Description: Bottom pressure, tilt, and seawater physical properties were monitored for a year using two instruments within the immerged Santorini caldera (Greece). Piggybacked on the CALDERA2012 cruise, this geodetic experiment was designed to monitor evolution of the 2011-2012 Santorini unrest. Conducted during a quiescent period, it allowed us to study oceanographic and atmospheric signal in our data series. We observe periodic oceanographic signals associated with tides and seiches that are likely linked to both the caldera and Cretan Basin geometries. In winter, the caldera witnesses sudden cooling events that tilt an instrument towards the Southeast, indicating cold water influx likely originating from a passage into the caldera between Thirasia island and the northern end of Thera island to the northwest. We did not obtain evidence of long-term vertical seafloor deformation from the pressure signal, although it may be masked by instrumental drift. However, tilt data suggest a local seafloor tilt event ~1/year after the end of the unrest period, which could be consistent with inflation under or near Nea Kameni. Seafloor geodetic data recorded at the bottom of the Santorini caldera illustrate that the oceanographic signature is an important part of the signal, which needs to be considered for monitoring volcanic or geological seafloor deformation in shallow water and/or nearshore areas.

Description: Highlights • Coseismic displacement documented and measured in a submarine fault following a 2004 Mw 6.3 event. • Coseismic deformation of hanging wall with cracking and fissuring of seafloor sediments. • High-resolution mapping, photomosaicing, and 3D video-derived terrain models to constrain earthquake rupture at seafloor. Abstract Properly assessing the extent and magnitude of fault ruptures associated with large earthquakes is critical for understanding fault behavior and associated hazard. Submarine faults can trigger tsunamis, whose characteristics are defined by the geometry of seafloor displacement, studied primarily through indirect observations (e.g., seismic event parameters, seismic profiles, shipboard bathymetry, coring) rather than direct ones. Using deep-sea vehicles, we identify for the first time a marker of coseismic slip on a submarine fault plane along the Roseau Fault (Lesser Antilles), and measure its vertical displacement of ∼0.9 m in situ. We also map recent fissuring and faulting of sediments on the hangingwall, along ∼3 km of rupture in close proximity to the fault's base, and document the reactivation of erosion and sedimentation within and downslope of the scarp. These deformation structures were caused by the 2004 Mw 6.3 Les Saintes earthquake, which triggered a subsequent tsunami. Their characterization informs estimates of earthquake recurrence on this fault and provides new constraints on the geometry of fault rupture, which is both shorter and displays locally larger coseismic displacements than available model predictions that lack field constraints. This methodology of detailed field observations coupled with near-bottom geophysical surveying can be readily applied to numerous submarine fault systems, and should prove useful in evaluating seismic and tsunamigenic hazard in all geodynamic contexts.

Description: A new geophysical data set off La Reunion Island (western Indian Ocean) reveals a large volcaniclastic submarine fan developing in an open-ocean setting. The fan is connected to a torrential river that floods during tropical cyclones. Sediment storage at the coast is limited, suggesting that the sediments are carried directly to the basin. The fan morphology and turbidites in cores lead us to classify it as a sand-rich system mainly fed by hyperpycnal flows. In the ancient geological record, there are many examples of thick volcaniclastic successions, but studies of modern analogues have emphasized mechanisms such as debris avalanches or direct pyroclastic flow into the sea. Because the Cilaos deep-sea fan is isolated from any continental source, it provides information on architecture and noncatastrophic processes in a volcaniclastic deep-sea fan.

Description: During the last few years, the radar images collected by the European satellites ERS1 and ERS2, the Japanese satellite JERS and the Canadian satellite RADARSAT have been used with success to create interferograms. This technique has been applied for geophysical applications like co-seismic deformation mapping, volcano deformation monitoring, landslides monitoring, mining subsidence detection, glaciers monitoring. Here we report the research carried out by our group on Etna volcano (Italy) and in the area of Naples (Italy) where are located several potentially active volcanoes (Vesuvius, Ischia) and where a subsidence of the caldera of Campi Flegrei is still on going in response to the 1982-1984 seismic crisis. Etna is the volcano that has been studied first using ERS SAR interferometry. Using this method, a large scale deflation of the volcano associated with the large 1991-1993 eruption was detected in data covering the second half of the eruption. Further studies showed that the local deformation fields located in Valle del Bove (East of the volcano) where associated with the compaction of the 1986-1987 and 1989 lava fields and also partly with a subsidence of the surrounding terrain in response to the load of the new deposited material. Other local deformation fields have been identified, corresponding to the 1983, 1981 and 1971 lava fields. However, due to its strong topography, interferograms of Etna are affected by tropospheric effects. Those effects must be eliminated in order to correctly interpret the fringes pattern. The problem of the troposphere has been first investigated from its theoretical point of view and using existing local meteorological data as well as radio-soundings data. Recently, thanks to the large amount of available interferograms, another approach has been investigated, consisting in the research of a correlation fringe/elevation in the interferograms themselves. This approach, operated either in automatic mode (automatic fringe unwrapping) or in manual mode proved to be efficient for most of the coherent interferograms. After removal of the tropospheric correction, the evolution of the deformation of the volcano at large scale between 1992 and 1998 has been inferred. The subsidence occurring during the second half of the 1992 eruption as well as the uplift preceding the 1995 unrest of the Southeast crater are visible, but their amplitude is less than previously estimated. The depth of the modelled source of subsidence/uplift related to the large scale deformation is of the order of 6 to 8 km, not well constrained by the data. The study of the correlation fringe/elevation was possible only after a detailed analysis of the spatial and temporal properties of coherence of the Etna area. Indeed, the technique of fringe unwrapping for fringe/elevation correlation analysis is possible only if the poorly coherent pixels are eliminated. A map of the most coherent pixels of the volcano was produced. The recent lava fields as well as the towns and villages surrounding the volcano are the most coherent areas. The quality of the interferograms is also enhanced when high accuracy DEM (Digital Elevation Model) are used. Using kinematic GPS data collected along more than 100 km of road around the volcano, we assessed the accuracy of several DEMs of Etna. The most accurate DEM was produced by digitising 1/25.000e maps of Etna. This DEM does not take into account the topographic changes due to the recent eruptions. Merging other more recent DEMs corresponding to those areas, we produced an updated relatively high accuracy DEM (±3 m) of Etna. In the Naples area, we analysed interferograms in the period 1993-1996 and show that the Campi Flegrei caldera is still subsiding at a rate of about 30 mm/year.

Description: Comité National Français de Géodésie et Géophysique

Description: Published

Description: 3.6. Fisica del vulcanismo

Description: open

Description: © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 5 (2015): 12152, doi:10.1038/srep12152.

Description: Natural CO2 releases from shallow marine hydrothermal vents are assumed to mix into the water column, and not accumulate into stratified seafloor pools. We present newly discovered shallow subsea pools located within the Santorini volcanic caldera of the Southern Aegean Sea, Greece, that accumulate CO2 emissions from geologic reservoirs. This type of hydrothermal seafloor pool, containing highly concentrated CO2, provides direct evidence of shallow benthic CO2 accumulations originating from sub-seafloor releases. Samples taken from within these acidic pools are devoid of calcifying organisms, and channel structures among the pools indicate gravity driven flow, suggesting that seafloor release of CO2 at this site may preferentially impact benthic ecosystems. These naturally occurring seafloor pools may provide a diagnostic indicator of incipient volcanic activity and can serve as an analog for studying CO2 leakage and benthic accumulations from subsea carbon capture and storage sites.

Description: This research was supported by the Eurofleets Caldera 2012 Project (EU), a NASA Astrobiology Science & Technology for Exploring Planets grant #NNX09AB76G (USA), a CAREER Award grant #OCE-0955674 to R. Camilli from the National Science Foundation (USA), and IPGP (France).