Département de Sismologie, IPGP, T14-24, Paris, France

A multidisciplinary project for the investigation of Mt. Vesuvius Structure was started in 1993. The core of the project is represented by a high resolution seismic tomography study by using controlled and natura1 sources. The main research objective is to investigate the feeding system of the vo1cano and to retrieve details of the upper crustal structure in the area. A first 2D using seismic experiment was performed in May 1994, with the aim of studing the feasibility of lIsing tomographic techniques for exploring the vo1cano interiors. Particularly, this experiment was designed to obtain information on the optimal sources-receivers configuration and on the depth extension of the volume sampled by shot-generated seismic waves. 66 three-component seismic stations and 16 single-component analogue instruments were installed by several Italian and French groups to record signals generated by three on-land, underground explosions. Sources and geophones were deployed along a 30-km NW-SE profile passing through the volcano crater. Receivers were placed at an average spacing of 250 m in the middle of the recording line and at 500 m outside. The arrival time data base was complemented by first P and S readings of micro earthquakes which occurred in the recent past within the volcano. The first arrival data set was preliminary used to determine the shallow structure of the volcano by applying Thurber's (1983) tomographic inversion technique. This analysis shows evidence for a high-velocity body which extends vertically from about 400 m below the crater down to at least 3000 m and for a shallow 300-500 m thick low-velocity cover which borders the edifice. Data from the distant shot show evidence for arrivals of deep reflected/converted phases and provide information on the deeper structure under the volcano. The results from the interpretation of 2D data are used for planning a 3D tomographic survey which will be cauied out in 1996.

At a local scale, travel-time tomography requires a simultaneous inversion of earthquake positions and velocity structure. We applied a joint iterative inversion scheme where medium parameters and hypocenter parameters were inverted simultaneously. At each step of the inversion, rays between hypocenters and stations were traced, new partial derivatives of travel-time were estimated and scaling between parameters was performed as well. The large sparse linear system modified by the scaling was solved by the LSQR method at each iteration. We compared performances of two different forward techniques. Our first approach was a fast ray tracing based on a paraxial method to solve the two-point boundary value problem. The rays connect sources and stations in a velocity structure described by a 3D B-spline interpolation over a regular grid. The second approach is the finite-difference solution of the eikonal equation with a 3D linear interpolation over a regular grid. The partial derivatives are estimated differently depending on the interpolation method. The reconstructed images are sensitive to the spatial variation of the partial derivatives shown by synthetic examples. We aldo found that a scaling between velocity and hypocenter parameters involved in the linear system to be solved is important in recovering accurate amplitudes of anomalies. This scaling was estimated to be five through synthetic examples with the real configuration of stations and sources. We also found it necessary to scale Pand S velocities in order to recover better amplitudes of S velocity anomaly. The crustal velocity structure of a 50X50X20 km domain near Patras in the Gulf of Corinth (Greece) was recovered using microearthquake data. These data were recorded during a field experiment in 1991 where a dense network of 60 digital stations was deployed. These microearthquakes were widely distributed under the Gulf of Corinth and enabled us to perform a reliable tomography of first arrival P and S travel-times. The obtained images of this seismically active zone show a south/north asymmetry in agreement with the tectonic context. The transition to high velocity lies between 6 km and 9 km indicating a very thin crust related to the active extension regime.At a local scale, travel-time tomography requires a simultaneous inversion of earthquake positions and velocity structure. We applied a joint iterative inversion scheme where medium parameters and hypocenter parameters were inverted simultaneously. At each step of the inversion, rays between hypocenters and stations were traced, new partial derivatives of travel-time were estimated and scaling between parameters was performed as well. The large sparse linear system modified by the scaling was solved by the LSQR method at each iteration. We compared performances of two different forward techniques. Our first approach was a fast ray tracing based on a paraxial method to solve the two-point boundary value problem. The rays connect sources and stations in a velocity structure described by a 3D B-spline interpolation over a regular grid. The second approach is the finite-difference solution of the eikonal equation with a 3D linear interpolation over a regular grid. The partial derivatives are estimated differently depending on the interpolation method. The reconstructed images are sensitive to the spatial variation of the partial derivatives shown by synthetic examples. We aldo found that a scaling between velocity and hypocenter parameters involved in the linear system to be solved is important in recovering accurate amplitudes of anomalies. This scaling was estimated to be five through synthetic examples with the real configuration of stations and sources. We also found it necessary to scale Pand S velocities in order to recover better amplitudes of S velocity anomaly. The crustal velocity structure of a 50X50X20 km domain near Patras in the Gulf of Corinth (Greece) was recovered using microearthquake data. These data were recorded during a field experiment in 1991 where a dense network of 60 digital stations was deployed. These microearthquakes were widely distributed under the Gulf of Corinth and enabled us to perform a reliable tomography of first arrival P and S travel-times. The obtained images of this seismically active zone show a south/north asymmetry in agreement with the tectonic context. The transition to high velocity lies between 6 km and 9 km indicating a very thin crust related to the active extension regime.