Tectonic Stress Indicators in the Iranian plateau by Determining the Focal Mechanism of the Recorded Earthquakes

This paper presents a study of focal mechanism of earthquakes and stress indicators for Iranian plateau and surrounding regions. All recorded earthquakes with magnitude of 4.5 and higher, occurred from 1964 to 2000, have been studied. The seismological data were taken from the International Seismological Centre (ISC) bulletin and Earthquake Data Report (EDR). More than 400 events of all earthquakes have been evaluated as good quality. Among them, 224 earthquakes are located in the Iranian plateau. Based on P-wave first arrival polarities, focal mechanisms, orientation of the principle stress axes (P and T), nodal planes, and their corresponding errors in cases where more than one solution may fit the data, have been determined. Reliability of the fault plane solutions has been evaluated by the values of the errors of P and T-axes orientations (less than 15), the minimum number of polarities (N>=30), and the score (S>=0.75). The average number of polarities for 224 earthquakes in Iranian plateau is 88. The fault plane solutions indicate that the strike-slip and oblique faulting movements characterize the majority of the earthquake focal mechanisms. 66 percent of the solutions correspond to strike-slip or oblique-slip with a strong strike-slip component faulting, while only about 18 percent of the solutions correspond to the mechanism of dip-slip faulting. Dip-slip movements are mostly reverse. This is markedly in effect of the Arabia-Eurasia convergence by strike-slip and reverses faulting in seismotectonics of Iran. The Frohlich triangle diagrams for Zagros, Alborz, the east of Iran and Makran zones were presented. There are two main alignments of the maximum compressional stress in each zone: NE-SW and NW-SE. The NE-SW direction is compatible with the main regional tectonic stress, whereas, the NW-SE direction seems to be associated with local tectonic movements. The focal mechanisms obtained in this study are in general agreement with those of Centroid Moment Tensor (CMT) solutions reported by Harvard. However, the epicenters given by CMT method are generally misplaced by tens of kilometers from the locations reported by ISC or EDR. As reported by Dziewonski et al. (1981), such a discrepancy is well expected because the location of epicenters in the Harvard CMT corresponds to the centroid of seismic moment release, rather than to the point of rupture nucleation.