G.F. Panza

Upper mantle properties of the Tuscan-Tyrrhenian area: a framework for its recent tectonic evolution

Abstract Within the Alpine-Mediterranean deformational system, several interesting features in the physical properties of the upper mantle have been recognized through the regional analysis of surface-wave dispersion. A large upper mantle anomaly (UMA), here called the Tuscan UMA, has been identified in the area northeast of Corsica, which includes part of the northern Tyrrhenian Sea and part of the North-Central Apennines. This anomalous upper mantle area together with the upper mantle characteristics of the Tyrrhenian Sea, located further to the south, provides a natural framework for comprehensive interpretation of the large data set of geological and geophysical observations, such as land and marine geology, volcanism, gravity, magnetic data, seismic data and seismology, deep temperatures, and surface heat flow.

Properties of the lithosphere in collisional belts in the Mediterranean—a review

Abstract A brief review of the lithosphere-asthenosphere properties is given for the Mediterranean area. The strongest lateral heterogeneities are found in the proximity of tectonically active areas, such as collisional belts. High S-wave velocities below about 70 km denote the presence of deep-seated, almost aseismic, lithospheric roots under the Western Alps, the North-Central Apennines and the Cordillera Betica. An alternative explanation of the data related to mechanisms of intermediate and deep-focus earthquakes in the Tyrrhenian Sea and Aegean Sea is discussed in the frame of the dynamic subduction model proposed by Freund et al. (1980), which postulates that the slab is disrupted in blocks and pulled down gravitationally.

Complete moment tensor retrieval for weak events: application to orogenic and volcanic areas

Abstract Aiming to study the mechanism and time history of weak local events we invert the dominant part of high-frequency seismograms (S and surface waves) by using two methods which implement moment tensor description of the focus. The point-source approximation is applied since we assume that the size of the focus with respect to the minimum wavelength of the analyzed signals is relatively small. Various constraints of the moment tensor are applied to cover local events of different origin - both the tectonic earthquakes and seismic events induced by volcanic activity. In the former case the double-couple constraint is applied, in the latter one a full moment tensor is decomposed into a volumetric part (V), representing volume changes, a compensated linear vector-dipole part (CLVD), describing opening of a fluid-filled lenticular crack, and a double couple part (DC), representing a shear slip. In the full moment tensor inversion the hypocentral depth and structural model may vary within pre-defined intervals. In the orogenic area of Friuli, Northern Italy, both the method looking for a DC only and the procedure implying the complete moment tensor arrive produce a DC mechanism, the orientation of which is consistent with the polarity readings. In the volcanic area of Phlegrean Fields, Southern Italy, the possible existence of fluid motion, which can be associated to volume changes and crack openings has to be taken into account, therefore, we used only the full moment tensor description to analyze several events in the magnitude range from 1.3 to 3. The obtained source durations vary from a few tenths of a second to about two seconds, suggesting that even small events may be characterized by relatively complex rupture history, although some of the retrieved complexities may be an artifact due to lateral inhomogeneities and other unmodelled structural effects. The percentage of the V component was found to be as large as 30% here, while it was negligible in the orogenic area of Friuli. Therefore, the retrieved V component, even though partly corrupted by imprecise structural modeling, can be used to distinguish between events with and without significant volume changes in the source. This might lead to a better understanding of the source processes in volcanic areas and to the use of source properties as possible precursors of explosive eruptions.

Hot/Cold Spots in Italian Macroseismic Data

Site effect is usually associated with local geological conditions, which increase or decrease the level of shaking compared with standard attenuation relations. We made an attempt to see in the macroseismic data of Italy some other effects, namely, hot/cold spots in the terminology of Olsen (in Bull. Seismol. Soc. Am. 90, 6B, 577–594, 2000), which are related to local fault geometry rather than to soil conditions. We give a list of towns and villages liable to amplify (+) or to reduce (–) the level of shaking in comparison with the nearby settlements. Relief and soil conditions cannot always account for the anomalous sites. Further, there are sites where both (+) and (–) effects are observed depending on the earthquake. The opposite effects can be generated by events from the same seismotectonic zone and along the same direction to the site. Anomalous sites may group themselves into clusters of different scales. All isolated anomalous patterns presented in this paper can be used in hazard analysis, in particular, for the modeling and testing of seismic effects.

Upper Mantle Structure in the Alpine Zone from Surface Wave Tomography

The present-day Alpine region began to form morethan 100 Ma ago with the subduction of a Tethyanbranch beneath the Adriatic plate, eventually arriving atthe collisional stage of the European continental litho-sphere at about 40 Ma ago. Presently, Africa is movingnorthward relative to Europe at a rate of about

A Contribution to the Assessment of the Seismic Vulnerability of Large Structures in Sofia City

An advanced modeling technique that allows us to compute realistic synthetic seismograms was used to create a database of realistic synthetic accelerograms in a set of selected sites in the Sofia urban area. The accelerograms can be used to assess the local site response in terms of the response spectra ratio (RSR). The results of this study, i.e. time histories, response spectra and other ground motion parameters, can be used for various earthquake engineering analyses. Finally, with the help of 3-D finite element modeling, a building’s structural performance is assessed.