Seismic Imaging of Volcanic Structures

Seismic Imaging of Volcanic Structures

Reference Projects

            TOMOVES (The Internal Structure of Mt.Vesuvius)

            SERAPIS (Seismic Reflection Acquisition Project in Campi Flegrei Caldera)

Research Objectives

         Massive use of active seismics for imaging the volcanic structure

         Integrate the seismic information with other geophysical data (gravity, magnetics, well,..)

         Develop and apply new tools for seismic imaging complex geological media

         Determine the 3-D structure of the volcano and define its feeding system at depth

Brief description

The most important volcanic structures inside the Bay of Naples (Southern Italy) are the island of Ischia, and Campi Flegrei and the Vesuvius volcano. All of the volcanoes have been dated to the late Quaternary, with Ischia island being the oldest volcanic complex in the area. The city of Naples is at the center of a much larger, very densely inhabited area that includes Campi Flegrei, the Vesuvius area and the northern Sorrentine Peninsula. A large part of the population is exposed to potential volcanic events in Campi Flegrei and in the Vesuvius area, and this thus makes the understanding of the volcanic system a crucial element for risk management in this area.

Previous researches on Mt. Vesuvius and Campi Flegrei volcanoes were directed to improve the knowledge of eruptive history, eruptive and depositional mechanisms and compositional characteristics of the feeding magma. The internal structure of both volcanoes was poorly known. A physical model of the crustal structure was needed for several reasons. The volcanic activity is rooted deeply in the Earth’s mantle, but the typology and dynamical evolution of the eruptions are determined by the size and depth of intracrustal reservoirs and conduits. A 3-D model of the shallow structure of the volcano would allow a better definition of the characteristics of the precursors of eruptions, through a substantial improvement of the accuracy of location of seismic and ground deformation sources.

The Italian Civil Defence and GNV decided to promote a project where the inner structure of Mt. Vesuvius was investigated by seismic methods, mostly based on active energy sources, because these methods  warrant the best resolution on a wide depth range. Although the emphasis was on the application of tomographic methods, particularly down to a depth of 3-4 (hence the acronym Tomo Ves), the project was more than this. It used first arrivals as well as delayed phases to identify major discontinuities using refracted and reflected waves and wave amplitudes for attenuation studies, and it verified its results with other geophysical data.

            The implementation of an acquisition lay out, fit to reach the planned resolution, in an area with a population of about 700,000 was a major effort. Three active seismic experiments have been carried out in 1994,1996 and 1997 where a largen number of sources and receivers have been deployed along the volcanic area and at sea. It was possible thanks to the joint collaboration of 25 different scientific institutions (most of them were from Italy and France, with groups also from Switzerland, Germany and U.S.A.) and to the support given by the Italian Civil Defence, by the Italian Army and several public authorities and public companies.

            The result was the acquisition of a huge amount of data. Its analysis, processing and interpretation constituted an “ad hoc” project, which was submitted to and approved by the European Commission. Its relevance for the European Commission policy of protection of people and territory from natural risks was one of the reasons moving the European Union to support it in the context of the 4th Framework Research Programme.

The project had many outcomes, including 3D models of the volcanic edifice and the underlying carbonate basement, the evidence of a wide horizontal mid crustal magma sill underneath Mt.Vesuvius, and the development of several innovative techniques of arrival time inversion to infer the properties of the propagating medium.

Few years later a new project was starter aimed at a high resolution imaging of the shallow and deep structure of Campi Flegrei, a huge, active caldera which has recently interested…

 

 

Seismic Evidence of an Extended Magmatic Sill Under Mt. Vesuvius

 

Mt. Vesuvius is a strato volcano near a densely populated area. It is located in a tectonic graben formed in the Plio-Pleistocene, and it is only a few km southeast of Fields, the active volcano on which the city of Napoli has been built (Fig.1). It experienced at least three violent explosive eruptions in historical times (79, 472, and 1631 A.D.). More frequent, less explosive eruptions have occurred from 1631 to 1944 (1). Mt.Vesuvius is presently in a quiescent state, characterized by low-temperature fumaroles (less than 100°C) and moderate seismic activity (about 100 earthquakes per year with magnitudes between 0.5 and 3.6), and it is difficult to predict when it may erupt explosively again. The definition of its structure and of the location and volume of the magma reservoir can be used to help prediction of the scenario of the next eruption and to interpret the pattern of the expected precursory seismic activity and ground deformation. The present volcanic edifice was built in a time span of about 40,000 years, and the total amount of erupted magma can be estimated to be about 50 km3. This would be the minimum volume of the magma reservoir if it was a closed system. However, Sr,Nd, Pb, and U-Th-Ra data indicate that the magma system underwent a complex, multistage evolution, which is not compatible with a closed magma reservoir (27).

(http://people.na.infn.it/~zollo/articoli/SCIENCE_2001/science.pdf)

 

 

 

 

 

 

 

 

Evidence for the buried rim of Campi Flegrei caldera from 3-d active seismic imaging

 

An extended marine, active seismic survey has been performed on September, 2001 in the gulfs of Naples and Pozzuoli by recording about 5000 shots at a network of 62 sea bottom and 72 on shore seismographs. 3-D images of the shallow caldera structure are obtained from the tomographic  inversion of about 77000 first P arrival times using the Benz et al. [1996] tomographic technique. The buried rim of the Campi Flegrei caldera is clearly detected at about 800–2000 m depth, as an anular high P-velocity and high density body. It has a diameter of about 8–12 km and a height of 1–2 km. According to stratigraphic and sonic log data from deep boreholes and tomographic P velocities, the rim is likely formed by solidified lavas and/or tuffs with interbedded lava. This study confirms the existence for a depressed limestone basement beneath the caldera at less than 4 km depth, while no evidence are found for shallower magmatic bodies.

(http://people.na.infn.it/~zollo/articoli/GRL_2003/2003GL018173.pdf)

 

 

 

 

 

 

 

The deep structure of the Larderello-Travale geothermal field from 3D microearthquake traveltime tomography

 

With the aim of exploring the deep structure of the Larderello-Travale (LT) geothermal field, a high resolution 3-D tomographic inversion of microearthquake traveltimes has been performed. Results show that the deep part of the Larderello-Travale field is characterized by the presence of a structure having a velocity range of 6.0–6.5 km/s and a convex shape deepening towards the northeastern and the southeastern sides of the field. Earthquakes are mostly concentrated on the top of the high velocity structure and below the ‘K horizon’ implying a transition of rheological properties at depth. The reported dependence on time of ts-tp observed at one station located above an earthquake cluster suggests that the variation in pore fluid pressure might be responsible for the transition of rheological properties along the contact. In such an area, changes in pore fluid pressure might be related to time-dependent hydraulic mechanisms

that are very effective in crustal rocks at elevated temperatures.

(http://people.na.infn.it/~zollo/articoli/GRL_2004/2004GL019432.pdf)

 

 

 

 

 

 

 

 

 

The Bay of Naples (southern Italy): Constraints on the volcanic structures inferred from a dense seismic survey

 

The bays of Naples and Pozzuoli host volcanic areas that have been active within recent history. In addition, the vicinities areas around the Vesuvius volcano and the Campi Flegrei caldera are extremely densely inhabited. Over the past decade, this situation has prompted several active seismic experiments focused on the Vesuvius volcano. More recently, the Serapis active seismic survey covered the entire Bay of Naples, with particular attention to Pozzuoli Bay. The processed Serapis P wave travel time collection, extended with a previously acquired data set, has allowed the computation of the most comprehensive three-dimensional P wave velocity distribution for the Bay of Naples, which includes a small-scale high-resolution model of the Campi Flegrei caldera. The joint interpretation of the velocity distribution together with both the available gravity measurements and models and the geothermal drilling information has allowed us to locate and map lithological units at depths. We propose a model of the Campi Flegrei caldera in relation to the underlying Mesozoic limestone unit. In this model, no magma chamber exists in the upper 6 km beneath the Bay of Naples. The feeding system of the Campi Flegrei caldera is directly controlled by the tectonic structures affecting the limestone unit beneath the Bay of Naples. Two NE-SW normal faults run beneath the Vesuvius volcano and Campi Flegrei, which are related to the pre-Pleistocene Apenninic tectonics.

(http://people.na.infn.it/~zollo/articoli/JGR_2004_a/2003JB002876.pdf)