GNGTS 2013 - Atti del 32° Convegno Nazionale

covariance information, we derived a continuous velocity gradient tensor on a regular 1° x 1° grid (whose nodes do not coincide with any of the GPS stations) using a “spline in tension” technique (Wessel and Bercovici, 1998). The tension is controlled by a factor T, where T=0 leads to a minimum curvature (natural bicubic spline), while T=1 leads to a maximum curvature, allowing for maxima and minima only at observation points; in our computations we set T=0.4. As a final step, we computed the average 2D strain-rate tensor as derivative of the velocities at the nodes of each grid cell. The estimated strain-rates are shown in Fig. 3b. As shown in Fig. 3a, available GPS data do not cover with the same density the investigated area. In particular, while the southern part (i.e. Central Zagros; CZ) shows a regular station density across the collisional belt, the northern part (i.e. North Zagros; NZ) is sampled by few data, mainly concentrated along its NE and SW borders. Beside this limitation, the geodetic velocity field clearly depicts two main features. NZ is affected by a prevailing right-lateral shear mainly concentrated along the Main Zagros Reverse Fault (MZRF): stations located NE of MZRF move toward SW with rates of ~12 mm/yr while stations located across the collision belt move toward SE with rates of ~3 mm/yr. Southward, the velocity field depicts a spectacular rotation passing from a South-directed motion (rates of ~10-13 mm/yr) NE of MZRF to a SW- ward motion across CZ (rates of ~1-3 mm/yr). The 2D strain-rate map highlights better these main features (Fig. 3b). In particular, the maximum contractional horizontal strain-rate shows a fan-shaped feature across CZ maintaining always an orthogonal orientation with respect to the curvature of the collisional mountain belt; across this area a shortening up to ~50 nanostrain/ yr can be recognized. Along NZ, the 2D strain-rate shows a complex pattern, probably due to the poor station density; on this area a general shortening up to ~25 nanostrain/yr is inferred. Conclusive remarks. Based on the data presented here and discussed in the previous section, we may draw the following conclusions: • A large amount of instrumental seismicity of the Zagros collisional belt ruptures a narrow belt located westward of MZRF. Seismicity appears mainly confined into the 10 and 25 km depth interval. • Focal mechanism solutions show prevailing high-angle reverse faulting features with NW-SE strikes, parallel to the folding and well depicting the contractional nature of the mountain belt. Preliminary results about the state of stress of the area infer a Sh MAX pattern showing trends always orthogonal with respect to the collisional mountain belt. • GPS data indicate that NZ is affected by a prevailing right-lateral shear mainly concentrated along the Main Zagros Reverse Fault (MZRF), while across CZ the velocity field depicts a spectacular rotation coupled with a decrease of the velocity values. These patterns are well recognized on the 2D strain-rate field. • A simple visual comparison of seismological stress and geodetic strain-rate directions shows that crust in the investigated area is contracting in the direction of maximum compression evidencing that, at this scale of observation, the release of elastic stress is at par with the tectonic loading of the crust. References Abdulnaby W., Mahdi H., Numan N., Al-Shukri H.; 2013: Seismotectonics of the Bitlis–Zagros Fold and Thrust Belt in Northern Iraq and Surrounding Regions from Moment Tensor Analysis. Pure and Applied Geophysics, doi:10.1007/ s00024-013-0688-4. Ambraseys N.N., Jackson J.A.; 1998: Faulting associated with historical and recent earthquakes in the Eastern Mediterranean region. Geophysical Journal International, 133, 390-406, doi:10.1046/j.1365-246X.1998.00508.x. Berberian M.; 1995: Master ‘blind’ thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. Tectonophysics, 241, 193-224. Bock Y., Wdowinski S., Fang P., Zhang J., Behr J., Genrich J., Williams S., Agnew D., Wyatt F., Johnson H., Stark K., Oral B., Hudnut K., Dinardo S., Young W., Jackson D., Gurtner W.; 1997: Southern California Permanent GPS Geodetic Array: Continuous measurements of crustal deformation between the 1992 Landers and 1994 Northridge earthquakes. J. Geophys. Res., 102, 18,013-18,033. Dewey J.F., Pitman W.C., Ryan W.B.F. and Bonnin J.; 1973: Plate tectonics and evolution of the alpine system. Bull. Geol. Soc. Am., 84, 3137-3180. 80 GNGTS 2013 S essione 1.1