Elsevier

Journal of Asian Earth Sciences

Volume 62, 30 January 2013, Pages 308-316
Journal of Asian Earth Sciences

Two-dimensional spatial analysis of the seismic b-value and the Bouguer gravity anomaly in the southeastern part of the Zagros Fold-and-Thrust Belt, Iran: Tectonic implications

https://doi.org/10.1016/j.jseaes.2012.10.010Get rights and content

Abstract

Environmental managers and protection agencies try to assess the magnitudes of earthquakes in regions of seismic activity. For several decades they have used the seismic b-values and Bouguer anomalies for evaluating the crustal character and stress regimes. We have analyzed geostatistically data on both variables to map their spatial distributions in the southeast of the Zagros of Iran. We found a strong correlation between the distribution of the b-value and the Bouguer gravity anomaly in the region. The large Bouguer gravity anomaly values and small b-values all accord with there being a thinner crustal root and a larger concentration of stress in the center. The small to moderate Bouguer gravity anomaly values and intermediate to large b-values accord with the thicker crustal root and the smaller concentration of stress in the northeast. We conclude the southeast of the Zagros, consists of heterogeneous crust, such that accounts for its varied tectonics.

Highlights

► The application of the spatial analysis for characterizing the spatial heterogeneity. ► Distinguishing the correlation between the quantitative distribution of b-value and Bouguer anomaly. ► Existing correlation between the quantitative distribution of the variables and the geological conditions.

Introduction

Orogenic processes comprise complex interactions of vertical and horizontal forces as tectonic plates converge and shear transpressionally over large regions. The Zagros orogenic belt is influenced by huge compressive stresses resulting from the transpression of the Afro-Arabian continent and the Iranian plate. Among the current active convergent zones, the Zagros continental collision zone is one of the youngest and most seismically active zones on Earth. It experienced several tectonic episodes of collisions during Late Mesozoic and Miocene times. The earthquakes that happen nowadays in the region show that the seismic activity that began in the Mesozoic era still continues (Nowroozi, 1976). Earthquakes occur throughout the 300-km width of the Zagros Mountains. Except in the northwest of the belt, most of the earthquakes with magnitude mb  5.0 on the Richter scale occur between the coast of the Persian Gulf and the 1500-m topographic contour (Jackson and McKenzie, 1984, Ni and Barazangi, 1986). The Zagros Fold-and-Thrust Belt is thus vulnerable to earthquake hazards.

There are several measures of seismicity, and in attempting to assess seismic hazard one must take into account the sources of the seismicity, such as resistant blocks (asperities), faults and lineaments, and thrusting zones. The estimated b-value, i.e. the relative abundance of large to smaller shocks in the well-known Gutenberg–Richter formula, identifies crustal heterogeneities. And the qualitative estimation of stress in relation to the release of seismic moment in tectonically active regions (Scholz, 1968, Khan and Chakraborty, 2007). In addition, tomography and gravity can help to identify heterogeneity in the crust and upper mantle, though they fail to account for the stresses associated with seismic sources. Therefore, it seems the monitoring of the seismicity in relation to the regional and local structural grain is the best scientific approach for anticipating hazards in any seismically active zone.

The early studies on the history and evaluation of plate collision and tectonics of the Zagros Fold-and-Thrust Belt by Stöcklin, 1968, Falcon, 1974, Berberian, 1976, Berberian, 1981 and Alavi (1994) were designed to understand. Other more specific studies on the seismicity of the region include those by Nowroozi, 1972, Nowroozi, 1976, McKenzie, 1972, Berberian, 1981 and by Ambraseys and Melville (1982). In addition gravity observations have been made by Dehghani and Makris (1983) and by Snyder and Barazangi (1986) in the region.

We have attempted to map, using geostatistics, and the spatial distribution of characteristics of seismic activity over the whole the area between the Qatar–Kazerun and the Minab faults. We have chosen two measures for the purpose. One is the estimated b in the Gutenberg–Richter formula and the other is the Bouguer gravity anomaly, both widely accepted for several decades for evaluating the tectonics.

As it is pointed out by Berg et al., 1964, Artyushkov, 1973, Assameur and Mareschal, 1995, Zamani and Hashemi, 2000, and Khan and Chakraborty (2007), measures of the seismicity and Bouguer anomalies can be used as possible indices of the stress level in the upper portion of the earth. It seems that the regional variations in lithospheric thickness and or in density might have significant effect on the occurrence of earthquakes. Therefore these factors can be considered in the studies of earthquake activity and hazard estimation in tectonically active regions.

Section snippets

Geological and tectonic settings

The Zagros Fold-and-Thrust Belt has formed as the Arabian craton has collided with the Iranian plate, beginning in the Miocene era and continuing to today (Stöcklin, 1968). The main feature of the Belt is a linear, asymmetrical folding, which forms a 200–300-km wide series of ranges extending for about 1800 km from the northern tip of the Arabian plate through Iraq and the southwestern part of Iran as far as the Strait of Hormoz (Blanc et al., 2003, Tatar et al., 2004). This orogenic belt

Seismicity

Fig. 3 is a map of seismicity of the eastern side of the Kazerun fault zone, the region between the Qatar–Kazerun and the Minab faults. The epicenters represent a selection from all events that occurred from the beginning of 1900 to the end of 2005 were taken from bulletins of the PDE (2000) and ISC (2000) and Ambraseys and Melville (1982). Our seismicity map, along with those published previously (e.g. Berberian, 1976, Nowroozi, 1976, Jackson and McKenzie, 1984), show that earthquakes are

Data analysis

For present purposes we first determined the boundaries of the area of study between the Qatar–Kazerun and the Minab faults in the southeastern part of the Zagros Fold-and-Thrust Belt. We identified three features, namely the Kazerun fault zone, the Zagros Thrust System and the Minab fault zone. Then we divided the study area into cells (geographic windows) of 0.5° × 0.5° on a regular grid. We overlapped the windows in steps of 0.25° along the directions of east, south and east south of the grid

Results

The estimated values of log b ranged from a minimum of −0.20 in the south-central parts of the region to a maximum of 0.6 towards the northeast between the Qatar–Kazerun and the Minab faults (Fig. 7). We divided the range of log b into three classes: small (log b  0.01), moderate (0.01 < log b  0.2) and large (log b > 0.2). We see that the northeastern and southeastern parts of the region are dominated by large values of b, while in the south-southwestern and central parts values of b are generally

Discussion

The data and the maps we have made from them match the historical record of seismic activity in the region between the Qatar–Kazerun and the Minab faults. Large values of b in the northeastern and southeastern parts of the region can well be accounted for by the collision between Afro-Arabian continent and Iranian plate and the regional tectonic grain. Towards the northeast the weak seismic activity and large values of b seem to be related to the thrust faults such as the Zagros Thrust System

Conclusions

This paper shows in map form that the region between the Qatar–Kazerun and the Minab faults consists of spatially heterogeneous of the seismic b-value and the Bouguer gravity anomaly, such that account for its varied tectonics. The parts of less active in the north–northeast of the region and the more active south–southwest. This confirms the migration of the Zagros deformation front southwestward and the increase the potential for the tectonic activities in that direction. It is suggested that

Acknowledgements

This research was supported by the Shiraz University Research Council. In particular, we thank from Dr. Sherkati (National Iranian Oil Company) and Dr. Hashemi (Damghan University of Basic Sciences) for their considerable help and guidance.

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