Zoning of Iran region based on changes in seismicity coefficient ratio (a/b)

 
1- Introduction
The geological, structural, and seismic characteristics are not the same in various regions in Iran. The difference in the magnitude and frequency of seismic events in this area indicates such variations. Different researchers have provided various studies and maps of the geologic, tectonic, and seismotectonic situation in this area over the past years (Stocklin, 1968; Berberian, 1981; Alavi, 1991; Alavi Naini,1972; Nowroozi, 1976; Berberian,1976; Nogol Sadat, 1993; Mirzaei et al., 1998; Tavakoli et al., 1999; Zare and Memarian, 2000; Ansari et al., 2009). The similarity of such maps indicates the close correlation of the geologic, tectonic, seismotectonic situation in this area.
Among the seismic parameters, the seismicity coefficients such as a and b in the Gutenberg-Richter (1954) equation are the most important parameters in determining the seismic pattern as well as tectonic and geologic characteristics of different regions. Determining these coefficients is necessary in the studies on the risk analysis and earthquake hazard zoning, and the value of these coefficients are assumed to be constant in the seismotectonic states. Therefore, providing a zoning map based on these coefficients will play an important role in better identification of the seismic characteristics of the region and applied studies. Providing a zoning map of the seismic coefficients of b-Value and a-Value and the ratio a/b- Value for Iran region is the main objective of this research.
In order to calculate these coefficients and provide zoning maps, initially a complete catalogue of earthquakes occurred from 1900 to 2007 in Iran region has been prepared. Then, the entire Iran region is divided into very small and regular zones, and the seismicity coefficients (b, a) from the Gutenberg-Richter (1954) equation are calculated individually from this network for each cell. Based on the numerical value calculated for each cell in the range of Iran, the zoning maps of a-Value, b-Value and a/b-Value have been provided. The changes in the value of seismic coefficients indicate different tectonic situations in the region. According to the b-Value and a-Value zoning maps, the regions with various seismicity coefficients values can be separated. In the study and comparison of the maps, the zoning map a/b-Value shows more comprehensive information on the seismicity and tectonic situations of the region. As a matter of fact, using this map, the effect of coefficients such as a, b can be seen together in a map. Accordingly, Iran zone is divided into three general regions; low seismic potential, moderate seismic potential, and high seismic potential regions.
 
2-Methodology
The first step in examining the seismicity of any region is collecting earthquakes that have previously occurred in that region. In this study, valid domestic and foreign sources were used for preparing the earthquake catalog. Before processing the seismic data, it is essential to refine the data of dependent events in order to obtain a poisson distribution of the data. In the present study, the window elimination method which is a standard method based on logarithmic time drawing of aftershocks based on the magnitude of earthquakes (Gardner and Knopoff, 1974). Finally, a catalog with 8090 earthquakes recorded from 1900 to 2007 (107 years) was prepared, serving as the basis and preliminary data of this study. 
Zoning of Iran based on seismic coefficients a-value, b-value, and a/b-value is the main objective of this study. These coefficients were prepared for the entire zone of the country, and then regions with equal values were zoned together. Thus, to perform numerical calculations and determine seismic coefficients, the entire zone of the country and adjacent regions were divided into 2°*2° cells with 1.5° overlapping. This overlapping among cells plays a vital role in the continuity of data. A total of 1354 cell were resulted, for each the a- and b-values were calculated and attributed to the central point of that cell. In fact, across the entire zone of Iran, there are 1354 points with 0.5° distance from one another, and each point has its specific a- and b-value. Due to the large number of earthquakes (8090 records) and the large volume of computations (1354 cell keys), a computer program was written using Visual Basic using which: 1) The recorded seismic data were read; 2) the data belonging to each cell were separated; 3) seismicity computations were performed for that cell, and 4) results were saved in a separate file. Therefore, for all the 1354 cells specified across Iran and for the data constrained in each cell, the Gutenberg-Richter line was drawn and a- and b-values were computed for them. Finally, after entering the preliminary data (seismic coefficients) in Arc GIS, zoning maps were prepared using the inverse distance weighting (IDW) method which is a common and frequently used method.
 
3-Result and discussion
Based on zoning maps, the a- and b-values of regions with different seismic activities and seismic coefficients were separated. In each map, nine zones with varying seismic coefficients are evident (Fig.1). In the zoning map of a-values, the largest numerical value of the frequency of earthquakes belongs to the Zagros region, East Alborz and Kopet Dag, and parts of Western and Northwestern regions, with the numerical value of 5.6 to 8.1. The b-value zoning map also showed the largest numerical value for the Zagros structural trend, East Alborz and Kopet Dag, and Western and Northwestern regions with the numerical value of 0.49 to 1.7.  These high-coefficient regions surround regions including parts of the Central and Eastern Iran with low seismic coefficients. The increase and decrease in b-value of various regions of Iran indicate the different tectonic conditions and behaviors of this zone (Scholz, 1968; Mori and Abercombie, 1997; Manakou and Tsapanos, 2000).
 
Figure 1. Right, the a-value zoning map across Iran; Left, the b-value zoning map across Iran
 
 
Figure 2. The a/b-value zoning map across Iran
Next, a zoning map of the a/b ratio was also prepared (Fig.2). According to Bayrak et al. (2002), this map demonstrates the seismic condition and regions with seismic potential better than a and b maps. Based on this map, the Iran zone includes regions with high (6.4 to 7.4), moderate (5.9 to 6.3), and low (0.0 to 5.8) seismic activity.
The high density of colors (high numerical value of a/b) in Zagros, Strait of Hormuz, some Eastern parts, Kopet Dag, Eastern and Western Alborz, and Azarbaijan indicate connected seismic belts of tectonically active regions surrounding regions with a low numerical value of a/b (low-density colors) or those with a low seismic potential (Fig. 2).
 
4-Conclusion
Based on zoning maps of a-value, b-value, and a/b value prepared for Iran, regions with varying seismic coefficients can easily be distinguished from one another. These seismic zones can be compared with sedimentary-structural zones of Iran separated based on geological characteristics. This can also indicate the close relationship between seismic and tectonic features in Iran.
Accordingly, Iran zone is divided into three general regions; low seismic potential, moderate seismic potential, and high seismic potential regions. The areas having high seismic potential have surrounded low seismic areas; of course, in form of a large belt of high seismic areas such as: 1-trend northwest to southeast of Zagros, 2- northern Strait of Hormoz with a northern southern trend corresponding to the Nayband-Sistan faults belt, 3- northeastern part of the Makran coast, 4- widespread sections of Azerbaijan, 5- Western Alborz, 6-Eastern Alborz, 7 Kopet Dagh, and 8-parts of the east of the country in the upper part of the Lut Desert (Khorasan).
 
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