field, located at the western rim of the summit crater lake. By combining space-based thermal flux and ground-based measurements (seismicity, sulfur emissions and lake temperatures), we suggest that the activity observed at Santa Ana between 2004 and 2005 was driven by the gradual intrusion of an undegassed magma body at a very shallow depth. Magma injection induced thermal anomalies associated with sustained degassing from the fumaroles field and promoted the interaction between the magmatic-hydrothermal system and the overlying water table. This process culminated into the VEI 3 phreatomagmatic eruption of October 2005 that strongly modified the shallow structure of the crater area. The subsequent three-years-long activity resulted from self-sealing of the fracture system and by the opening of a new fracture network directly connecting the deeper hydrothermal system with the crater lake. Our results show that satellite-based thermal data allow us to detect the expansion of the high-temperature fumarolic field. This may precede an explosive eruption and/or a lava dome extrusion. In particular, we show that thermal records can be analyzed with other geochemical (i.e. SO2 emissions) and geophysical (seismicity) data to track a shallow magmatic intrusion interacting with the surrounding hydrothermal system. This provides a remarkable support for volcano monitoring and eruption forecasting, particularly in remote areas where permanent ground data acquisition is hazardous, expensive and difficult

Estudio estructural y del sistema hidrotermal de los volcanes Sabancaya y Hualca-Hualca mediante el método de Potencial Espontáneo

El volcán Sabancaya, considerado el segundo volcán más activo del Perú forma parte del complejo Volcánico Ampato-Sabancaya (CVAS), está ubicado a 80 Km en dirección NNO de la ciudad de Arequipa (15°47’ S; 71°72’W; 5976 msnm) en el sur del Perú. El presente estudio tiene como finalidad determinar estructuras importantes que se encuentran ocultas por material volcánico y el efecto que generan estas estructuras sobre la señal del Potencial Espontaneo (PE); además, estudiar el sistema hidrotermal del volcán Sabancaya, aplicando uno de los métodos geofísicos más antiguos y conocidos, pero poco usado en la vulcanología, como es el PE. La aplicación de este método nos ha permitido conocer la estructura interna del área del CVAS y volcán Hualca-Hualca, así como determinar las dimensiones del sistema hidrotermal

Asymmetrical structure, hydrothermal system and edifice stability: The case of Ubinas volcano, Peru, revealed by geophysical surveys

Ubinas volcano, the historically most active volcano in Peru straddles a low-relief high plateau and the flank of a steep valley. A multidisciplinary geophysical study has been performed to investigate the internal structure and the fluids flow within the edifice. We conducted 10 self-potential (SP) radial (from summit to base) profiles, 15 audio magnetotelluric (AMT) soundings on the west flank and a detailed survey of SP and soil temperature measurements on the summit caldera floor. The typical “V” shape of the SP radial profiles has been interpreted as the result of a hydrothermal zone superimposed on a hydrogeological zone in the upper parts of the edifice, and depicts a sub-circular SP positive anomaly, about 6 km in diameter. The latter is centred on the summit, and is characterised by a larger extension on the western flank located on the low-relief high plateau. The AMT resistivity model shows the presence of a conductive body beneath the summit at a depth comparable to that of the bottom of the inner south crater in the present-day caldera, where intense hydrothermal manifestations occur. The lack of SP and temperature anomalies on the present caldera floor suggests a self-sealed hydrothermal system, where the inner south crater acts as a pressure release valve. Although no resistivity data exists on the eastern flank, we presume, based on the asymmetry of the basement topography, and the amplitude of SP anomalies on the east flank, which are approximately five fold that on the west flank, that gravitational flow of hydrothermal fluids may occur towards the deep valley of Ubinas. This hypothesis, supported by the presence of hot springs and faults on the eastern foot of the edifice, reinforces the idea that a large part of the southeast flank of the Ubinas volcano may be altered by hydrothermal activity and will tend to be less stable. One of the major findings that stems from this study is that the slope of the basement on which a volcano has grown plays a major role in the geometry of the hydrothermal systems. Another case of asymmetrical composite cone edifice, built on a steep topography, is observed on El Misti volcano (situated 70 km west of Ubinas), which exhibits a similar SP pattern. These types of edifices have a high potential of spreading and sliding along the slope owing to the thicker accumulation of low cohesion and hydrothermally altered volcanic products

Estudio estructural del volcán Ticsani-sur del Perú, mediante potencial espontáneo

El volcán Ticsani (329700 O, 8146400 S, 5415 msnm) está ubicado a 60 km al NE de la ciudad de Moquegua. Este volcán está emplazado en la Zona Volcánica Central (ZVC) de los Andes (De Silva y Francis, 1991), y geopolíticamente pertenece a la Provincia General Sánchez Cerro, Región de Moquegua. En este volcán, se ha efectuado un estudio geofísico basado en mediciones de Potencial Espontáneo (PE) con el objeto de ubicar el sistema hidrotermal e identificar fallas no visibles en superficie