G. Biella

Seismic Evidence for a Low-Velocity Zone in the Upper Crust Beneath Mount Vesuvius

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower (P-wave velocity VP ∼ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a P to S phase converted at a depth of about 10 kilometers at the top of a low-velocity zone (VP < 3 kilometers per second), which might represent a melting zone.

Crustal shortening and duplication of the Moho in the Northern Apennines: a view from seismic refraction data

Abstract A reappraisal of the DSS refraction seismic campaigns of 1978 and 1974 in the Northern Apennines and the northern Tyrrhenian Sea, after digitization of the original analog data, and a new interpretation of crustal structures are presented in this paper. The layering of the Adria crust consists of a low-velocity lower crust topped by a 6.7 km/s horizon and an upper crust again formed by a low-velocity layer capped by a faster one. In the Tuscany sector the crust-mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the Adriatic counterpart. These differences are due to the extensional tectonics, restricted to Tuscany and occurred in a high heat flow regime related to an uplifting astenosphere. It has been confirmed that a deep thrust is responsible for Moho doubling in correspondence with the zero-Bouguer anomaly line: the Tuscan Moho overlaps the Umbro-Marchean one for a length of about 30 km. It is hypothesized that another shallower thrust involves upper crust in the Mid-Tuscany Range, Mt. Cetona and Perugia Massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks. No conclusive proof of the existence of a European Moho below a Tuscan one in the area underneath Elba has yet been found. Revision of DSS data excludes the presence of a refractor-reflector 60 km deep; the same data, however, are compatible with the existence of a crust-mantle boundary at 30–35 km depth.

An image of Mt. Vesuvius obtained by 2D seismic tomography

Abstract A high-resolution seismic tomography of Mt.Vesuvius was started in May 1994, with the aim of reconstructing the detailed shallow crustal structure underneath the volcano and define its feeding system. The first phase of the experiment was to perform a 2D profile, using three underground explosions as active sources. Data from controlled sources and microearthquakes were jointly used to determine the shallow structure of the volcano. A high-velocity body (Vp=3.5–4 km/s) was identified at about 2 km beneath the Somma-Caldera. It is likely to represent a sub-volcanic structure, formed by a dense network of solidified dikes. A prominent converted P-to-S phase at about 10 km of depth indicates the occurrence of a sharp transition to a very low-velocity zone. This may represent the top of an extended magmatic reservoir.

Velocity structure of the Vulsinian Volcanic Complex (Latium, Italy) from seismic refraction data and three‐dimensional inversion of travel times

We have interpreted three reversed seismic refraction profiles and three related fans in the Monti Vulsini Volcanic Complex, modelling travel times and amplitudes calculated using the asymptotic ray theory. The interpretation of the refraction lines revealed complex structure in the shallow crust (0–7 km), characterized by strong lateral heterogeneities. The three-layer seismic model of the volcanic area is characterized by relatively high P wave velocities, generally ranging from 4.0–4.5 km/s in the uppermost layer to 6.7–7.1 km/s at a depth of about 7 km. The upper layer (0.5–2.7 km thick) corresponds to the volcanic cover and the upper part of the flysch sequence. The depth to the top of the middle layer, corresponding to the lower part of the flysch unit and to the Meso-Cenozoic carbonate sequence, is very irregular and is strongly controlled by the tectonic evolution of the area. The thickness of this layer ranges between 2.2 and 5.0 km. The third layer extends beneath the whole Vulsinian region and is characterized by high P velocity (6.7–7.1 km/s) at relatively shallow depth (5–7 km). We interpret the high velocity observed in the third layer as being caused by mafic intrusive rocks, such as gabbros, or by high-grade metamorphic rocks, such as schists, granulates, or metatuffs. The results of the refraction modelling have been compared with a three-dimensional (3-D) P velocity model calculated by inverting travel time residuals from explosions and local earthquakes. Generally, close agreement is observed between refraction and inversion models. The fan profiles, as well as three unreversed refraction profiles and 3-D inversion, showed a velocity decrease, as low as 5%, within the deepest layer in the central part of the volcanic complex. The low-velocity zone in the central region could be related either to high temperatures and/or partial melt in the intrusive (or metamorphic) body, or to the deepening of the top of the third layer.

Deep and Shallow Solid-Earth Structures Reconstructed with Sequential Integrated Inversion (SII) of Seismic and Gravity Data

In this paper, the possibility of using simultaneously seismic and gravity data, for the reconstruction of solid-Earth structures, has been investigated through the use of an algorithm which allows joint efficient and reliable optimisation of compressional velocity and mass density parameters, We view the measured data as a realisation of a stochastic process generated by the physical parameters to be sought and we construct a “probability density function” which includes three kinds of information: information derived from gravity measurements; information derived from seismic travel time inversion and information on the physical correlation among density and velocity parameters. We show that combining data has a beneficial effect on the inversion since: it makes the problem more stable and as a consequence, providing that the quality of data is sufficiently high, enables more accurate and reliable reconstruction of the unknown parameters. In this context, we look forward the GOCE mission, which promises high spatial resolution (100–200 km) and accurate (1–2 mGals) gravity data.