The new release of the database of Earthquake Mechanisms of the Mediterranean Area (EMMA Version 2)

Gianfranco Vannucci (1) and Paolo Gasperini (2)

(1) Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy
(2) Dipartimento di Fisica, Università degli Studi di Bologna, Italy

We present here the second release of a database, running on MS-Access platform, of the earthquake focal mechanisms of the Mediterranean area, published in the literature. For all of the mechanisms (more than 6000) the published parameters, taken from about 190 papers, have been checked for consistency and if necessary corrected when possible. The MS-Application also allows the importing (without checking) and the use of major moment tensor catalogs available for the Mediterranean area.

The motivation of this work originates from the difficulties that any investigator, interested in seismotectonics and seismic hazard analysis of Italy and surrounding regions, encounters when collecting fault plane solutions from the literature. Italy is in fact a region with a relatively moderate seismicity, where the earthquakes above the magnitude threshold of the Harvard CMT catalog (about Mw>5.5) are only few dozens since 1976, hence the contribution given by first pulse mechanisms published in the literature is also necessary to well characterize the tectonic styles of the various seismic source areas.  The difficulties in using such data mainly come from the fact the they were very dispersed over a large number of papers as well as that, due to the papery format, they were not easily available for analyses by computer codes. Moreover, a number of mistakes can be occasionally found on some of them, going from the incorrect use of terms (i.e. strike instead of dip-direction) to inconsistencies of the reported data (i.e. non-orthogonality of fault planes and deformation axes, inconsistency between plane and axes, etc.).

Therefore, taking advantage of a call for proposals within the Framework Project 2000/2002 of  the Italian Gruppo Nazionale Difesa dai Terremoti (GNDT), our group submitted a  where one of the tasks was
Coordinate Project ("Revision of the theoretical and observational grounds of seismic hazard estimates at a national scale")concerned to the development of a reliable catalog of focal mechanisms to be used in new hazard estimates. We thus started to collect focal solutions, from the national and international literature. Initially our interest was limited to Italy and surrounding region, but later the area has been extended to include the whole Mediterranean Sea and surrounding regions, from the Atlantic Ocean to Iran. The first version of the database, published more than one year ago (Vannucci and Gasperini, 2003), was containing about 5000 mechanisms. Such version was recently used to constrain the tectonic styles of the seismogenic source areas of the new seismic zonation ZS9 (Working Group, 2004) which has been developed within the framework of the INGV initiative following the Italian Prime Minister Ordinance of March 20, 2003 for the assessment of the new seismic hazard map of Italy.
The database is embedded in a MS-Access application allowing the visual comparison between original and recomputed focal mechanisms data, the importing of the data from the on-line Global CMT Harvard catalog and from two regional RCMT catalogs (at INGV and ETH), the selection of mechanisms on the basis of various criteria as well as the exporting onto ASCII files of the data to be used in further computations.
An added value of the EMMA database is the checking for consistency of all the mechanism parameters and, in case of problems, their correction so that the mechanisms are immediately usable for drawing maps or making further computations like the ones shown in the previous article of this issue. We checked the angles between planes and between axes to not differ from 90 degrees by more than three degrees. Also, we checked the consistency between fault planes and axes and between moment tensor components and planes and/or axes. To make all of these checks as well as to homogeneously re-compute all relevant parameters (fault planes and deformation axes angles, moment tensor components) for each mechanism, we have developed a structured package of Fortran 77 subroutines (FPSPACK, Gasperini and Vannucci, 2003) performing the most common computations and checks on focal mechanism data. This package is freely available from the ftp server of Computers & Geosciences journal: and includes, among the others, routines to compute nodal planes from P and T axes and vice versa as well as to compute moment tensor components from planes or axes or best double couple parameters from moment tensor components. All definitions conventions and formulas we have used are reported in Appendix A.

To make reliable selections on the basis of the earthquake size as well as to compute the seismic moment tensor, we uniformly computed the scalar seismic moment, using empirical regressions with available magnitude estimates, for all of the mechanisms for which this parameter is not reported on the original paper.

Another feature of the EMMA database is the choice of the best mechanism among the available duplicates. We weighted each solution on the basis of a series of objective criteria based on: i) the correctness of the solution (presence or absence of errors in the published FPS parameters), ii) the originality of the source (original sources are preferred with respect to indirect ones), iii) the "authoritativeness" of the source (chosen roughly proportional to the impact factor of the journal where the mechanism is published), iv) the recentness of the publication (most recent papers override previous ones). We did not consider any kind of quality estimators (solution quality factor, number of stations, etc) as these are only given by a minority of the published solutions. The "best solution" is the one with higher weight and is marked in the database by a specific flag that can be included in selection.

References and related papers

Gasperini, P., Vannucci, G., 2003. FPSPACK: A package of simple FORTRAN subroutines to manage earthquake focal mechanism data, Computers & Geosciences, 29, 893-901.

Vannucci, G., Gasperini, P., 2003. A database of revised fault plane solutions for Italy and surrounding regions, Computers & Geosciences, 29, 903-909.

Vannucci, G. Gasperini, P. 2004, The database of Earthquake Mechanisms of the Mediterranean Area (EMMA): a call for contributions, CSEM/EMSC Newsletter, N. 21, 3-6.

Vannucci, G. Gasperini, P. 2004, The new release of the database of Earthquake Mechanisms of the Mediterranean Area (EMMA Version 2), Annals of Geophysics, Supplement to V. 47, N.1, 307-334.