GNGTS 2013 - Atti del 32° Convegno Nazionale

Seismic data. Historical seismicity catalogues, probably incomplete due to political and social conditions in the area, reveal 5 events with M >7 before 1960 (Ambraseys and Jackson, 1998; Mirzaei et al. , 1997). Seismicity is anyway spread over the width of the Zagros (Fig. 2a), but it is characterized by moderate magnitude, generally less than 5. Instrumental catalogues are devoid of earthquakes with M ≥ 7 but only 6 events with M b ≥ 6. Instrumental crustal seismicity (since 1960; M ≥ 2.5) registered on the investigated area is mainly distributed along the Zagros collisional belt, and defines elongated NW-SE-trending lineaments parallel to the fold axes (Fig. 2a). Seismicity decreases considerably toward the NW Arabia-Eurasia boundary line. In this gap, seismicity appears quite spread and earthquakes are little clustered. In the southern Iran and along Iran-Iraq southern boundary, seismic events show homogeneous behaviour in terms of magnitude and depth, which is generally confined in the first 35 km but most frequent between 10 and 25 km. In order to study the pattern of seismic deformation and obtain a detailed map of stress ori- entations later, we have compiled a database of 387 fault plane solutions (with M ≥ 3) merging data from public catalogues and literature. We collected focal mechanisms from existing on- line and literature catalogues. In particular, we use the following on-line and free available mo- ment tensor catalogues: Global Centroid Moment Tensor catalogue (Dziewonski et al. , 1981; Ekström et al. , 2012; GCMT; http://www.globalcmt.org ); European-Mediterranean Regional Centroid Moment Tensor catalogue (Pondrelli et al. , 2002, 2004, 2007, 2011; RCMT; http:// www.bo.ingv.it/RCMT/searchRCMT.html) and U.S. Geological Survey catalogue (USGS; http://earthquake.usgs.gov/earthquakes/eqarchives/sopar/ ). To extend back in time the dataset and enlarge the magnitude range, we use published first- motion FPSs collected in the “Earthquake Mechanisms of the Mediterranean Area” database (EMMA; Vannucci and Gasperini, 2003, 2004; Imprescia, 2010; Vannucci et al. , 2010 and references therein) which contains in the last version (3.1) more than 12000 first-motion polarity FPSs of earthquakes from the Mediterranean area since 1905 and published until 2007. The final dataset count ca. 300 FPSs. Focal mechanisms depict dominant reverse faulting and their nodal planes strike generally in a NW-SE direction which gradually changes toward EW approaching the south-east Persian Gulf (Fig. 2b). Nodal planes are parallel to regional structures and strikes behaviour follows folding directions of BZFTB, in accord with previous studies (e.g. Hatzfeld et al. , 2010). A significant fraction of reverse focal mechanisms are associated with hypocentral depth < 20 km, probably due to seismicity in the basement and not in the sedimentary succession (Berberian, 1995), and high dip angle, maybe related to reactivation of Mesozoic normal faulting (Jackson, 1980) inherited from the opening of the Tethys Ocean. The shortening is also accommodated by frequent strike-slip faulting processes, rupturing along the major tectonic structures. In addition, we present a preliminary inversion of the state of stress in the investigated area by taking into account our dataset of earthquake focal mechanisms. For the stress inversion method, we adopted the Spatial And Temporal Stress Inversion (SATSI) program (Hardebeck and Michael, 2006) available from the USGS webpage. The SATSI method is a damped grid- search inversion for stress tensor orientation. The damping helps to decrease the artificial noise or isolated data singularities associated with stress field orientation inversion (Hardebeck and Michael, 2006). We divided the mapped area into 1°×1° squares, requiring a minimum of 10 earthquakes in each square to be included into the inversion procedure. The smoothed results, reported in Fig. 2c, well evidence a regional trend of the maximum horizontal compressional stress (Sh MAX ) which systematically changes from a NNE-SSW attitude, along NZ and the western sector of CZ, to a NS attitude on the eastern sector of CZ, maintaining always an orthogonal orientation with respect to the collisional mountain belt. GPS data. Data acquired by continuous GPS sites installed across the Zagros collisional belt and freely availably on-line archives (i.e. SOPAC, UNAVCO, NGS) were processed by using the GAMIT/GLOBK software (Herring et al. , 2010) following the strategy reported 77 GNGTS 2013 S essione 1.1