عنوان مقاله [English]
نویسندگان [English]چکیده [English]
We have analyzed the state of stress in Fars Province, Iran, based on systematic inversion of available focal mechanisms of the earthquakemethod of Angelier (2002).Fars Province makes up a significant portion of the âFars salientâ in the Zagros fold-thrust belt.The main purpose of this study was to evaluate the uniformity of the seismotectonic stress regimes and to investigate the dominant and possible kinetic mechanism of the faults that cause earthquakes in different parts of the study area. We analysed the data using two main methods. First, the application of the right dihedral method quickly provided a robustgraphical expression of the mechanicalcompatibility within a setof focal mechanisms. Second, a direct inversion gave an accurate quantitative account of the stress state. This method is based on consideration of the SSSC (Slip Shear Stress Component) criterion. The SSSC is the component of stress acting in the slip direction of a fault. The intrinsic characteristics of the adopted criterion results in two main technical properties of the method. First, no choice between the nodal planes is needed prior to or during the inversion. Second, the numerical aspects are reduced to a minimum so that the runtime is negligible regardless of the size of the data set. The major advantage of this method is that, after the inversion, three main estimators enable one to evaluate the mechanical consistency of a data set in terms of both the individual and the average misfit levels obtained from the best-fitting stress tensor. These estimators are the estimator of the stability of the stress regime, the estimator to determine the accuracy of the analysis level and the estimator of mechanical stability stress tensors. The focal mechanism of the earthquake data was collected from 1935 to 2013 from a variety of sources. Some of these sources were online moment tensor catalogs and other sources were extracted from the literature. Due to the very different characteristics of tectonics in Fars Province, we deemed it necessary to divide the area into zones with relatively similar stress regimes, and then follow these methods for each zone. Therefore, in view of the tectonic features and earthquake characteristics, the best linear unbiased geostatistical estimator method (Kriging) was used which provided the ability to predict under study variables in each unsampled coordinates based on a spatial correlation function. Therefore, using the spatial prediction method of Kriging, we could predict the spatial axes of the stress variables P and T to pre-identify the zones. This pre-identification of zones improves and facilitates using the method of systematic inversion of focal mechanisms. To choose the best spatial model, the Leave-one-out cross validation method was used. In addition,prediction residualswere evaluated to select the best spatial Kriging method.Evaluating and cross validation results showed that the Ordinary Kriging method presented the best spatial prediction method to pre-identify the zones. According to the Ordinary Kriging results, the five relatively distinct zones of stress characteristics in Fars Province wereidentified. The results of the systematic inversion of focal mechanisms indicated that the mean principal stress axesoptimized zones do not change significantly, and the common trend of NNE-SSW (N21E to N34E) was determined.The uniformity of the stress regime of the area(Fars Province) increased as we moved from the northwest (the range of Kazeroun shear fault zone) to the southeast and the southwest areas. The nonuniformity of the stress regimes implied heterogeneity and heterogeneous kinetic mechanisms for seismic faults and the stress distribution in the mentioned areas. The kinetic mechanism of the northwest and southeast fault zoneshavea tendency to strike slip and general Â mechanism of southwest area is revers for many faults.