An Atlas of Mediterranean seismicity

Gianfranco Vannucci (1), Silvia Pondrelli (1), Andrea Argnani (2), Andrea Morelli (1), Paolo Gasperini (3), and Enzo Boschi (1, 3)

(1) Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy
(2) ISMAR, Consiglio Nazionale delle Ricerche, Bologna, Italy
(3) Dipartimento di Fisica, Università degli Studi di Bologna, Italy

We present a description of the characteristics of the seismic deformation occurring in the Alpine Mediterranean belt, and outline its association with tectonic and geologic features. We map seismic activity using several catalogues. Hypocentral data are retrieved from the Catalog of the International Seismological Center, the most comprehensive compilation of global data. Earthquake size and source geometry are instead evaluated from catalogs of earthquake mechanisms. These include seismic moment tensor catalogs (mainly the Harvard CMT catalog and the Euro-Mediterranean regional centroid-moment tensors RCMT catalog), and a recent extensive compilation of literature solutions (EMMA), that provides consistency-controlled fault-plane solutions where mechanisms based on waveform fitting are missing.

The study area follows the Africa-Eurasia margin from the Central Atlantic to Iran, and it has been divided into several provinces for the sake of presentation and graphic purposes. For each province, a brief geologic and tectonic description complements the outline of the pattern of seismicity, illustrated by several maps made by GMT (Wessel and Smith, 1991). In places of high seismicity, focal mechanisms are grouped together to yield average mechanisms, and enable synoptic views. A rather comprehensive bibliography is referenced.

Rather than an attempt at presenting a comprehensive seismotectonic model of the Mediterranean, this contribution aims at offering a panoramic view of the active tectonics, as it is imaged by seismicity and focal mechanisms. Its scope may be seen as similar to that of an atlas, as a broad reference and a support for possibly more specific studies.

The CD-ROM contains an extensive collection of summary cards that report,for each given zone,the mechanisms resulting from the sum of all of the moment tensors (without duplications) available from the EMMA database alone, the moment tensor catalogs alone (also with duplicates removed), and from a combination of both EMMA database and moment tensor catalogs.

Summary cards give a visual and quantitative comparison of the relative contribution given by the EMMA database alone, the moment tensor catalogs, and a combination of the two kinds of data source. They also show maps of the focal mechanism, as well as some summary indicators, useful to evaluate the relative coherency of the different datasets. In particular, we indicate the total number of mechanisms available for the given area, the number of actually «used» mechanisms (i.e., belonging to a given depth interval), the average source depth and the cumulative scalar moment, computed as the scalar moment of the double couple which best fits the cumulative moment tensor (the average of the two largest moment eigenvectors in modulus).

To evaluate the reliability and coherency of the different datasets, we also compute and report on the cards some further quantities like the average depth difference, the ratios between cumulative scalar moments and the minimum rotation angle between best double couples (Kagan,1991). The latter is the measure of the misalignment between the dihedrons (couple of planes) representing the best double couples of two mechanisms. Its value ranges from 0°,corresponding to a perfect coincidence,to 120°,indicating the maximum divergence (Kagan, 1991). An acceptable agreement is represented by angles of the order of some tens of degrees,while a strong variance is given by angles larger than 50°-60°.

We compiled a summary card for each of the seismogenic zones described in the article, and for all of the non-empty rectangular cells drawn over a regular grid with a 1°×1°mesh, both in latitude and in longitude. A sensitive map allows them to be accessed by clicking on the red cell

s including mechanisms from both EMMA and catalogs (green cells contain mechanisms from EMMA only, blue cells mechanisms from online catalogs only). For the two areas with highest density of data (Italy and Greece) a further subdivision with cell size by half a degree is also provided. For Italy we make summary cards accessible for the boxes of the most recent seismogenic zonation ZS9 (ZS9 Working Group, 2004).

Geodynamic models and detailed descriptions of seismogenic areas at the scale of fault zones are outside the goals of this contribution. We instead restricted ourselves to presenting the information content of available datasets of earthquake data. These data provide a wealth of information on active tectonics, that the interested reader can easily retrieve using databases and tools from the CDROM, zooming in areas of specific interest.

References and related papers

Vannucci G., Pondrelli S., Argnani A., Morelli A. Gasperini P. and Boschi E., 2004. An Atlas of Mediterranean seismicity, Annals of Geophysics, Supplement to V. 47, N.1, 247-306.

Kagan Y.Y., 1991. 3-D rotation of double-couple earthquakes sources, Geophys. J. Int., 106, 709-716.

Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data, Eos Transactions of the AGU, 72, 441.

ZS9 working group (2004). Redazione della mappa di pericolosità sismica prevista dall'Ordinanza PCM del 20 marzo 2003. Rapporto Conclusivo per il Dipartimento della Protezione Civile, INGV, Milano-Roma, aprile 2004, appendice 2. Available at