Silvia Pondrelli

European–Mediterranean regional centroid-moment tensors: 1997–2000

The Mediterranean region is seismically very active and undergoing rapid deformation. Earthquakes as strong as M≥6 are present, but relatively rare. On the other hand, moderate-magnitude seismicity (4.5<M<5.5) is widespread and, often, damaging. An improved knowedge of moderate-magnitude earthquakes can contribute significantly to a better understanding of active tectonics in the region. Harvard centroid-moment tensors (CMT) today represent the most widely used reference for reliable source mechanisms, but modelling seismic sources with the CMT technique for earthquakes with magnitude less than M≈5–5.5 is quite difficult. Since 1997, we are using an extension of the standard CMT algorithm to compute regional centroid-moment tensors (RCMT) for moderate-magnitude earthquakes (MW as low as 4.0), analyzing intermediate-period surface waves from regional and teleseismic seismograms. We routinely apply the algorithm to European–Mediterranean seismicity to produce a catalog of seismic moment tensors for events with magnitude between 4.5 and 5.5, which generally are not included in the Harvard CMT catalog. In this paper, we show the results for earthquakes that occurred between 1997 and 2000, consisting of 252 solutions, and we discuss the characteristics of the catalog and the areas where these new data are most relevant.

European–Mediterranean Regional Centroid Moment Tensor catalog: Solutions for years 2003 and 2004

Abstract We present an update, for years 2003 and 2004, of the European– Mediterranean Regional Centroid Moment Tensor (RCMT) catalog that we maintain since 1997. This update adds 177 new solutions to the database, relating to moderate-magnitude ( 4.5 ≤ M 5.5 ) earthquakes. About 30% of our new solutions also appear in the global CMT catalog: comparison of the two independent determinations generally shows very good agreement. Within the time span covered, the most noticeable seismic sequences occurred in Northern Africa (Boumerdes, Algerian coast in 2003 and Al Hoceima, Morocco in 2004) and in Southern Greece and Dodecanese region (August 2003 and 2004). The complete RCMT catalog for the European and Mediterranean region now covers 8 years, and contains about 600 solutions.

SKS splitting measurements in the Apenninic‐Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection

[1]xa0We present a compilation of singular shear wave splitting measurements in the Italian region. We used the method of Silver and Chan [1991] to determine the splitting parameters of fast direction and delay time from broadband SKS waveforms. The stations analyzed were mainly deployed in temporary experiments; a few are permanent MedNet broadband stations. The splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath Italy. The fast polarization directions define two anisotropic domains: a Tyrrhenian domain with E-W prevalent fast directions and an Apennines-Adriatic domain with NW-SE to NNW-SSE prevalent fast directions. The stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths. This variability is a sign of a complex upper mantle structure (also shown by seismic tomography) but allows us to constrain the depth of the anisotropic layer in the 100–250 km range, at least in the boundary zone between the two domains. Data at station VSL (Sardinia) suggest the presence of depth-dependent anisotropic structure, where the shallower signature is consistent with the Tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670-km discontinuity. We argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding Tyrrhenian plate.

Seismotectonic re-evaluation of the 1976 Friuli, Italy, seismic sequence

The 1976 Friuli seismic sequence is known for itscomplexity, with several unusually large aftershocksoccurring in the epicentral area. The source region, where the northeastern part of the Alpine chain meetsthe northern Dinaric chain, is characterized by a complexcompressional tectonic regime. Previous studies have not clearly identified which of the two main tectonic systems was activated, in part due to the limited precision of derived earthquake parameters, such as hypocentral coordinates and focal mechanisms. We review the locations for the 10 largest events of the sequence, including theSeptember 16, 1977 and April 18, 1979 earthquakes andwe compute their centroid moment tensors. Source parameters are calculated using intermediate period surfacewaves and the modification of the Harvard centroid-moment tensoralgorithm proposed by Arvidsson and Ekström (1998). A summary of all available geological, geodetic and seismological data show that most of the earthquakes may be associated with the Periadriatic overthrust and other related thrust faults. Based on their locations and focal mechanisms only two ofthe early aftershocks (May 7, 1976 and May 9, 1976) appear instead to have occurred on Dinaric structures.

Hints on the deformation penetration induced by subductions and collision processes: Seismic anisotropy beneath the Adria region (Central Mediterranean)

Adria is a small region surrounded by three mountain belts: the Alps, the Apennines, and the Dinarides, built up by long evolution of subduction and collisional systems. We present 253 shear wave splitting measurements obtained by studying more than 100 teleseismic events for 12 stations. SKS splitting measurements show 3-D complexity and quite strong upper mantle deformation. We carefully analyzed results in terms of back azimuthal coverage and interpret measurements as related to Adria rotation and to subductions evolution. In the northern part of Adria, the anisotropy pattern follows the arcuate shape of the Alps; the same pattern, parallel to the mountains, occurs along the Apennines, but fast directions show a sudden change in the Adria foreland. This lateral variation has been analyzed to isolate a distinct Adria mantle anisotropic pattern, which is identified as NE-SW fast direction along the western microplate boundary and as N-S fast direction at Trieste. This pattern might be induced by drag effect of the counterclockwise rotation of Adria lithosphere that behaves as an independent microplate as identified by GPS data. Our measurements suggest that the geodynamic process that generated the Alps is more efficient deforming a larger volume of mantle than its Apennine counterpart. Moreover, the mantle circulation we hypothesize looking at the regional-scale patterns of anisotropy requires the existence of an escape route beneath the Alps-Apennines transition, through which the mantle flows and feed circulation in the Tyrrhenian mantle system as suggested by previous geodynamic models and as seen by some tomographic studies.

Standardization of seismic tomographic models and earthquake focal mechanisms data sets based on web technologies, visualization with keyhole markup language

We present two projects in seismology that have been ported to web technologies, which provide results in Keyhole Markup Language (KML) visualization layers. These use the Google Earth geo-browser as the flexible platform that can substitute specialized graphical tools to perform qualitative visual data analyses and comparisons. The Network of Research Infrastructures for European Seismology (NERIES) Tomographic Earth Model Repository contains data sets from over 20 models from the literature. A hierarchical structure of folders that represent the sets of depths for each model is implemented in KML, and this immediately results into an intuitive interface for users to navigate freely and to compare tomographic plots. The KML layer for the European-Mediterranean Regional Centroid-Moment Tensor Catalog displays the focal mechanism solutions or moderate-magnitude Earthquakes from 1997 to the present. Our aim in both projects was to also propose standard representations of scientific data sets. Here, the general semantic approach of an XML framework has an important impact that must be further explored, although we find the KML syntax to more emphasis on aspects of detailed visualization. We have thus used, and propose the use of, Javascript Object Notation (JSON), another semantic notation that stems from the web-development community that provides a compact, general-purpose, data-exchange format.

Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis

Teleseismic data recorded by temporary and permanent stations located in the Northern Victoria Land region are analysed in order to identify the presence and location of seismic anisotropy. We work on data recorded by 24 temporary seismographic stations deployed between 1993 and 2000 in different zones of the Northern Victoria Land, and by the permanent very broad-band station TNV located near the Italian Base “M. Zucchelli”. The temporary networks monitored an area extending from Terra Nova Bay towards the South beyond the David Glacier and up to the Indian Ocean northward. To better constrain our study, we also provide an analysis of data recorded by TNV in the same period of time and we take into account also SKS shear wave splitting measurements performed by Barruol and Hoffman (1999) on data recorded by DRV. This study, to be considered as preliminary, reveals the presence of seismic anisotropy below the study region, with a mainly NW-SE fast velocity direction below the Terra Nova Bay area and rather large delay times, that mean a deep rooted anisotropic layer.