Raffaella De Matteis

1-D P-velocity Models of Mt. Vesuvius Volcano from the Inversion of TomoVes96 First Arrival Time Data

Abstract—We applied a revised version of the 1-D τ–p inversion method to first P-arrival times from the active seismic experiment performed at Mt. Vesuvius (southern Italy) in 1996 (TomoVes96 Project). The main objective of this work is to obtain 1-D velocity models of Mt. Somma-Vesuvius volcano complex and surrounding area. Moreover we show that combining the 1-D information we provide a reliable 2-D initial model for perturbative tomographic inversions. Seismic and geological surveys suggest the presence of a refractor associated with the contrast between carbonate basement and volcanic/alluvial sediments; synthetic simulations, using a realistic topography and carbonate top morphology, allowed us to study the effect of topography on the retrieved velocity models and to check that the 1-D τ–p method can also approximately retrieve the refractor depth and velocity contrast. We analysed data from 14 on-land shots recorded at stations deployed along the in-profile direction. We grouped the obtained models in three subsets according to the geology of the sampling area: Models for carbonate outcrop area, models for the Campanian Plain surrounding the volcano edifice and models for Mt. Somma-Vesuvius volcano complex. The found 1-D P-velocity models show important vertical and lateral variations. Very low velocities (1.5–2.5 km/s) are observed in the upper 200–500 m thick shallow layer. At greater depths (3 km is the maximum investigated depth) P velocities increase to values in the range of 4–6 km/s which are related to the presence of the carbonatic basement. Finally we interpolated the 1-D models to demonstrate an example of misfit for a 2-D interpolated model whose residuals are confined in a narrow band around zero.

P‐wave arrival time inversion by using the τ‐p method: Application to the Mt. Vesuvius Volcano, southern Italy

In this paper we have applied a r-p in- version method based on a non linear approach to a set of first P-arrival times from a 2-D active seismic exper- iment performed at Mr. Vesuvius in 1994. The initial seismic investigation is devoted to the study of the shal- low structure and the r-p method has been used for modelling velocity variations of a spherically concentric medium which is a better approximation than a flat layer of the Vesuvius geometry. We are able to obtain detailed 1-D velocity models along different specified sections of the recorded seismic line. These sections span different depth ranges and indicate strong lateral and vertical P-velocity variations in the first 1 km of the volcano structure with values ranging from about 1.5 km/s at the surface to 3.5-4.0 km/s at depth. The average 1-D concentric model obtained in this study can be used as a reasonable starting model for itera- tive high-resolution tomographies of Vesuvius velocity structures.

The structural setting of the Ischia Island Caldera (Italy): first evidence from seismic and gravity data

Ischia Island is one of the active volcanoes of the Neapolitan area (Italy). Hazard assessment of active, densely populated volcano is primarily based on knowledge of the volcano’s past behaviour and of its present state. As a contribution to the definition of the present structural setting of Ischia Island, we constructed a new model of the shallow crust using geophysical data: seismic wave travel times and Bouguer anomaly data. We analysed these data sets through seismic tomography and gravity data inversion. The main results inferable from the 3D seismic and gravity images are the definition of the caldera rim along the perimeter of the island, as hypothesized by many authors, and the presence of a high velocity and density area inside the caldera consistent with extension of the resurgent block that characterizes the recent deformation of the island.

A Strong Motion Attenuation Relation for Early-warning Application in the Campania Region (Southern Apennines)

For early-warning applications in particular, the reliability and efficiency of rapid scenario generation strongly depend on the availability of reliable strong ground-motion prediction tools. If shake maps are used to represent patterns of potential damage as a consequence of large earthquakes, attenuation relations are used as a tool for predicting peak ground-motion parameters and intensities. One of the limitations in the use of attenuation relations is that these have only rarely been retrieved from data collected in the same tectonic environment in which the prediction has to be performed. As a consequence, strong ground motion can result in underestimations or overestimations with respect to the recorded data. This also holds for Italy, and in particular for the Southern Apennines, due to limitations in the available databases, both in terms of distances and magnitude. Moreover, for “real-time” early-warning applications, it is important to have attenuation models for which the parameters can be easily upgraded when new data are collected, whether this has to be done during the earthquake rupture occurrence or in the post-event, when all the strong motion waveforms are available.

BISTROP: Bayesian Inversion of Spectral‐Level Ratios and P‐Wave Polarities for Focal Mechanism Determination

The analysis of earthquake focal mechanisms provides information about the stress regime, fault geometry, and deformation processes acting in a given region. Generally, the techniques aimed at determining focal mechanism are designed to work in a specific magnitude range operating both in the time and frequency domain and using different data (e.g., P polarities, S ‐wave polarization, S / P ‐amplitude ratios, etc.). In this article, we present a new method, Bayesian inversion of spectral‐level ratios and P ‐wave polarities (BISTROP), that can be applied to both small and moderate‐to‐large magnitude events. BISTROP uses a Bayesian approach to jointly invert the long‐period spectral‐level P / S ratios and the P polarities to infer the fault‐plane solutions. We apply this method to analyze synthetic data as well as those generated by real earthquakes. We find that the obtained solutions for moderate earthquakes are comparable with those obtained using moment tensor inversion, and they are more constrained with respect to the solutions obtained using only P ‐polarity data for small earthquakes.

Rupture characterization of a low magnitude earthquake of central Apennines (Italy)

Abstract This paper describes a detailed study of the rupture mechanism of a M L = 3 aftershock of the 1984 Abruzzo (Central Italy) earthquake ( M s = 5.8). To study the rupture extension, shape and directivity, the time durations of P and S far-field pulses were analyzed in different directions corresponding to six recording stations. The path corrected far-field displacement P and S pulses at six seismic stations were determined using the empirical Green function method. The fault plane solution has been estimated by joint inversion of P polarities and S polarizations measured on three component records. Finally, we used a method based on the plot of ‘isochrons’ associated with the observed pulse widths and projected on each nodal plane. The comparison of isochron plots for the two nodal planes suggests a rupture fault plane striking along the Apenninic direction. A weak source directivity up and west-ward on the fault plane was observed. A maximum ruptured surface of 0.4 km 2 was estimated assuming a sub-shear rupture process. The estimate of the seismic moment from spectral measurements gives the static stress drop ranging from 1 to 66 bar for the rupture velocity ranging between 0.2 β and β, where β is the shear wave velocity at the source.