Spatial distribution of seismicity parameters in the Persian Plateau
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 15 June 2012
Accepted: 26 February 2013
Published: 17 September 2013
In this paper, seismicity parameters in the Persian Plateau are estimated and represented as colored maps. We depict the b-value variations as an indication of either changing material properties, or the heterogeneity of the stress field in this region. The lower b-values in some regions indicate that the seismic energy release in these areas is mostly provided by large magnitude earthquakes. Yet the b-value around northern CentralIran and Eastern Alborz seems to be near unity, which implies that the seismic energy release is compatible for larger and smaller earthquakes. We also compared our results with some existing seismological information on Iran. Regarding attenuation: low-Q (high attenuation) regions approximately correspond to relatively lower b-value regions, and areas with nearly medium/high b-values correspond to high-velocity anomaly zones inferred from tomographic inversions. Meanwhile, regions associated with thrust and strike-slip faulting was associated with low/medium b-values. The higher b-values in NW Iran agree with the regional extensional tectonics and low/medium b-values were in agreement with regional compressional/strike-slip tectonics. In terms of geodynamics, this may be interpreted as an indication of a weak crust in NW Iran versus stronger crust (zones under stress) in other parts.
Key wordsSpatial variations a-value b-value regional tectonics faulting-style
The nature of a spatial-temporal distribution of earthquake occurrence is a complicated phenomenon with complex nonlinear dynamics, so seismicity parameters (“a” and “b” values) can help reveal some aspects of the complexities. Gutenberg and Richter have shown that seismicity parameters may follow a power law. The a-value depends on the size distribution, and the time and volume window considered, and is a measure of seismic activity. The b-value is a tectonic parameter which indicates a measure of the relative abundance of greater to smaller earthquakes, and the stress and material conditions in the focal depth (Mogi, 1962; Scholz, 1968). As the b-value plays a key rule in present seismic hazard study theories and procedures, the results will be very useful for seismic hazard analysis in Iran.
In this study, we have only considered events after 1996, so it may be helpful to take a look at the development and installation of seismic stations in Iran. Several seismic stations have been in operation in Iran from the early sixties to monitor seismicity for seismic hazard studies. Mash-had, Shiraz, Tabriz and Tehran stations were among the first seismic stations in the country. Later, the events recorded by the Iranian Long Period Array (ILPA) have also been used to study regional seismicity. The ILPA comprises seven stations equipped with broadband seismometers located southwest of Tehran (Akasheh et al., 1976). ILPA data was not appropriate for studying the precise seismicity all around Iran because it enclosed only a 30-km-diameter circular area, and other sparsely distributed stations suffered the same problem. After the 1990 catastrophic Rud-bar earthquake, the Institute of Geophysics of the University of Tehran decided to provide a better station coverage over Iran. Since late 1995, several stations have been installed in different regions of the country based on digital technology and radio transmission. Each station is equipped with three-component short-period seismometers, digitizer and transmitter. Continuous data are sent by radio links to the central station, where they are time-stamped and a STA/LTA algorithm detects the events and stores them on permanent storage media. The recorded events of these stations can be extracted from the IGUT (Institute of Geophysics, University of Tehran) for further processings and constructing regional catalogues.
2. Tectonic Settings
The seismicity of the Persian Plateau has been studied by many different authors (Wilson, 1930; Niazi and Bas-ford, 1968; Nowroozi, 1971, 1972, 1976; McKenzie, 1972; Kaila etal., 1974; Berberian, 1981, 1995; Shoja-Taheri and Niazi, 1981; Ambraseys and Melville, 1982; Ahmadi et al., 1989; Engdahl et al., 2006). This Plateau is located within the continental collision between the African and Eurasian plates, and includes recent volcanic activity, high mountain ranges and active faults. The collision between these two plates has uplifted mountain ranges like the Alborz and Za-gros. This Plateau has undergone different mountain building phases that is characterized by concurrent magmatism and metamorphism, especially within the early Paleozoic, middle Triassic, early Jurassic and early Cretaceous (Berberian, 1981; Sengor etal, 1988).
3. Data and Method
As evident from Figs. 3 and 4, the seismicity parameter maps before, and after, declustering provide comparable results. The magnitude of completeness maps show that it is around Mn = 2.5 for inner parts of the Plateau and about Mn = 3.0 for outer parts (Figs. 3(a) and 4(a)). The a-value maps (Figs. 3(b) and 4(b)) indicate that values generally larger than 4.5 that may be taken as high seismicity rates in the Persian Plateau and surrounding area. The b-value estimations are about 0.8 to 0.9 for inner regions, and lower than 0.8 for outer regions, of this Plateau (Figs. 3(c) and 4(c)). Based on Figs. 3(d) and 4(d) that represent significant b-value maps according to Wiemer and Schorlemmer (2007) the b-values are generally lower than unity and there is no significant difference between b-values. The lower b-values in some regions indicate that the seismic energy release in these areas is mostly provided by large magnitude earthquakes. Yet the b-value around northern Central-Iran and Eastern Alborz seems to be near unity which implies that the seismic energy release is compatible for larger and smaller earthquakes.
Several studies have revealed spatial variations in the frequency-magnitude distribution of different tectonic regimes (Wyss et al., 1997; Wiemer and Wyss, 1997). Some studies have indicated spatial and the temporal changes in the b-value before large earthquakes (Murase, 2004; Nakaya, 2006). Material heterogeneity and thermal gradients (Mogi, 1962; Warren and Latham, 1970) may also cause changes in b-values. Wyss et al. (2001) showed that a low-velocity zone started around a volume with a high b-value in the subducting slab at 140 to 150 km in depth and suggested that this may be an indication of magma generation at high b-value anomalies. The b-value of the deep earthquake zones of Alaska and New Zealand is high at a 95 km depth (Wiemer and Benoit, 1996). This observation was interpreted as being due to high pore pressure in this depth range, due to dehydration in the subducting slab. Therefore, high b-value anomalies may be found in a descending slab. The low-velocity zone in a mantle wedge was thought to be a path of magma ascent in northeastern Japan (Nakajima et al., 2001). Several studies have shown that the b-value is different for small and large earthquakes (Hamilton and McCloskey, 1997; Ikeya and Huang, 1997). High and low shear stresses may cause earthquakes having low and high b-values (Wyss, 1973; Schorlemmer et al., 2005). Along the creeping zone of the San-Andreas fault, earthquakes have high b-values which may indicate a low stress (high pore pressure). Other observations propose that the spatial variation in the b-value of aftershocks is related to the rupture process of the main shock (Bayrak and Oz-turk, 2004). Almost all seismic hazard studies which are reflected in seismic hazard maps rely on b-value calculations, so temporal and spatial variations in the b-value will affect existing seismic hazard maps (Schorlemmer et al., 2005).
Different regions in the Persian Plateau experience a significant variation of heterogeneity laterally and vertically, but it seems that vertical tectonics, caused by the convergence between Arabia and Eurasia, is controlling the seis-micity of Iran. Tectonic adjustment between different structural features (mountains, hills, valleys, etc… ) has been increasing the seismic activity in various regions of the same Plateau. Although the seismicity of the Persian Plateau has not been deeply studied (to provide clear seismogenesis results) so as to provide sufficient information for clarifying the mechanisms for b-value variations, we can state that the different stress regimes are among the main reasons for the changes in seismicity parameters within different regions of this Plateau.
The uneven distribution of the a-value may clearly be an indication of the wide fluctuations of stress level and heterogeneity in the crust of the Persian Plateau. Large a-value regions (i.e. Alborz, Central-Iran, Kopeh-Dagh and Zagros) may be considered for greater rock fracture densities caused by strong structural movements and/or a low rheological strength of the crust causing brittle failure at lower stress levels. Higher a-values may also be related to the mutual tectonic adjustment between Zagros with the African Plate, and/or Alborz-Kopeh-Dagh with the Eurasian Plate. Lower a-values are also related to lower heterogeneities/fracturing of the crust caused by the tectonic processes of eastern Iran against the Indian Plate, and/or Azarbaijan against the Caucasian Plate.
Based on the range of the computed measures, the Plateau can be divided into low (b ≤ 1.0) and high (b > 1.0) zones. Higher b-values can possibly be accounted for by low strength rocks that experience brittle failure at lower stress levels. They can also represent a relatively low stress in the seismogenic zones caused by episodic changes in the state of the tectonic coupling in each region. Lower b-values usually indicate areas including strike-slip and reverse faulting (Narteau et al., 2009) which is compatible with the typical features of the tectonics of Iran. An important result based on the b-value estimations (Figs. 3(c), 3(d), 4(c) and 4(d)) is that these values in Alborz, Azarbaijan, East and Central-Iran, Kopeh-Dagh and Zagros are around low values (b ≤ 1.0). Evidently these regions are those associated with thrust and strike slip faulting that tends to be under higher stress regimes (Schorlemmer et al., 2005). The world-wide subduction zones show b-values having a low value in a range from 0.53 to 0.74 (Bayrak et al., 2002) and this has been found to be the same in Makran. Masson et al. (2006) provide an explanation for the extensional tectonics in NW Iran based on a dense GPS network installed there. We interpret the b-values (Figs. 3(c) and 4(c)) in Azarbaijan and the surrounding area to be due to this extensional deformation. Meanwhile, the low/medium b-values in other parts of the Persian Plateau are under the influence of the convergence of tectonic forces from the African, Indian, and Eurasian Plates, presented regionally by thrust and strike-slip deformations. In terms of geodynamics, therefore, this configuration depicts a weak crust in NW Iran while other parts show a relatively stronger crust which is under higher compressional/sliding forces. Accordingly, it can be concluded that these values are closely related to the existing high deformation in the Persian Plateau.
Some of the existing seismological information enable us to compare our results with them. Comparing our b-value map (Figs. 3(c) and 4(c)) with the LgQ attenuation map of Pasyanos et al. (2009) reveals that low-Q (high attenuation) regions approximately correspond to relatively lower b-values, and vise versa. Koulakov (2011), and Alinaghi et al. (2007), demonstrated body and shear wave anomalies beneath Iran along different cross-sections resulted from tomographic inversions. The high-velocity anomalies of their cross-sections and maps are almost in agreement with medium/high b-value areas of this study (Figs. 3(c) and 4(c)).
I would like to thank Tomomi Okada and anonymous reviewers for their constructive comments and valuable suggestions. We acknowledge IGUT/IRSC (irsc.ut.ac.ir) for providing the IGUT catalogue. Some figures have been drawn by GMT (Wessel and Smith, 1991) and zmap (Wiemer, 2001) has been used for data processing and seismicity parameter estimations.
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