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
V
P
≃ 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 (
V
P
< 3 kilometers per second), which might represent a melting zone.
Summary
A multi‐2‐D non linear traveltime tomography of the shallow (3–4 km deep) structure of Mt Vesuvius volcano was performed. Data have been collected during two recent active seismic experiments using a total of 17 on‐land shots and about 140 three‐component digital seismographs. A newly developed technique for imaging the volcano velocity structure has been applied, based on an adaptive model space investigation where the number of grid nodes is progressively increased (multi‐scale approach). The optimal model parametrization is chosen according to the minimum of the Akaike Information Criteria (AIC) parameter. This corresponds to finding the best compromise between the data misfit and simplicity of the model. The model parameter estimate is performed through the computation of an a posteriori probability density function (pdf), defined following the Bayesian approach. The maximum likelihood model is searched by an optimization technique which combines the genetic and simplex algorithms. The evaluation of the a posteriori pdf is based on traveltime computations using ray tracing techniques. Constraints on the model parameters are inserted in the form of prior pdf and error maps are inferred from cross‐sections of the posterior probability around the found best fit solution. The retrieved images of Mt Vesuvius volcano show variable P‐velocities in the range 1700–5800 m s−1. A fairly detailed image of the top of the Mesozoic carbonate rocks forming the basement of the volcanic area is obtained. A 9 km long, 1 km deep depression was detected at the N side of the volcano. The presence of a shallow high velocity body is evidenced underneath the Mt Somma caldera and it can be interpreted as a sub‐ or palaeovolcanic structure.
The present active faults and stress field acting in the southern Apennines (Italy), a structurally complex area characterized by high seismic potential, are studied by analyzing the background microseismicity (M <= 3). We used a microearthquake data set consisting of 1312 events that occurred from August 2005 to April 2011 by integrating the data recorded at 42 seismic stations of various networks. The refined seismicity locations and focal mechanisms delineate a system of northwest-southeast striking normal faults along the Apenninic chain and an approximately east-west oriented strike-slip fault transversely cutting the belt. The seismicity along the chain does not occur on a single fault but in a volume, delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake on subparallel predominantly normal faults. Results show that the recent low magnitude earthquakes belong to the background seismicity, and they are likely generated along the major fault segments activated during the most recent earthquakes, suggesting that they are still active today, 30 years after the main-shock occurrences. The stress inversion from the whole data set suggests that a unique anti-Apenninic extensional regional stress field could explain the two different faulting styles characterizing the earthquakes that occur along the chain and the east-west fault dissecting the belt. On the other hand, the results obtained by separately inverting the Irpinia and the Potenza clusters indicate a more complex model that would predict a change from a normal-faulting regime, acting in the inner sector of the chain, to a strike-slip regime moving eastward and down-depth in the Potenza area
We use a multidisciplinary approach to gather preliminary evidence for a Quaternary east-dipping extensional detachment in Central Italy. This structure crops out in the Sabini-Eastern Simbruini (SES) and would be hidden at mid-crustal depths beneath the L'Aquila 2009 (M w 6.3) epicentral area. The SES geometry is reconstructed through geological mapping, structural analysis and seismic line interpretation. The geometry of the mid-crustal segment, referred to as the Ocre Segment (OS), is interpreted through seismological analyses of the largest aftershock (M w 5.4) of the L'Aquila 2009 sequence. The kinematic compatibility between the SES and the OS under a common SW-NE tensional field is tested through stress inversion of both geological and seismological data. The reliability of OS activation is tested through slip tendency analysis. Like other Italian cases, the SES and the OS are preliminarily interpreted as expressions at different depths of the same unknown east-dipping extensional detachment, characterized by a ramp-flat-ramp geometry.
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