محمدرضا قیطانچی

Green's empirical function method has been used to simulate the strong motion of the Damavand earthquake. One of the conventional methods of simulation to produce strong ground motion is the use of aftershocks and foreshocks of large earthquakes using the empirical Green's function method; As a result, the ground motion simulation algorithm based on Green's empirical function, presented by Hutchings and Wu (1990), has been used to simulate the Damavand earthquake waveform.On 02/18/2019, an earthquake with a magnitude of 5 occurred in Damavand, located in Tehran province. In this study, with the aim of estimating the parameters of the source and the mode of fracture in this earthquake, the empirical Green's function method has been used to simulate the powerful movement of the earth. To simulate the main earthquake, a foreshock with a magnitude of 2.9 near the center of the main earthquake, as well as aftershocks with a magnitude of 3.9 and 4.1 recorded at the Masha and Rodhen accelerometer stations, have been considered as experimental Green's function. The length and width of the fracture in this earthquake are 2 and 3 km, respectively, in the direction of extension and slope. Calculations show that the direction, slope and tilt of the fault that caused the earthquake were 196, 76 and 156.5 degrees, respectively. The focal mechanism of this earthquake is of reverse type with strike-slip component, which is consistent with the mechanism of earthquakes in this area.

Iran has been affected by the convergence of the Arabian and Eurasia plates. For this reason, it has experienced destructive earthquakes throughout history. In terms of tectonic and seismic properties, Iran can be divided into 5 general seismotectonic provinces; Zagros, Central-East Iran, Alborz-Azerbaijan, Makran, and Kopeh Dagh. One of the facts that best describes the characteristics of earthquakes is their focal mechanism. In this article, the focal mechanisms of these 5 provinces were collected and classified with FMC software. For each zone, the frequency percentage of each group of faults was determined. Most faults are in the reverse class or compression type with the strike-slip component. Most of the faults of the reverse type are related to Zagros province, which includes 58% of its earthquakes and 42% of Iran's earthquakes. Thus, it seems the Zagros has been more affected by the convergence of the Eurasian and Arabian plates than other regions. Normal faults in all provinces have a small frequency so that only about 3% of the total earthquakes in Iran are in this class.

The aim of this study is to present the modified Mercalli intensity map in a probabilistic way based on seismic studies in Mako Quadrangle. The studied area is located in the northwest corner of Iran. The occurrence of several destructive earthquakes and the presence of active faults indicate a high level of seismicity in this region. Based on tectonic earthquake surveys and b parameter values, all active seismic springs were identified and seismic parameters were calculated for them. Then the modified Mercalli intensity map was calculated using four reduction relations. The results showed that the maximum intensity is in the form of a strip with a northwest-southeast trend, which shows a good compatibility with the Igdeer and Blikglu faults. The present results are in good agreement with the intensity map of the earthquakes that occurred in the region.

Earthquakes are one of the natural hazards that have caused many casualties and financial losses around the world. This is why earthquake risk analysis studies need to be conducted more seriously. Iran is located in one of the seismogenic regions of the world, the Himalayan-Alpine belt, which is subject to many earthquakes every year. The Arias intensity, as one of the important seismic parameters, helps in seismic hazard analysis, and can be used to estimate structural performance, slope stability, and liquefaction during an earthquake. The region under study is in Khoy area in the northwest, near the Iran-Turkey border. The geographical area covers 38.5°N, 43.5°E to 39°N, 45°E. The main features of the region are the presence of the major lakes of Van and Ercek in the west, and also the Iran-Turkey border in the middle of the map.Arias (1970) created an equation for measuring the intensity of an earthquake based on the time-integral of the of the square of the ground acceleration, which was later used by some researchers to evaluate the damage potential. Harp and Wilson (1995) found out that the Arias Intensity is reliably linked with the distribution of landslides caused by an earthquake. Later on, Kayen and Mitchell (1997) proposed a method for evaluating the soil liquefaction during an earthquake’s strike with the help of the Arias Intensity. In another work (Cabañas et al., 1997), a noticeable correlation was found between the Arias Intensity and MSK scale. It was also revealed that for certain structures, such as the ones found in villages and adobe and brick buildings, the damage could be linked to the Arias Intensity. More recently, Borja et al. (2002) used the Arias Intensity as a measure for comparing the results of two seismic response analyses. Additionally, for determining damage indexes, such as the value of destructiveness potential factor , where la is the Arias Intensity, and V0 is the amount of the zero crossings of the acceleration–time history, this parameter is used (Araya and Saragoni, 1984).

The Arias Intensity is one of the seismic parameters which is usually used in seismic hazard analysis to shed light on the potential damage earthquakes may cause. The Arias Intensity, as a scale of the shakings linked with an earthquake in terms of the amount of cumulative energy, is defined as an infinite set of single degree-of-freedom oscillators of unit weight with the frequency of zero to infinity (Elnashai and Sarno, 2004). This parameter correlates with the integral of the square of the module of ground acceleration over the time history of an earthquake. The Arias Intensity is a parameter which includes characteristics such as the domain and duration of ground motion for a wide range of recorded frequencies. Thus, compared to the parameters depending on the maximum value of ground motion, it is far more effective for evaluating earthquake impacts on engineering targets. In fact, evidence (Khademi, 2002; Mamseyedeh et al., 2021) supports that the Arias Intensity is to correlate proportionately with the damage caused by an earthquake, making it a reasonable choice when describing shakings capable of causing instability in structures, landslides (Chousianitis et al., 2014; Travasarou and Bray, 2003), and liquefaction (Kayen and Mitchell, 1997; Orense et al., 2015). 

The a and b values were calculated using the Gutenberg and Richter law (Gutenberg and Richter, 1956) in MapSeis software (Figure 3.). To complete the information about the regions seismic sources, the iso-potential maps shown in Figure 3 were used. As is seen in the figure, the a-b values of the center of the region of the study were higher. These higher values indicate more seismic activity in the area (Kanamori, 1981; Kanamori, 2013; Wyss, 1975; Wyss et al., 2001). On the other hand, the accumulation of energy in the areas with a low b value shows their potential for future earthquake events. Of course, along with the consideration of this variable, other variables such as a and λ, in addition to the seismotectonics of the region, must be observed to locate future earthquakes with minimized error (Jarahi, 2017). Observing Figure 2 and Figure 3, along with noting the location of seismic sources in the region, reveals that three areas (A, B, and C) are to be expected to be subject to future earthquake events. Altogether, the mentioned revelations prove that the region is  highly seismic. The information formed the basis for the analyses done by Ez-Frisk for further calculations. Figures 4 to 6 show the Arias Intensity iso-potential maps for probabilities of 20%, 10%, and 5%, respectively in 100 years.

The Arias Intensity map can be used as one of the most important indexes for measuring the destructiveness of an earthquake. In this study, for the three return periods of 475, 975, and 2475 years, the Arias Intensity was examined and analyzed. During the study, the main seismic sources of the region were identified and their seismic parameters were calculated. Three areas, marked A, B, and C, were introduced as the areas with potential for future earthquake events. In the Arias Intensity of the region, some specific points were noticed. In general, the further the area is from the seismic source, the less is the intensity of the earthquake there. Nevertheless, it is not always true. That is because another important parameter, known as shear wave velocity, can control this pattern or even reverse it. In various regions, such as the land surrounding the Shkriazy fault, this reverse pattern was detected and that displays the effect of shear wave velocity on the Arias Intensity distribution. Considering the history of liquefaction during earthquakes in this region, the results of the current study could serve as the basis for liquefaction and landslide studies conducted there. From among the three recommended areas, area B is adjacent to a relatively large sedimentary basin (northeast of Salmas), where the very low shear wave velocity makes it prone to liquefaction. Therefore, the potential for the occurrence of future earthquakes, on the one hand, and the conditions increasing the likelihood of liquefaction and resonance, on the other hand, make area B the most dangerous one in the region. Moreover, it is recommended that in the three areas discussed in this study, measures be taken for reinforcing and strengthening structures to prepare them for future earthquake events.

The Makran zone in southeastern Iran and southern Pakistan is the result of subduction of oceanic crust of the Arabian Plate under the Eurasian Plate. From seismic behavior point of view, there is a distinct segmentation between the western and eastern parts of the subduction zone. The western part of the Makran has an abnormally very low level of deep seismicity with lack of recorded great earthquakes, while the eastern part has experienced many great earthquakes. Another difference between the western and eastern parts of the Makran region is that the distance between the Quaternary volcanic arc and fore-arc setting is larger in the east than in the west. Understanding the nature of unusual behaviors of the Makran subduction zone has long been one of the biggest challenges in seismotectonic investigations of this region. The present study aims at producing high-resolution love-wave velocity structure maps of the crust and the upper mantle in the Makran subduction zone using ambient seismic noise. To achieve this purpose, a large dataset has been provided to produce tomographic maps. Empirical Green’s functions were obtained from cross-correlations of broad-band seismic noise records at different stations inside and outside the region. Love-wave velocity dispersion curves were then extracted from the ambient noise, and finally converted into a 2D group velocity image (or tomography map) for crustal and upper mantle structures of the region.

Natural frequency of soil is an important factor in site effect studies. In order to determine this parameter, microtremor measurements were applied as a fast, simple and economic tool. For this purpose, microtremors were recorded in 74 points with time durations of 5 to 10 minutes throughout the city of Shiraz. Natural frequency and amplification factor of soil were then estimated considering the main peak in spectral ratio between horizontal and vertical components (H/V) that is also known as Nakamura’s technique. Fourier and power spectrums were also used to determine this spectral ratio. The results demonstrate that natural frequency varies from 0.5HZ to 6HZ in different parts of the city, so that it increases in the east and north part of the city where alluvial deposits over the bedrock are thin. Also, amplification factor of the alluvial deposits obtained by this method has values between 1.3 and 5 throughout the city. In order to investigate time-dependence of the natural frequency, after 8 years of data recording new data were recorded in five stations. The results indicate acceptable correlation and consistency between old and new data results in these five stations.

In the last 150 year, most destructive earthquakes of Kopeh Dagh occurred near Quchan. These earthquakes caused large damages to Quchan (Tchalenko, 1975; Ambraseys and Melville, 1982). The Kopeh Dagh zone accommodates a motion, by a combination of slip-partitioning in the NW, thrust faulting in the SE, and anticlockwise block rotation in the Central Kopeh Dagh (Hollingsworth et al. 2006, 2008). The system of NNW–SSE right-lateral strike-slip faults in the Bakharden–Quchan fault zone between Bojnurd and Quchan is one of the most prominent structural and topographic features of the central Kopeh Dagh (Hollingsworth et al., 2006). The Kopeh Dagh is made up of a sequence of mostly conformable and complete Mesozoic–Tertiary sedimentary rocks (Stocklin, 1968; Berberian, 1976). The Kopeh Dagh form a linear intracontinental fold and thrust belt trending NW–SE between the stable Turkmenistan platform and Central Iran (Hollingsworth et al., 2006). Shortening in Iran accommodates the northward motion of the Arabian shield into Eurasia. Recent GPS measurements (McClusky et al., 2003; Vernant et al., 2004) indicate that Arabia moves approximately northwards, with respect to Eurasia, at ∼ 23 mma−1 at the longitude of the Kopeh Dagh. The Kopeh Dagh fold belt as a part of Alpine-Himalayan mountain belt in western Asia, constitutes the north-eastern border of the Iranian plateau and lies on the south-western margin of the Turan (Turkmenistan) continental crust, forming its epi-Hercynian (Early Kimmerian) cover (Berberian, 1981; Nabavi, 1983).
In this study, the Double-Difference earthquake location algorithm was applied to the relocation of a large set of seismic events that occurred in Quchan region and recorded by Quchan and Mashhad seismic networks affiliated with the Iranian Seismological Center (IRSC) during the period from 1996 to 2012. The study area extends from 35.5°N to 39°N and 56°E to 60.5°E and is located in the Kopeh Dagh major seismotectonic province. The purpose of this study is to improve earthquakes location by using Double-Difference method developed by Felix Waldhauser and William Ellsworth (2000). Relative earthquake location methods can locate earthquakes with higher accuracy by removing effects due to unmodeled velocity structure. HypoDD program determines relative locations within clusters using the Double-Difference algorithm. In order to estimate capability of Double-Difference technique in the area, we performed synthetic tests by which four datasets, each including 10 synthetic earthquakes, were considered along the Kashafrud, Quchan, Binalud and Robat-e-Qarabil faults. The single event method by hypo71 program was applied to determine initial locations. Then, Double-Difference technique by hypoDD program was used for relocating the events. The results showed significant decrease in errors using Double-Difference technique. By using the synthetic tests, capability of Double-Difference algorithm was demonstrated. Then, by putting constraints on primary data, a number of 2516 earthquakes, recorded by Quchan and Mashhad's seismic networks from 1996 to 2012, were chosen to be relocated by the latest version of hypoDD program using the Double-Difference algorithm and Mehraban’s 3D velocity model (2012). The distribution of the events in the central part of the Kashafrud fault shows that the fault is dipping northeast and the occurrence of earthquakes at different depths can be the representation of a high-angle thrust fault and the activity in the entire fault plane of this reverse fault. According to the relocation of the earthquakes and the cross sections in the north of the Shandiz-Sangbast and the west of the Quchan faults, the existence of seismic activities can represent hidden fault activity. The linearity of earthquakes to the south of Baghan-Garmab fault can be also the representation of the continuation of seismic activity in this fault, though the surface trace of this activity is not visible in the geologic maps. In the present study, the average RMS is 0.27 s in the initial locating and reaches to 0.09 s in the relocation by hypoDD using the 3D velocity model. The average of the relative horizontal and vertical uncertainties stood at 686 m and 721 m for relative relocation. The relocation using a 3D model could improve the depth distribution of earthquakes, which is more accurate than initial location. This means that it reveals the concentration of the events between the depths of 5 to 23 km. As for the constraints imposed on the initial data, we considered a minimum depth of 3 km, but the Double-Difference relocation of earthquakes using the 3D model shows that 73 earthquakes occur at depth less than 3 km with least errors.

On the 11thMay 2013 at 2:08:08 GMT, a moderate earthquake (Mw=6.2) occurred in Hormozgan province, near the Goharan village (southeast Iran). The largest aftershock of magnitude 5.8 occurred on 12th May 2013 at 12:07:02 GMT near the mainshock, which produced more destruction. According to the reports, one person died and 17 people injured. This earthquake is the first noticeable recorded event in this area. By this time,the most important seismic activity had occurred in 1983 with magnitude Ms=5.7 (ISC). Aftershock sequences provide valuable information about the Earth’s crust and source properties of large earthquakes. This is because a large number of events occur during a short time in a small area. This earthquake had a lot of aftershocks, after 3 months Iranian seismological center (IRSC) recorded 284 events.    Generally, initial location of earthquakes produce inevitable error because of algorithms we use to locate earthquakes. But this error will be decreased by relocating. One of the most common methods used is double-difference algorithm developed by Felix Waldhauser (2000). The double-difference is one of the relative earthquake location methods in these methods, effects of errors produced by deviation from real structure velocity model can also be minimized. According to the theory of double-difference, if the hypocentral separation between two earthquakes is small (compared to the event station distance and the scale length of the velocity heterogeneity), then the ray paths between the two sources and a common station are similar along the entire ray path. In this case, the difference in travel times for two events observed at one station can be related to the spatial offset between the events with high accuracy.    Previous studies have shown that the direction of causative fault (related to mainshock) can be obtained by the distribution of aftershocks, but it does not exactly assess motion on the fault occurred during the earthquake. So obtaining focal mechanism of the mainshock and largest aftershocks could help us to solve this important problaluablem. There are some methods to obtain focal mechanism and one of the most powerful methods is moment tensor inversion. This method has developed for a local earthquake by Zahradnik et al (2005). This algorithm has been written in Fortran language programing named ISOLA. The ISOLA GUI has been created using the Matlab GUIDE tool.ISOLA is based on multiple point-source representation and iterative deconvolution method, similar to what Kikuchi and Kanamori (1991) have done for teleseismic records, but here the full wavefield is considered, and Green's functions are calculated by the discrete wavenumber method of Bouchon (1981). Thus, the method is applicable to regional and local events.   In this study, we use double-difference algorithm to relocate the mainshock and aftershocks of this earthquake. During this process, the error of location improved (compare to initial location). By using spatial-temporal analyse of aftershock sequence we derived that the causative fault of this earthquake is in an east-west direction. All the aftershocks are distributed in the area with length 40 km and width 25 km, but the most of them are being in a cluster with length 15 km and width 10 km. We determined the focal mechanisms of this earthquake, and 15 other large aftershocks by moment tensor inversion method (Isola software) and the results show that all of the events have a predominatinely strike-slip mechanism. According to the distribution of the aftershocks we determined, fault plane is distinguished from the nodal plane. The fault plane is a left-lateral strike-slip in the SW-NE direction. According to the Geological evidence, this earthquake is related to the unknown fault in this region, but topographic map shows that probably this earthquake is related to linear structure parallel to aftershocks distribution and the fault plane that obtained in this study using CMT(Centroid Moment Tensor) method.

Summary: Rapid and accurate assessment of earthquake source parameters is extremely useful in seismology from different perspectives including hazard assessment and also for rapid response services. Nowadays، there exist various automated phase detection and location algorithms that provide near real-time seismic bulletins. Such algorithms rely on explicit phase identification and complex event association techniques that must be reformulated for different velocity models accordingly. These algorithms often use only P and S phases (Withers et al.، 1999). In many cases، the signal-to-noise ratio (SNR) is small، making it difficult to use the classical methods based on phase identification. However، event location based on a migration approach has recently been proposed in seismology (e. g.، Kawakatsu and Montagner، 2008; Larmat et al.، 2009; Kim et al.، 2011; Gharti et al.، 2011) mostly because phase identification is not required in this method. Time-reversal (hereafter referred to as TR) is a migration-based approach، in which the observedseismograms are back propagated in time in order to determine the location of the events. The TR method can potentially provide more accurate information on earthquake processes especially for small earthquakes where phase-identification-based methods are hardly used due to a low SNR. In general، classic earthquake location is a nonlinear problem and linearization oftravel time equations in earthquake location is mandatory based on a Taylor series expansion around some prior estimate (or guess). The locations of events also contain random errors، for example errors associated with the arrival time، as well as systematic biases due to manual phase picking. On the other hand، the most important error in earthquake locations with the TR method is only the inherent dependence of earthquake locations on an assumed seismic velocity structure of the Earth. The history of the TR method، which takes the advantage of none-phase identification، can be traced back to automated local and regional seismic event detection and a location system using waveform correlation by McMechan (1985) and Whiters et al. (1999). In recent years، thesame theory of waveform correlation is used by various researchers to investigate earthquake location in local (e. g. Maggi and Michelini، 2010)، regional (OBrien et al.، 2011) and telesimic distances (e. g. Ekstrom، 2006; Larmat et al.، 2006) using the TR method. The time reversal algorithm has been recently used to study the location and characterization of seismic sources in elastic media. This algorithm is related to the back propagating of the recorded data in earth in reverse time to find the parameters of an event. In this study، we investigate the location of small seismic events in south-eastern of Iran in an automatic mode. Most of the events are linked with structures of east of the Lut Block occurred after the main shock on 2010، December 20 with Mw 6. 3. Data from a temporary network was used to investigate the P phase clarification using a kurtosis analysis on records and then it was migrated to the source in reversed time to find thelocation of events without human bias. We tested several time reversal imaging conditions by synthesizing events for location. Finally، we used the best obtained parameters from a synthetic test to apply the same procedure to a real database observed after the main shock in the Rigan region which had been recorded by the network. We showed that although the obtained parameters were used in the real data، simple limitations such as number of contributed station in location should be used to improve the location. The results showed that well-constrained centers of events were in good agreement with the trend and dip of faults in the region and the focal mechanism of the main shock.

An important aim of seismology is to determine focuses of earthquakes. The more precisely the process is carried out، the better results will be achieved in various studies، for example، in vestigation of the crustal structure and seismogenic zone or the determination of the plane of faults causing earthquakes. The classic methods of locating are based on Geiger''s equations (1910). The relation between the arrival time and coordination of the earthquake is linearized in this method using the first term of the Taylor''s expansion and a velocity model available for the study area. The optimum responses in the linearized methods are obtained by minimizing the differences between the observed and calculated data using iterative Least Squares (L. S.) method. A considerable error is usually observed in determination of earthquake focus because of the absence of a proper azimuthal bearing، as well as the mere use of the travel time data. With the purpose of reduction of the errors، researchers have used azimuth and ray path parameters for optimization of their performance. The study area is located within 44-50o E and northern latitude 36-40oN، which is located in northwestern Iran، forming part of the central Alborz-Azerbaijan tectonic region. In this research the earthquakes of a magnitude equal or greater than 1. 4 in the body wave scale occurred in the area within the period from 2006 to 2013، included 11،200 events، first were located using only P wave traveltimes and then process was repeated by adding ray path and P wave azimuthal parameters data. To this effect، the aforesaid earthquakes were classified into two general classes، consisting of earthquakes of magnitudes 1. 4-3. 4 and earthquakes of magnitudes exceeding 3. 4. Concerning the first class of earthquakes، processing was made only for the data obtained from the stations in the local seismic network of the Institute of Geophysics، University of Tehran. The velocity model used in this part of the work was local Velocity Model (Bayramnejad، 2008) that is specific to the study area. Because of a large number of earthquakes، each of the aforesaid classes of earthquakes were subdivided into two subclasses consisting of the earthquakes with azimathal gaps less than 180 degrees and those with azimuthal gaps greater than 180 degrees. The results obtained by the earthquakes locating are represented by histograms which indicate that utilization of ray parameter and azimuthal parameter considerably reduce the horizontal error in relocating of earthquakes، specially for the earthquakes with the azimuthal gaps exceeding 180 degrees compared to utilization of mere travel time data. For the second class of earthquakes، we have used all the data from the stations located within the whole seismic range of Iran. It is evident that locating of earthquakes is carried out with a greater precision where more data is available. The velocity model used in this part was similar to the one used for the whole. Optimization of relocating of earthquakesare indicated by histograms for the magnitude of 3. 4-4. 6earthquakes. Results indicated reduction in the horizontal error when the azimuthal and ray path parameters have been used. For the magnitude 4. 6-6-2 earthquakes، it is proved، by drawing a certainty ellipse، that the use of azimuthal and ray parameters may optimize locating of earthquakes،and lower the dimensions of the certainty ellipse.

One of the most important purposes in seismology is determination of crustal velocity using earthquakes data that have been recorded by regional and local seismic stations. The more precise the process is carried out، the better shall be the results reached in various studies in the area including earthquake locating، seismicity of area، seismic zone mapping or the determination of the plane of faults causing earthquakes Imaging velocity structure of Earth''s interior using travel times inversion commonly called seismic tomography which is usually done two or three-dimensionally.. This method has extensively been used in recent decades by researchers. Since the seismic waves are associated with valuable information about direction of propagation and environmental properties، using earthquakes as natural seismic sources are very useful in seismic tomography as well as the artificial sources such as limited and controlled explosions، air guns and bore-hole sources.. The seismic tomography characterizes the size، geometry and extent of velocity anomalies. In this method subsurface structure is modeled initially by several parameters and improved by the inversion of seismic travel times data. In this study the crustal velocity structure was determined using three-dimensional inversion of local earthquakes travel times recorded by seismic networks of Institute of Geophysics University of Tehran (IGUT) occurred within the period from 2000 and 2012 in the study area. The study area is bounded within 31oE to 34oE and 50oN to 53oN. We used the VELEST software in one-dimensional modeling section. The procedure of study is minimizing the differences between observed and calculated travel time by applying the initial obtained model. This software simultaneously optimizes the earthquake locations، crustal velocity model and station corrections using the Joint-Hypocenter-Determination (JHD) method. The initial estimates for P waves velocity and crust thickness of the region are achieved using the travel-time curve of primary phases of all earthquakes occurred in the area. Then the larger relative earthquakes are selected and the best crustal one-dimensional model was derived by simultaneously inverse modeling method using this data set and VELEST algorithm. This method can be considered as one of the useful methods in study of 1D crust structure. The results proposed a 4 layers model of crust in which the P wave velocity is equal to 5. 4 km/s for depths less than 5 km، 6. 0 km/s for depths from 5 km to 20 km، 6. 2 km/s for depths of 20 km to 32 km and finally 6. 9 km/s for depths of 32 km to 47 km. The thickness of crust and Pn velocity are respectively obtained 47 km and 7. 9 km/s. The aim of this work is obtaining an optimal crust model that can aid to improve seismic data and can be used to determine the next earthquake locating. Then the obtained crustal model is used as an initial model to study of three-dimensional inverse modeling of crust in the region by using FAST algorithm. All the earthquakes relocated using new obtained model. In this study a data set recorded by the 8 seismic stations of Isfahan and Shahrekord networks were used. The resolution of final solution of 3D model was investigated using synthetic dataset (checkerboard model) that shows fair resolution for various depths. The lateral variations of the main resolved structures in the model obtained are highly correlated with the faulting systems in the region.

In this study, applying a tomography method to local earthquakes, a three-dimensional (3D) image of the crust of central Alborz is obtained. The result is not only consistent with the tectonics features in the region but also abale to interpret the tomography of Moho and crustal structure beneath the volcanic mountain of Damavand. More than 11000 local earthquakes, with a magnitude of 1.7 or higher, recorded by three-component short-period seismic stations of Tehran, Mazandaran, and Semnan networks between 1996 and 2006, bounded by 34-37N and 39.7-54E, were used to image the crust in central Alborz. These raw pieces of data, on one hand, were used as input for the 1D inversion method to obtain the variation of Vp and plot the velocity versus depth diagram whose rms was smaller than 0.15. On the other hand, they were used in a relocation process and when their locations were improved, a 3D model was generated based on them. After determining the preliminary 3D and forward models using the finite-difference method, the refracted and wide-angle reflected travel times were inverted and the horizontal and vertical sub-structures in our determined region were investigated. Based on these results, the discontinuities of the crust and Moho were mapped and analyzed. The final outputs showed that the resulted crust model is consistent with some of the recent geological studies. These outputs illustrat that the upper layer is thicker than the middle and lower ones as these two layers become thinner and even disappear below Alborz. It seems that the upper layer fills some hollows in the other ones. The depth of Moho increases below Damavand mountain; also, the area around the volcanic conduit of Damavand, between 6km and 18km depths, has a high velocity and is colder than the other areas.The P-velocity model resulted by using 1D tomography facility of Velest with RMS values less than 0.15, are compatible with the previous models. The resulted depth is 45 ± 2 km for the Moho and 7 km for the sediment layer. Frequency and distribution diagrams of the earthquakes show that about 75% of earthquakes have happened in depths less than 24 km and consequently the most seismogenic layer of the crust is estimated to be located at this depth. The 3D tomography, performed through the Zelt routines, has acceptable results with less than 2.5% error. Although an enough number of earthquakes overcome the problem of scarcity of the stations, the high depths of earthquakes cause a low resolution in shallow layers. However, this problem can be solved by increasing the density of stations.

The Shahroud fault system plays important role in seismotectonic of the eastern Alborz. In this paper we have surveyed the seismicity of the middle-eastern Alborz and its southern area. At this investigation، the data of the Geological Survey of Iran local seismological networks، the seismological networks of the Institute of Geophysics of the University of Tehran and the International Institute of Earthquake Engineering and Seismology of Iran were used for processing the focal mechanism of micro-earthquakes and the south of Damghan earthquake and its greatest aftershock. Distribution of the micro-earthquakes and the south of Damghan events epicenters indicate intense activity of the Shahroud fault system and the Toroud fault. Focal mechanisms of them shows near vertical dipping of the faults and left lateral mechanism of the western segments of the fault system and the Toroud fault. The focal mechanisms suggest the Astaneh، Chashm and Firouzkuh faults from the system fault behave in a same manner with no deference between them at depth and have seismic potential proportion to their total length. Also due to left lateral mechanism of the south of Damghan earthquakes، Toroud fault treats like of the eastern Alborz seismotectonically and this area could cover Toroud fault.

Study crust and upper mantle structure below the surface of Earth is one of the important objectives of geophysics. Teleseismic body waveforms have been used to infer crust and upper mantel structure. The Iranian plateau is part of the Alpine-Himalayan orogenic belt. The Zagros mountain belt in southwestern Iran has resulted from the collision of Arabian Plate with the continental crust of Central Iran after the closure of the Neotethys Ocean. The region referred as central Zagros of Iran in this study includes the area located between 50.9°-54° longitude and 28.1°-30.8° latitude. In this study we use teleseismic receiver function method to determine the Moho depth variations and Vp/Vs ratio beneath Shiraz Telemetry Seismic Network, located in the central Zagros using teleseismic data (30°  95°, Mb ≥ 5.5) which have been recorded in five 3C stations short period from 2002-2009. Teleseismic events with relatively high signal-to-noise ratio (>4) have been carefully selected at each station. We considered a time window of 110s, starting 10 s before the P-onset arrival time. Firstly, to broaden the response of short-period instruments into a more useful teleseismic frequency band, the instrument response is denconvolved from the original records. ZNE components are then rotated into the local LQT ray-based coordinate system (using theoretical back azimuth and incidence angle), in which L shows the direction of P wave incident to the surface; Q is perpendicular to L and T is perpendicular to both L and Q forming the third axis of the right-hand LQT system. To isolate the P-to-S conversions on the Q component, the L component is deconvolved from the Q component. A band-pass filter of 2-10s is applied to the P receiver functions (PRFs). They are stacked after move out correction for reference slowness of 6.4 s/°. P-RFs are sorted by increasing back azimuth. A notable feature, which can be observed underneath all stations, is the presence of a significant sedimentary layer at about 0.2-1.2s delay time. The middle crustal layer at about 2.3-4.0s delay time can be also seen beneath all stations. The most coherent conversion is however the conversion at the Moho boundary arriving between 5.6-6.6 s delay time. This conversion can be clearly followed in the individual traces as well as in the stacked traces. The minimum arrival time of the Moho converted phase (5.6s) is observed beneath the station MOK located in the northeastern part of the area. Even though, the largest arrival time (6.6 s) is seen beneath the station PAR located in the northwestern part of the region. Moho depths are obtained by using an average crustal P wave velocity of 6.3 km/s and a Vp/Vs ratio of 1.73. This keeps our depth values independent from possible errors of preliminary shear wave velocity models. However, we estimate the deviation to this model to be less than 5%. Therefore, this procedure results in a ±2 km error in the Moho depth determination. For station KAZ, we couldn’t obtain P-RF because of low signal-to-noise ratio. We have used the arrival times of crustal multiples for determination of crustal thickness (H) and Vp/Vs ratio. This was done using Zhu and Kanamori method, which performs a grid search through the H and Vp/Vs space and searches for the largest amplitudes at the predicted times of direct conversions and multiples. We used weight factors of 0.5, 0.25, 0.25 for the Moho conversion and multiples, respectively. The average Moho depth is 49.5 km and varies between 46.0 km and 56.5km. We observed that Moho depth decrease from north to south. The thinnest crust was found beneath MOK station whereas the deepest crust was observed beneath PAR station. The Shiraz region crust has an average Vp/Vs ratio of 1.73, with higher ratio of 1.74 in MOK station and lower ratio of 1.70 in SHI station. 2D migrated PRF section depth-distance obtained from a profile perpendicular to the strike of Zagros (NE-SW), shows the Moho boundary varies from 45 km to more than 50 km.

Crustal Velocity Structure Model has a significant role in truly understanding of seismicity and also in relocating earthquakes. On the other hand, it can be used for recognizing major and potential seismic sources which is very critical for seismicity and earthquake hazard assessment studies. Seismic parameters simultaneous inversion modeling is one of the most prevalent methods in the study of seismic velocity structure. This approach optimizes the coordinate parameters, time of the events and the velocity structures simultaneously by processing the initially assumed values. The resulted velocity model can be used for relocating the seismic events, registered on the local seismic network, and locating future seismic events as well as establishing the future seismic tomography studies. In this study, the simultaneous inversion modeling and VELEST software were used in order to find an optimum crustal velocity model. located in east of Caspian Sea, north east of Iran and south of Touran plate tectonics, the Kope Dagh region lies within a broad zone of deformation and forms part of Alpine-Himalayan orogenic belt which is actually the conjunction zone of Touran and Iran plates. This region is separated from Touran plate by Main Kope Dagh Fault from the north, and its southern boundary is assumed Sabzevar Reverse Fault and Mayamey Reverse Fault. Our study area covers Sabzevar, Mashhad, Shirvan and Quchan cities. Quchan is located in the central Kope Dagh region and has experienced four destructive earthquakes in the past centuries (1851, 1871, 1893 and 1895). These earthquakes caused widespread devastation and heavy human loss in Quchan and many surrounding villages. To estimate the layer configuration, velocities and thicknesses; we used first arrival travel times. More than 14000 first arrival data related to 2200 seismic events, registered by the local seismic network, were considered. They are all registered in the stations located less than 400 meters of epicenters. First arrival times were plotted versus their distances. The chart suggests three major layers; therefore we decided to fit three lines on three sections of the chart which are 50-100 meters, 120-180 meters and 190-350 meters, using least square method. By inversing the slopes of these three lines, we calculated the mean velocities for the three layers which are 6.01, 6.36 and 8.10 km/sec related to 0-20 km, 20-46 km and more than 46 km respectively. The second anomaly is corresponding to Moho Depth so it shows thickness of the crust in the study area. We used this resulted information as an initial velocity model to prepare numerous models needed for VELEST software runs. In the next step of our work, in order to improve resulted velocity model, we used VELEST for seven run groups, each containing 20 independent runs using 20 initial models. These initial models are created by a FORTRAN program in a way that 20 initial models have all same thickness but different velocities which are restricted in defined intervals. We considered the convergence of the resulted models in each group to select one as the best run and then to determine a proper velocity model and Moho Depth as well. Hence the third group was selected as the best run group and therefore the related Moho Depth is 46. It is exactly the same as Moho Depth resulted from the first arrival travel times. In the final step, we used the resulted velocity models in the previous step and calculated their mean values as the mean velocity model. This model was used as another initial model for the final run of VELEST program, but in this run we added several layers to initial model so that their velocities increase regularly. The calculated model, by the VELEST, is very similar to the mean resulted model of the second step. We determined this three layers model as an optimum velocity model for the study area in which the thicknesses of layers are 5, 10 and 31 and velocities of P-wave in these layers are 5.95, 6.1 and 7.97 km/sec respectively. Quchan station is determined as our origin station, therefore its time correction was assumed zero. The most time correction resulted by the final VELEST run is related to Moghan Station.

The investigation of this paper focuses on eastern part of Shahroud fault system in middle-east Alborz. This fault system is an important part in the seismotectonic map of the area. We used two local temporary dense seismological networks data installed around the fault system for several months during 2007 and 2008 and simultaneously micro-earthquakes data recorded by the permanent seismological network of the Geophysics Institute of University of Tehran were used. The seismicity of both networks has overlapping with the surface outcrops and the depth of Shahroud fault system faults, mainly Astaneh fault. Processing the data provided us a P wave velocity range within the east Alborz which resulted discontinuities like seismogenic zone thickness, 24 km. An initial estimation of Moho depth located at 34 km, near vertical and north dipping seismicity dips corresponding with the Astaneh and North Semnan faults respectively were the other results. Faults dipping and seismogenic zone thickness do not support the flower structure hypothesis at the east Alborz in spite of some author's idea. A few focal mechanisms indicated left-lateral motion and confirm high angle of the faults planes. The crustal movement directions resulted from P and T vectors show well correspondence with the GPS measured direction in the area. The b-value which could be considered as inverse short term background seismicity intense, was determined about 0.9.

In this study we use the P receiver function technique to determine the Moho depth and Vp/Vs ratio for 8 short period stations of Qochan and Mashhad seismic networks and map the variations of Moho depth under Kope Dagh region. It is shown that a receiver function can provide a relatively good point measurement of Moho depth under a short period station. The crustal thickness estimated from the delay time of the Moho P-to-S converted phase trades off strongly with the crustal Vp/Vs ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We use a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios (zhu & kanamori,2000). This transforms the time domain receiver functions directly into the H-Vp/Vs domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and Vp/Vs ratio are found when the three phases are stacked coherently. Applying this technique to 8 stations in Kope Dagh region reveals that the Moho depth is approximately 45 km on average and varies between 41 and 49 km. Thick and thin crust are found under the southern and northern Rang, respectively. These results are in good agreement with the geology and tectonic setting of this region.

The Alborz mountains, as a folded and faulting region, is the northern region of crust shortening in Iran. Dominant mechanism (Harvard, 2002) of Alborz earthquakes is left-lateral strick-slip paralleled to the range. East Alborz is one of the active regions that plays a significant role in the tectonics of its neighbors like South Casbin Basin. Regarding the geological maps of the area, the faults have clearer outcrops in the east than the west. The strike of mountain range is changed from N110°E in the western part to N80°E in the eastern part. The Astaneh is one of the important faults in the Shahroud fault system in the east Alborz. Astaneh fault, the case study of the local networks, with about 150 km length is clearly seen in satellite pictures especially at the eastern segment. and is about 100 km in length, based on 1:250000 geological map of Semnan (Samadian et al., 1975) and Sari (Vahdati and Saidi, 1990). This fault has made a 30-40 km left lateral pull-apart basin near 53.6°E; this is the same with total left lateral offset, found by reconstructing Cambrian sandstone of the Lalun Formation with the slip rate of 3-5 mm/year. There is 45 (m) left lateral alluvial fan deposit displacement near 54°E (Hollingsworth et al., 2008). The offset dated from the last incision event at about 12 ka in the east (e.g., Regard et al., 2005; Fattahi et. al., 2006). The fault had initially introduced thrust with southward dip (Berberian, et al., 1996) and now is considered as left lateral strike slip (Jackson et al., 2002). The local government responsible say that about 2 million people live in and around the mentioned faults in Semnan, Damghan, Astaneh, Kiasar, Firuzkuh and Veresk cities, which may be affected by the activity of the fault in future. The faults in the Shahroud Fault system were mapped and their surface mechanisms are known, but their related geometry at depth and seismicity are not known. Many important historical earthquakes have occurred in the east Alborz. The most destructive earthquake was historical A.D. 856 Qumes with the estimated magnitude of 7.9 (Ambraseys and Melville, 1982). The distribution of the IGUT recorded earthquakes, because of large station spacing, can not show accurate relationship between seismicity and the faults, also IGUT large station spacing is not appropriate for computation of focal mechanism of the local earthquakes. The master event technique helps increase the accuracy of the teleseismic earthquake location (Engdahl, et al., 1998), but there are only a few events have been relocated in the studied area (small circles in fig.1), because this technique requires large earthquakes with the magnitude greater than 6.5 (Jackson and MacKenzie, 1984). Also the World Wide Seismological Station Network (WWSSN) has just been installed since 1966. Several attempts have been made to solve the earthquakes smaller than 6.5 (Shirkova, 1972; Akasheh and Breckhemer, 1984); but their solutions are not stable, so consequently they could not give a correct explanation because the majority of original data were not available. Therefore the remaining reliable solutions are Centriod Moment Tensor (CMT) at Harvard University (Harvard, 2002) and body waveform modeling (Priestley, et al,. 1994) for the earthquakes with a magnitude greater than 5.5. But there is no earthquake in the studied area with known mechanism except the earthquake of 1990/01/20 with left lateral strike slip mechanism related to the Firuzkuh fault at a longitude of 52.9°E and latitude of 35.8°N (Harvard, 2002). This paper investigates crustal velocity structure and micro-earthquake locations to explain kinematics and seismic activity of the faults in the studied area. We used IGUT network with 10 permanent stations and two local networks each with 10 temporary stations to investigate a selected area in the east Alborz. The duration of the local networks, 2007-2008 network and the 2008 network, was in total months. The temporary stations were visited every week for maintenance, checking their power supplies and internal time against the time of the external GPS of the instruments. First, we estimated the Vp to Vs ratio (1.71) by the Wadati method (Wadati, 1933). Then we determined the crust velocity model using local earthquake arrival times by 1D inversion (Kissling, 1988). Totally 121 events recorded with minimum 8 phases, maximum azimuthal gap of 180°, RMS less than 0.3 s and both horizontal and vertical uncertainties less than 2.0 km were used for computing velocity structure. We processed the inversion in two steps, after testing a few thousand multilayer models, in order to see the convergence of the inversion to a unique velocity model, first 50 random models were tested. These models were stacked with 15 layers of 2.0 km thickness from the surface to 30 km depth, with maximum 0.5 kms-1 velocity change for each layer and with the uniform starting velocity of 6.0 kms-1. Those thin layers allowed us to determine the approximate depth and velocity of the real layers. We suggested a three-layer model with two velocity contrasts located at 4 and 12 km depth over a half space. After merging these layers with a similar velocity then the starting model was repeated with the mentioned layers and the same velocity of the first step. The final selected velocity model is 5.4 and 6.0 km/s for the mentioned layers over a half space with a velocity of 6.3 km/s. Because the majority of the well located events were located shallower than 20 km, we could not determine any layer beneath this depth using inversion. We selected 834 events with a minimum of 6 phases, a RMS less than 0.5 s and a horizontal and vertical uncertainty less than 5.0 km from a total of 1443 events that were recorded by the IGUT network during this period. Regarding the distribution pattern of these earthquakes, the seismic activity is located near Astaneh, Firuzkuh, Mosha, Garmsar, Khazar, North Alborz and North Semnan faults. Also the temporary networks totally recorded 1972 earthquakes during this period. The statistics of the local network earthquakes show that the location error and RMS of about 60% of them are less than 3 km and 0.3 s respectively, but only about 40% of them are inside the networks (with an azimuthal gap less than 180°). We selected 339 earthquakes which were recorded with minimum 6 phases, a RMS value less than 0.3 s and both horizontal and vertical uncertainties less than 3.0 km. For this selection the average of location uncertainties are 1.25 km, depth of seismicity is 8.5 km, the number of the phases read for locating is 12 and RMS of time residuals is 1.4 s. Distribution of these earthquakes shows that the horizontal dimension of the two located clusters with the new model is almost the same as the length of the fault, about 100 km, has good correspondence with the fault and shows the activity of Astaneh two segments. The depth histogram of the earthquakes shows that the majority of the events have been located between 4 and 14 km. To constrain the geometry of the Astaneh fault, we plotted two cross-sections both perpendicular (in section points) to the Alborz range tectonic structures and Astaneh fault. All of the seismicity dips concluded from the depth distribution have high angle and southeast dipping but have more vertically beneath the southwest segment of the Astaneh fault than the east segment. Because of having no earthquake deeper than 23 km, the seismo-genic zone in the area was not greater than 23 km deep. We were also able to estimate the sedimentary cover thickness about 4 km.

North-East Khorasan is one of the most active regions in the world because of is setting on the Alpine-Himalayan belt. Historical and geological backgrounds suggest that this region has experienced many destructive earthquakes throughout history. Compared with the historical background, the seismicity of the region, in the present century, is better known both from the macroseismic and instrumental point of view. The instrumentally located earthquakes suggest that seismic activity in the present century has increased remarkably. A better understanding of crustal velocity model could help to improve the location of earthquakes and to find out the active faults and the tectonic evolution in the region. In this study we used the travel times of local earthquakes recorded by the seismic networks of Quchan and Mashad operated by the Institute of Geophysics, University of Tehran, to investigate the crustal velocity structure in the Khorasan region. In this study, among all recorded data during 1997-2006, we selected and used the records of 103 earthquakes that were recorded by at least four seismic stations. For these selected earthquakes, the azimuth coverage was less than 270 degrees; RMS less than 1 second and the location error was less than 5 km. The travel times obtained from these earthquakes were used to find out an appropriate velocity model. First we applied the VELEST method and used the travel time data to obtain the one dimensional velocity model. We applied random velocity variations of about ±0.5 Km/s in each crustal layer and produced fifty preliminary models. We selected those models that indicated acceptable convergence during the inversion process. Then, by inversion, we obtained a preliminary three layer model. Next, we used this model as initial value to find out the appropriate velocity model. The final result indicated a simple two layer model. This model contains a first layer having a thickness about 10 km and a velocity of 4.5 km/s over the second layer that has a velocity of 6.2 km/s. As we used local data and the earthquakes had shallow depths, we could investigate the structure down to 20 km. This result is in good agreement with the results of other studies in this region In general, the one dimensional inversion of travel time data for crustal velocity structure is sensitive to the number of seismic stations and the distance between the successive two stations. The results of this study indicate that if a good data set is available, the one dimensional inversion of travel time data is an appropriate method for the study of crustal velocity structure.

In this study, the 2005 Dahuieh (Zarand) locally recorded aftershock sequence has been analyzed. Having the distribution of aftershocks and the source extension, a W-E trending near vertical faulting with an extension of about 15-20 km could be estimated. The rupture causing the powerful Dahuieh earthquake apparently initiated in the modified epicentric area and propagated unilaterally towards the west. The cross section of aftershocks perpendicular to the fault suggests that the aftershocks had a depth range about 20 km, indicating that the seismic activity took place within the upper crust and the seismogenic layer, in this region, which had a thickness not greater than 20 km. The focal mechanism of the main shock and right lateral motion of the Kuh-Bannan fault suggested that the earthquake fault must be reverse and the northern block acted as a hanging wall during the source process of the main shock. The epicenteral distribution of aftershocks showed a lack of activity that was interpreted as the modified location of the main shock. Our results are in agreement with waveform modeling. The time frequency pattern of the aftershock decay followed the Kisslinger stretched exponential descending formula.

Characterization of the detailed structure of the crust and upper mantel is an importantcontinuing goal of geophysical studies. There are a variety of geophysical methods(seismic refraction, seismic vertical reflection and seismic tomography) to investigatesubsurfaces. The teleseismic P Receiver Function (RF) method has become a populartechnique to constrain crustal and upper mantle velocity under a seismic station.Teleseismic body waveforms recorded at a 3-component (Z, N-S, E-W) seismic stationcontain a wealth of information on the earthquake source, the earth structure in thevicinity of both the source and receiver, and mantle propagation effects. The resulting RFis obtained by removing the effects of the source and mantle path. The basic aspect of thismethod is that a small percentage of the incident P wave energy from teleseismic eventswith significant and relatively sharp velocity discontinuities in the crust and upper mantlewill be converted to S wave (Ps), and arrive at the station within the P wave coda directlyafter the direct P wave. To obtain P-RF, the following steps are generally used: to utilizedata recorded with different types of seismometers, the instrument responses have to bedeconvolved from the original records. ZNE components are then rotated into the localLQT ray-based coordinate system (using the theoretical back azimuth and incidenceangle).To eliminate the influence of the source and ray path, an equalization procedure isapplied by deconvolving the Q component seismogram with the P signal on the Lcomponent. The resulting Q component data are named P-RF. An advantages of the RFmethod is that, because the P-to-S conversion point is close to the station (usually within10 km laterally), the estimation is less affected by lateral velocity variations. Theestimation provides a good point measurement at the station because of the steepincidence angle of the teleseismic P wave. Since the direct P arrival is used as a referencetime, it can be shown that the result is not sensitive to crustal P velocity. We compute P receiver functions to investigate the crustal thickness and Vp/Vs ratiobeneath the East of Iran (Birjand) and map out the lateral variation of Moho depth underthis region. We selected data from teleseismic events (Mb ≥ 5.5, 30˚<Δ<95˚), recordedfrom 2005 to 2009 at 4 three-component short period stations from Birjand SeismicTelemetry Network. These stations are equipped with SS-1 seismometers with a naturalfrequency of 1 HZ. The data is recorded at 50-samples-per-second. First of all, iscalculated 247 P-RFs for TEG, KOO and DAH stations and then estimated the Mohodepth solely from the delay time of the Moho P-to-S conversion phases. Then, we used an H-Vp/Vs stacking algorithm to estimate crustal thickness and the Vp/Vs ratio under each station. The best value for the H and Vp/Vs ratio are found when the three phases (Ps and crustal multiples) are stacked coherently. The results obtained from the P receiverfunctions indicate clear conversions at the Moho boundary. A notable feature, which can be observed underneath all stations, is the presence of a significant sedimentary layer at about 0.7-1s delay time. The middle crustal layer at about 1.9-3.3s delay time can also be seen beneath all stations. The most coherent conversion, however, is the conversion at the Moho boundary arriving between 4.7-5.4s delay time. As a result of measurements using the Zhu and Kanamori (2000) method, the average Moho depth is found to be approximately 41 km and to vary from 38.5 to 44 km. The crust is relatively thin beneath the DAH station, whereas the thickest crust was observed beneath the KOO station, located southwest of the study area. The crust of Eastern Iran has an average Vp/Vs ratio of 1.76, with a higher ratio of 1.84 in the TEG station and lower ratio of 1.76 in the KOO station.

The Bojnourd earthquake occurred in a mountainous area in North Khorasan provinc. The mainshock produced extensive destruction. Field investigation and aftershocks distribution suggest a NW-SE trend faulting. The distribution of locally recorded aftershocks was extended to a length of about 40-50 km and a depth of about 30 km. Aftershock activity was scattered indicating a complex mode of faulting. The result of waveform inversion indicated that the mainshock followed mainly strike-slip mechanism and the source process included at least two main fault slip. The source time functions indicates that the major amount of seismic energy was released within the first 10 seconds. Considering the field observation, the distribution of aftershocks and the source mechanism, an average source dimension of about 45 km, a NW-SE strike and a SW dipping fault plane could be estimated. The mechanism for the total source is obtained as (strike, dip, rake) = (323, 89, 178). The total seismic moment was calculated to be M0= 6.7×1025 dyne cm. The calculated maximum dislocation was about 50 cm and the obtained moment magnitude was Mw = 6.5. The average stress drop was estimated to be 25 bar and the average dislocation was 25 cm.

On 26 December 2003 at 05:26LT (01:26GMT), a catastrophic earthquake Mw 6.6, seismic moment (6 – 9 ×10 Nm) struck the city of Bam in the Kerman province of southeast Iran. The intense shaking in the city caused the complete collapse of nearly every building in the central parts of the city including many of the newer buildings, killing about 40000 people officially. The city lies to the east of the Nyband – Gowk – Sarvestan fault system and on which several powerful earthquakes have occurred over the past 23 years [Berberian et al., 1984; Berberian and Qorashi, 1994; Berberian et al., 2001]. There are no recorded historical earthquakes at Bam. Most of the citadel of Arg-e-Bam, one of world heritage sites inscribed by UNESCO, which was constructed by mud brick about 2000 years ago was destroyed in this earthquake.

In this article, we studied the dynamic fracture process of Bam earthquake. In two presented models stress heterogeneity on the fault plain was modeled as barrier or asperity and friction included as slip-weakening relationship. Results of models were constrained by near field ground motion recorded in Bam station. In the first model, fracture starts form a weak asperity which waves surround the neighbor barrier and break it down. In the second model, another asperity is included in southern part of the fault. Breaking barrier releases two fracture fronts traveling in two different regimes. One of them travels faster than shear waves and goes to the intersonic velocity. The other front travels with 0.74 shear wave velocity and makes the largest pulse of the record. Both models predict the slip rate successfully, but the second model is more consistent with the real data.