نوربخش میرزایی

Iran is a wide compressional deformation and seismic activity zone along the Alpine-Himalayan orogenic belt resulting from the conversion motion between the stable Arabian and Eurasian plates. Northwestern Iran is part of a complex tectonic system within the Arabia-Eurasia collision zone. The main active fault of northwestern Iran is the Gailatu–Tabriz strike-slip fault system (GTFS) that extends ~400 km in length from north of Mianeh (a town in the East-Azarbaijan province of Iran) to the southwest and south of Kaghsman (in Turkey) to the northwest. It has a conspicuous history of seismicity and a controlling role in the geodynamics of the region. In this study, we utilize satellite images, DEM images, field evidence, earthquake information, and GPS data, to investigate the active tectonic characteristics of the GTFS. From the southeast to the northwest, GTFS consists of three main fault zones, named: North Tabriz Fault, Mishu-Tasuj Fault, and Gailatu-Siah Cheshmeh-Khoy Fault. Near Kaghsman, the northwestern end of the GTFS forms a horsetail splay structure, with many faults having normal components, and to the east, GTFS merges with the Bozghush fault zone. GTFS shows a variety of transtension and transpression tectonic structures (stepovers, bendings, pull-apart basins, and splay structures) formed in the dextral shear zone. North Tabriz fault zone is characterized by three main NW striking right-stepping en echelon segments (Bostan Abad, Shebli, and Tabriz fault segments) and is known as the causative fault of three destructive historical earthquakes on 1042/11/04 (Mw 7/6), 1721/04/26 (Mw 7/7), and 1780/01/08 (Mw 7/7). To the east, it joins the Bozghush thrust fault zone that caused the 2019/11/07 (Mw 6/0) earthquake on its Shalgun-Yelimsi left-lateral strike-slip fault segment. In the central part of the GTFS, the Mishu-Tasuj fault zone is formed as a transpressional bend. The macroseismic epicenter of the 1786/10 (Mw 6/2) earthquake is located near this fault zone. Thrust faults in the southern part of the Mishu-Tasuj fault zone are parallel with close distances, and have uplifted the land masses; probably representing the migration of thrust faulting into the southern plains; similar to the Esfarayen and Sabzevar thrust faults in northeastern Iran. Four pull-apart basins have been created due to the movement of fault segments along the Gailatu-Siah Cheshmeh-Khoy fault. The current kinematics of the GTFS plays a key role in the tectonic of northwestern Iran and accommodates part of the convergence movement between the Eurasian and Arabian plates. Earthquake history and geometry of different segments of the GTFS imply seismic gap, especially on the North Tabriz fault, and faults interaction (e.g., between Shalgun-Yelimsi left-lateral strike-slip fault and south Bozghush thrust fault, and Gailatu-Siah Cheshmeh-Khoy right-lateral strike-slip fault and Tasuj thrust fault) which are important issues in seismic hazard in northwestern, especially for Tabriz City with a population of about 1.5 million.

The area bounded in 46º-48º E and 34º-36º N is selected for probabilistic seismic hazard analysis of Kermanshah region utilizing the Monte Carlo method. Potential seismic sources model includes 7 clusters obtained from the combination of weighted K-means clustering and particle swarm optimization (PSO) method. The peak ground horizontal acceleration (PGA) and spectral acceleration (SA) for 5% damping ratio at 0.2 and 2 seconds corresponding to 10% and 63% probability of exceedances within 50 years are calculated for the region.

The amplitude of seismic waves is generally reduced while passing through the earth under the influence of the two main factors of geometrical spreading and apparent attenuation of seismic waves. Scattering attenuation, as an elastic phenomenon, redistributes seismic wave energy due to the collision of seismic waves (P, S and surface waves) with randomly distributed heterogeneities. The attenuation of coda wave, backscattered waves form heterogeneities, is one of the most important parameters in the estimation of seismic wave attenuation. In this study, the attenuation of seismic coda waves as scattered body waves has been estimated using single back-scattering model.

The most common method to estimate attenuation is single back-scattering model. Here, we use short-time Fourier transform (STFT) instead of bandpass filter. For each individual frequency, the envelopes of STFT coefficients of extracted waveforms for lapse times of 30, 40, 50, 60, 70, 80 and 90 seconds are measured as the BPF method. The obtained envelopes have been used in order to estimate attenuation at each individual frequency. Since the length of the window is constant in the STFT method, it is possible to determine fairly accurate frequency in a short window. However, to this purpose, each waveform is divided into 1 s windows (50 samples) overlapped by 90% (45 samples), and the STFT has been calculated for each window.

In this study, attenuation parameter has been estimated in northwestern Iranian plateau using the STFT method. The results show good correlation between seismicity and tectonic of the study area for lapse times greater than twice the S- wave travel time, especially for lapse time of 40 s. At lapse time of 40 s, the frequency-dependent relationship has been estimated as Q = (103±1)f(0.88±0.04) using the STFT method. It is concluded that the STFT model can be used as an appropriate time-frequency tool to study the energy attenuation of high-frequency coda waves due to the high correlation coefficients and low standard deviations of the relationship. Furthermore, the relatively low values of quality factor in frequency attenuation relation show that there is high heterogeneity among shallow layers of Iran northwestern region that is possibly due to the relatively high seismicity of the region.

Earthquake early warning parameters play an important role in issuing of a warning and reducing casualties caused by earthquakes. Site effects are one of the factors that affecting the values of earthquake early warning parameters (, , ,  and B). In this study, in order to investigate the site effects on the values of earthquake early warning parameters, 1830 earthquake accelerograms recorded at the six KiK-net accelerometery stations in Japan between 2008 and 2020, were used. These accelerograms were recorded at the surface and borehole stations. Short time Fourier transform (STFT) as well as deconvolution tool were used to remove the site effects from the accelerograms in the time-frequency domain. After removing the site effect from the waveforms of the surface stations, the values of all the earthquake early warning parameters, especially ,  and B, were improved and became closer to the values obtained from the bedrock. The results show that the early warning parameters  and  have the minimal influence from site effects and also behave on surface and borehole stations similarly. In general, the values of the frequency-dependent early warning parameters  and  increase after modifying the site effect on the vertical component. In contrast, the parameters , and  were reduced at all stations. After modification of the site effect, the difference between the calculated values of all parameters on the surface and bedrock are decreased, which indicates the improvement of the results of earthquake early warning systems in estimating the earthquake source parameters.

There are two different seismotectonic zones in around of the Zendan- Minab- Palami (ZMP) fault system and the Oman Line, in south of Iran (Makran subduction zone in the east and Zagros collision zone in the west), which led to the complexity of this region. Since studying the stress field is important for accurate perception from elastic features of environment, surveying the exerted the tectonic stresses to the tectonics plates and their magnitude, and description the geodynamic of this region, in this study considerd to assessment of stress field and also, maximum horizontal stress (SH) in around of ZMP fault system. To receive this purpose, amount and direction of stress is calculated by iterative joint inversion of earthquake focal mechanism. From east to west of ZMP fault system, with transition from Makran subduction to Zagros collision, direction of SH is reduced from 5.09º in east to 0.9º. To surveying the strain field, we used Global Positioning System (GPS) data. Maximum variance between velocity vector and direction of SH is determined in Bandar-Abbas (BABS) station, that located in adjacent of ZMP fault system. The friction coefficients which obtained in this study show that friction in Makran zone is more than Zagros zone.

Presently, seismic hazard assessment (SHA) is highly conducive to seismic-resistant building designs and seismic regulations. Seismic hazard maps and other seismic hazard products such as spectral acceleration (SA) are prerequisites for the preparation of building codes and earthquake risk mitigation plans, which are used in making public decisions and policy. Therefore, it is indispensable to use the best available data and methods in SHA. In cases of incomplete historical records like Iran, and in intracontinental areas like Central-East Iran, where fracture boundaries interact slowly, large earthquakes may recur every 1000–5000 years or even with a longer period. In such areas, it is useful to employ geological inputs like slip rates. In the present paper, the slip rate of the faults is used for the first time, in both direct and indirect approaches in time-independent probabilistic seismic hazard assessment (PSHA), so as to evaluate spectral acceleration (SA). To this end, Kerman region (Southeastern Iran) is selected as an example, and Kerman and Ravar cities located between 54-59° N, and 28.5- 34° E are considered as specific regions to show the effects of slip rate on the seismic hazard. With the purpose of using slip rates in PSHA, indirectly, we have defined a new factor denoted as the fifth factor (K5) to specify the effects of slip rates in calculating spatial distribution function (SDF). On the other hand, the mean annual occurrence rate of each source may be directly calculated based on the slip rate of the faults for a direct use of slip rates in PSHA. In the first time-consuming stage, the slip rates of the faults or fault segments and the seismological data are assembled using available resources and literature. Seismicity parameters in the targeted region are calculated using a unified, homogenized and complete catalog in the method proposed by Kijko and Sellevoll (1992), in which one can consider the magnitude uncertainty and completeness of data in calculations. Through the use of geological maps with scales of 1:100000 and 1:250000, and with the experience of previous studies, we have determined 26 potential seismic sources in the region. The comparison of the SDFs calculated based on four factors (K1-K4) and SDFs calculated based on slip rate factor (the fifth, K5) accompanied with the previous four factors indicates that the most differences occurred for sources No. 111 and 121 for the magnitude of 7

To assess seismic hazard, it is essential to estimate the potential seismicity, particularly estimation of size of the largest earthquake that can be identified by a distinct fault or earthquake source. One of the methods applied to estimate magnitude of earthquake is use of empirical relationships between magnitude and fault parameters. Fault parameters for the earthquakes from early instrumental period to 2014 are compiled to develop a series of empirical relationships among M_w and M_s with surface rupture length, horizontal and vertical displacements. The objective of this study is provision of an accurate piece of information about previous earthquakes through studies and investigations as well as a comprehensive and through catalog of the recent earthquakes so as to represent precise empirical relationships between the magnitude and earthquake fault parameters. Moreover, the information about earthquakes with magnitudes greater than M_w and M_s ≥5.5 were selected. In the beginning, the relationships between magnitudes and fault parameters were acquired, then the database on the basis of three slip types, consisting of strike-slip, normal and thrust, were manipulated and eventually relations were obtained. One also should not overlook the fact that three regression models, including SR, ISR and OR, were employed in this study. In SR and ISR methods, no error are considered for independent variables, whereas the mentioned error is taken into account in OR method. The OR method is obtained by minimization of the squares of the orthogonal distances to the best-fit line, whereas SR is derived by minimizing the squares of the vertical offset and also inverted standard least-squares regression (ISR) is derived by minimizing the squares of the horizontal offsets. However, roughly equal uncertainties for the two variables are regarded in the SR and OR methods. According to the obtained results, the best relationship between M_w and M_s with surface rupture length was established with correlation coefficients of 0.87 and 0.86, respectively. Also the relationships between M_s with maximum horizontal and vertical displacements with correlation coefficient of 0.63 and 0.62 in a respect way, are far better than the relationships between M_w with maximum horizontal and vertical displacements with correlation coefficient of 0.59 and 0.59. In addition, the relationships between magnitudes (M_w or M_s) and maximum horizontal displacements indicate a better fit than the relationships between the magnitudes and maximum vertical displacements. It is also worth mentioning that the best fit between M_w or M_s and surface rupture length was acquired with correlation coefficients of 0.87 and 0.86, respectively, by separating the database based on slip type. Whatever was mentioned about relationships between magnitudes and maximum horizontal and vertical displacements, is still valid by separating the database on the basis of slip type. According to the results of the direct relations, the SR and OR regression methods are far better than the ISR regression method with less error than ISR regression method. What’s more, in inverse relations, the ISR regression method estimates the coefficients with the lowest error rate in comparison with other methods. As an outcome, the findings were established from the current study are better than the global relationships for Iran and its adjacent regions. For example, in the light of the relationship between M_s and surface rupture length, the utilized global relation has been overestimated and then underestimated up to M=7.2 with respect to OR regression. The obtained relations have been simultaneously compared to two global relations within relationship between MS and surface rupture length. The results would seem to suggest that both global relations are overestimated and then underestimated up to M=6.5 in comparison with other regression methods.

In this study¡ a modified “probabilistic seismic hazard assessment” (PSHA) method is used to estimate the level of the potential seismic ground motion in Firoozkouh. A problem that may be encountered in probabilistic studies of seismic hazard for a specific site¡ for engineering purposes¡ is the selection of design earthquakes corresponding to a given hazard value. In order to derive a seismic scenario consistent with the results of PSHA for a site and determine the relative contribution of events to the overall seismic hazard¡ the concept of disaggregation was introduced. The disaggregation of seismic hazard is an effective way to identify the scenario events that contribute to a selected seismic-hazard level. In other words¡ the disaggregation process separates the contributions to the mean annual rate of exceedance (MRE) of a specific ground motion value at a site due to scenarios of given magnitude M¡ distance R¡ and the ground motion error term¡ ε. Disaggregation results could change with the spectral ordinate and return period¡ thus more than one single event may dominate the hazard especially if multiple sources affect the hazard at the site. These results can provide useful information for better defining the design scenario and selecting corresponding time histories for seismic design. In most cases¡ as the probability decreases¡ the hazard sources closer to the site dominate. Larger¡ more distant earthquakes contribute more significantly to hazard for longer periods than shorter periods. In this study¡ the seismic hazard disaggregation process is performed to identify dominant scenarios in “peak ground acceleration” (PGA) and 5% damped 0.2 and 2.0 s spectral accelerations corresponding to mean return periods (MRPs) of 50 yr¡ and 475 yr (hazard levels of 63% and 10% probability of exceedance in 50 yr¡ respectively) in Firoozkouh city. In this regard¡ potential seismic sources and their seismicity parameters have been estimated based on the concept of spatial distribution function in 34º–37ºN latitudes and 52º–55º E longitudes in grid intervals of 0.1º. For each point using proper attenuation relationships¡ PGA¡ 0.2 and 2.0 s spectral acceleration values with 63% and 10% probabilities of exceedance in 50 yr have been calculated using the EZ-FRISK (version 7.43) code. The hazard can be simultaneously disaggregated in different types of bin. The result of seismic hazard disaggregation are presented in terms of 1-D M¡ R and ε bins and 2-D M-R bins. Bins of width 0.4 in magnitude¡ 10 km in distance¡ and 0.2 in ε are selected. The disaggregation results in terms of probability density function (PDF) are reported¡ which is obtained by dividing the probability mass function (PMF) contribution of each bin by the bin’s size¡ thus the PDF representation is independent of the bin’s amplitude. The results identify the distribution of the earthquake scenarios that contribute to exceedance of PGA and 5% damped 0.2 and 2. s spectral accelerations for 50 yr and 475 yr MRPs¡ in terms of magnitude and distance (M-R). Dominant scenarios are identified for interest hazard levels in Firoozkouh city.

In this research, using data from earthquakes recorded in Central Kopeh Dagh range, by the Iranian Seismological Center  (http://irsc.ut.ac.ir), we have interpreted Tectonic of the range by calculating seismicity parameters a-value, b-value and fractal dimension. Kopeh Dagh range, is northern zone of deformation caused by the convergence of the Arabian and Eurasian plates. It forms an active tectonic zone trending northwest - southeast, on the border between Turan platform to the North and Lut-Central Iran Blocks to the south. The Map of  b-value, shows that this parameter changes between about 0.6 to 1.1, that is usually seen in tectonically active areas. The fractal dimension of region is 1.75 that expresses the complex and multiple stresses on the range. Maps of  b- value and fractal dimension of earthquake epicenters, shows a clear change in the Bakharden–Quchan fault zone, so that it can be divided into three distinct parts. Parts A and C forms the asperities and part B is located between them. The results show that using the seismicity parameters a-value and b-value, fractal dimension and  local stress distribution maps, provide valuable information about the mechanism of faults and fault systems changes over time.

By growing the population and need for settlement، many cities have been built on soft sediments and seismic areas. It emphasizes the need for a careful and reliable assessment of site effect phenomena. Beyond the methods of studying site effects، microtremor recordings has become popular over the last decades as it offers a convenient، practical and low cost tool to be used in urbanized areas. Besides، in areas of low to moderate seismicity which gathering a significant number of recordings with satisfactory signal to noise ratio is a time-consuming task، microtermor studies are more useful. Lack of accurate knowledge about the nature of microtremor wave field، would lead to misinterpretation of site effects، hence، investigating microtremor wave field is an important goal to achieve. Two techniques are predominantly used to determine microtremor wave field: the array techniques (such as SPAC and F-K methods) and the single station horizontal to vertical spectral ratio (H/V). Array studies have shown that surface waves dominate microtremor wave field، but the relative proportion of Rayleigh and love waves has still been unclear. In this study، single station horizontal to vertical spectral ratio method was applied to investigate the nature of microtremors in south of Tehran. Theoretical aspects of this method، has always been a considerable issue for researchers in this field. Regarding the dominance of fundamental Rayleigh wave mode on vertical component of microtremors، some researchers believe that، if impedance contrast between surface layers and the bedrock tend to be high، ellipticity curves (ellipticity at each frequency is defined as the ratio between the horizontal and vertical displacement eigenfunctions in the P-SV case، at the free surface) of fundamental Rayleigh wave mode shows a conspicuous peak around the site resonance frequency. It is due to the vanishing of vertical component corresponding to reversal rotation of fundamental Rayleigh wave from retrograde to prograde، In contrast، some other researchers do believe that the SH resonance in surficial layers (removing the effects of surface waves) accounts for H/V ratio peaks. Data used in this study was recorded by Haghshenas et al. (2003) using continuous recording for a period of five months in 13 seismological stations. The results of two stations are shown here. Geopsy software (www. geopsy. org) is used to analyze microtremors. First، particle motion behavior in microtremor wave filed was studied. The results showed an elliptical behavior that can be related to predominance of Rayleigh waves in microtremor wave field. It should be mentioned that if body waves dominate the wave filed، the particle motion will show a linear behavior which is not observed in our study. Then، spectrum amplitude curves were obtained. To compute H/V for each time window، root mean square of two horizontal amplitude spectra is divided by vertical amplitude spectra at each point. H/V curves showed that site resonance frequency varies from 0. 3 to 5 Hz in south of Tehran. Our study revealed that the peaks at site resonance frequency were localized by minima in vertical amplitude spectra as well as by maxima in horizontal amplitude spectra. To study dispersion property of layers beneath each station، shear wave velocity variation with respect to depth was investigated. Dispersion curves were obtained based on earth models. It was shown that the under-structure layers are dispersive. Take into consideration that Tehran has a complicated geological state and lack of borehole information، Jica & Cest report (2000) was used to obtain earth models. Jica & Cest report (2000) includes only thickness and type of layers، while، the shear and longitudinal wave velocity is needed. Jica & Cest report (2000) contains Standard Penetration Test values according to the type and thickness of layers and experimental relations between these values and shear wave velocity. These relations were used to compute shear wave velocity under each station. Longitudinal velocities were computed by the relation proposed by Lay and Wallace (1995). Finally، ellipticity curves of Rayleigh waves were modeled and compared with H/V curves. The same earth models of previous step were used to model ellipticity curves. The ellipticity curves showed a conspicuous peak around the site resonance frequency. These could be due to the reversal motion of fundamental Rayleigh wave mode from retrograde to prograde. To sum up، it could be said that in south of Tehran، fundamental mode of Rayleigh wave accounts for H/V ratio peak.

The spectral acceleration، SA، is used as the most commonly tool for the building response analysis. In many international or national engineering standards/codes such asInternational Building Code (IBC) and Iranian Building Code (standard 2800)، the buildings are desiged on the basis of spectral acceleration method. This method is based on orthogonal functions and has received much attention because of its simplicity. The target of this study was to estimate the spectral accelerations of ground motions due to earthquakes and compare them with the results of the standard No. 2800 and IBC in Isfahan and the adjoining regions (the area between 49. 5-54° E and 31-34. 1° N). The spectral accelerations، which are the input data for building design، are determined via spectral attenuation relationships by multiplying the response factor given in Iranian Building Code (standard 2800) to the Peak Ground Acceleration (PGA) determined from attenuation relationships. To this end، first، the seismicity parameters in the interest region for each seismotectonic province were calculated using a unified، homogenized and a complete catalog in the method proposed by Kijko and Sellevoll (1992)، in which one can consider magnitude uncertainty and completeness of data in calculations. Geological maps with scales of 1:100000 and 1:250000 were used to provide the fault map of this region. We determined several probable faults in the interest region to help us to introduce the potential seismic sources more precisely. Based on the fault maps، the potential seismic sources of the interest region were determined (twelve and six potential seismic sources for Zagros and Central-East Iran provinces، respectively). Then، the maps of the maximum and the spectral acceleration seismic zonation were prepared using a modified probabilistic approach (Shi et al.، 1992) as well as the spectral attenuation relationships of Campbell and Bozorgnia (2003) and also Ambraseys et al. (2005). In the modified probabilistic approach، the concept of spatial distribution function is introduced. By calculating the spatial distribution function، the contribution to annual mean occurrence rate of seismotectonic province is made for each potential seismic source. In other words، a spatial distribution function characterizes the seismicity differences among potential seismic sources. The spectral acceleration seismic zonation maps are produced for Peak Ground Acceleration (PGA) and periods of 0. 2، 0. 4، 0. 6 and 1 seconds using the EZ-FriskTM computer program. By multiplying the spectral response given in the standard 2800 to the zonation map of PGA، zoning maps for other periods were obtained based on the spectral acceleration and compared with the results of the direct estimation method. Macrozonation probabilistic seismic hazard maps of the interest region for 10% and 63% probability of exceedance were produced for each attenuation relations. It was shown that the maximum horizontal acceleration in Isfahan city for a 10% exceedance in 50 years، via Campbell and Bozorgnia (2003) and Ambraseys et al. (2005) relations was equal to 0. 1g and 0. 19g، respectively. As the spectral acceleration curve for the Isfahan city showed، the largest horizontal acceleration using Campbell and Bozorgnia (2003) relation obtained as 0. 35g in the period 0. 1 sec، while with the use of the Ambraseys et al. 2005) relations، it was obtained as 0. 61g at period of 0. 11 sec. It means that the frequency content of acceleration derived from both attenuation relationships was the same. In addition، a comparison between the spectral acceleration obtained via attenuation relationships and the one derived from the IBC and standard 2800 of Iran in different periods، exclusively for the city of Isfahan showed that up to 0. 1 sec، the spectral acceleration of standard 2800 of Iran and the values obtained from Ambraseys et al. (2005) relations were substantially close to each other. However، the spectral accelerations derived from Campbell and Bozorgnia (2003) relations have a lower value than those obtained based on standard 2800 and Ambraseys et al. (2005) relations. Considering these results as well as the less involvement of Iranian event data in Campbell and Bozorgnia (2003) attenuation relations than Ambraseys et al. (2005)، the second one was more reliable at the interest region of this study. The spectral acceleration obtained from standard No. 2800 of Iran for the periods larger than 0. 1 sec showed some values more than those obtained from other methods. According to the comparison made in the present study، this discrepancy may be due to a lack of sufficient accuracy of the relations proposed in standard 2800 of Iran. All calculations in the present study have been conducted for soil type I (according to the standard 2800).

Growing environmental and social concerns، both on the part of decision makers and public opinion، have brought a new perspective to the perception of hazard assessments a valid alternative in the long-term، and an effective complement in short and medium terms، to traditional design procedure for a resistant and safe environment. Results of the gradual development of research on the probabilistic seismic hazard assessment (PSHA) in the past 40 years make a framework that could be used for estimation of probability of occurrences of earthquakes، at certain return periods on each site. The primary advantage of the PSHA over alternative representations of the earthquake threat is that PSHA integrates over all possible earthquake occurrences and ground motions to calculate a combined probability of exceedance that incorporates the relative frequencies of occurrence of different earthquakes and ground-motion characteristics. Features of the PSHA allow the ground-motion hazard to be expressed at multiple sites consistently in terms of the earthquake sizes، frequencies of occurrence، attenuation، and associated ground motion. Potential seismic sources، seismicity models، ground motion prediction equations (GMPE) and site effects، are the most important factors in seismic hazard studies. In this research، a modified probabilistic seismic hazard assessment، developed by Chinese researchers، is used to estimate the level of the potential seismic ground motion in Iran. A unified catalog of de-clustered earthquakes containing both historical and recent seismicity until late 2012 in the area encompassed by 22-42ºN and 42-66ºE is used. An area source model which contains 238 potential seismic sources within 5 major seismotectonic provinces in the study region has been delineated. Considering magnitude uncertainty and incompleteness of the earthquake data، the seismicity parameters of the seismotectonic provinces are determined. Spatial distribution function is used to determine occurrence rates of potential seismic sources for different magnitude intervals. Also، the background seismicity has been determined for each province. Seismic hazard assessment of Iran for a grid of over 40،000 points with 10 km interval is carried out using OpenQuake software by three different GMPEs and two models of seismicity for potential seismic sources in a logic tree. The peak ground horizontal acceleration (PGA) and spectral accelerations (SA) for 5% damping ratio at 0. 2 and 2 seconds corresponding to 10% and 63% probability of exceedances within 50 years (475- and 50-years mean return periods، respectively) are calculated. The resultant seismic hazard maps display a probabilistic estimate of PGA and 0. 2 and 2 sec SA for different mean return periods of 50 and 475 years. Resultant peak ground horizontal acceleration for 475-years return period varies from 0. 63g in North-East of Lorestan to 0. 1g in central Iran. The resultant PGAs for the 475-year return period in provincial capitals indicate the maximum value (0. 35g) in Bandar Abbas and Tabriz، and the minimum one (0. 11g) in Esfahan and Yazd. Comparison of the results of this study with the last map of seismic hazard in the Iranian code of practice for seismic resistance design of buildings، seismic macrozonation hazard map of Iran، Standard 2800، shows significant differences. Seismic hazard levels estimated in this study in southern Iran، Sistan-Baluchestan، Hormozgan and Fars provinces، show significantly higher values.

Ground vibrations during an earthquake can severely damage structures and equipments housed in them. Many factors including earthquake magnitude, distance from the fault or epicenter, duration of strong shaking, soil condition of the site, and the frequency content of the motion define the properties of ground motion and its amplification. A deep understanding of the effects of these factors on the response of structures and equipments is essential for a safe and economical design. Some of these effects such as the amplitude of the motion, frequency content, and local soil conditions are best represented through a response spectrum, which describes the maximum response of a damped single-degree-of-freedom (SDOF) oscillator with various frequencies or periods to ground motion. Earthquake ground motion is usually measured by strong motion instruments, which record the acceleration of the ground. The recorded accelerograms, after corrections for instrument errors and baseline, are integrated to obtain the velocity and displacement time-histories. The maximum response of a SDOF system excited at its base by a time acceleration function is expressed in terms of only three parameters: (1) the natural frequency of the system, (2) the amount of damping, and (3) the acceleration time-history of the ground motion. Response spectrum analysis is the dominant contemporary method for dynamic analysis of building structures under seismic loading. The main reasons for the widespread use of this method are: its relative simplicity, its inherent conservatism, and its applicability to elastic analysis of complex systems. Since the detailed characteristics of future earthquakes are not known, the majority of earthquake design spectra are obtained by weighted averaging of a set of response spectra from records with similar characteristics such as soil condition, epicentral distance, magnitude and source mechanism. The design spectrum specifies the design seismic acceleration, velocity or displacement at a given frequency or period if it is derived from ground acceleration, velocity or displacement time histories. For practical applications, design spectra are presented as smooth curves or straight lines. Smoothing is carried out to eliminate the peaks and valleys in the response spectra that are not desirable for design because of the difficulties encountered in determining the exact frequencies and mode shapes of structures during severe earthquakes when the structural behavior is most likely nonlinear. Since the peak ground acceleration, velocity, and displacement for various earthquake records are different, the computed response cannot be averaged on an absolute basis. Thus, normalization is needed to make a standard basis for averaging. Various procedures are used to normalize the response spectra before averaging is carried out. Among these procedures, one has been the most commonly used, which is normalization with respect to peak ground motion to make the same peak ground motion for all ground motion time histories. Building codes commonly present design spectra in terms of acceleration amplification as a function of period on an arithmetic scale. In this study, the data from Accelerographic network stations are deployed on rock sites of Iran with shear wave velocity larger than 750 m/s, which is equivalent to site Type I in the Iranian seismic building code. The Seismosignal software is used to do both baseline correction and filtering for all the dominant horizontal and vertical components to reduce the inherent error of the motion. Among all the ground motions, only 103 vertical and 109 dominant horizontal time histories are accepted after baseline correction and filtering. The data are classified considering different combinations of the range of magnitude and distance. The epicentral distance is classified as near field (0-35 km), medium distance (35-65 km) and far field (65-100 km), while the earthquake magnitude is classified as small earthquake (4.5

Keywords: Peak ground acceleration, Time history, Damping, Response spectra, Design spectra

Palaeomagnetic analysis has been applied worldwide on active faults for decades. The palaeomagnetic investigation on the Sahneh Fault, at middle part of the Zagros Main Recent fault, is the main objective of this research. The length of the Sahneh fault, which is about 100 km in study area, cuts the gabbroic blocks exposed on the both sides with NW-SE trend and connects the Morvarid fault in the NW to the Nahavand fault in the SE of the study area. Tectonically, the mechanism of the Sahneh fault is high angle reverse with dextral strike slip component, and is compatible with the earthquakes focal mechanism solution, movement of the Arabian plate towards the Central Iran and the results of palaeomagnetic data. The paleomagnetic analysis results are based on the drilled oriented samples of 17 selected sites along and on both sides of the Sahneh fault. Nine to eighteen oriented samples were collected from each site. The conducted paleomagnetic analysis includes measurement of NRM, magnetic mineralogy (high temperature), and thermal/ AF demagnetizing. The declination, inclination and ChRM directions of each site separated from the overprint directions by means of the thermal demagnetization method. The mean direction of ChRM and VGP for each site is determined using statistics and palaeomagnetic analysis. For 7 sites the ChRM mean direction is calculated. The obtained mean ChRM directions then compared with the reference palaeomagnetic pole position of the ophioliths of Central Iran, and the sense of rotation were determined for all sites. The dextral strike-slip Movement of the Morvarid and Nahavand faults imposed a compressional and shear stress components on the Sahneh fault, resulted in the formation of transpressional stress regime in the study area. Under this tectonic stress regime, the Riedel of shear systematic fractures may be helpful to interpret the palaeomagnetic data. According to this model, the whole synthetic shear fractures (P, R, D) caused clockwise rotation and the antithetic shear fractures (Ŕ) caused counterclockwise rotation in gabbroic blocks.

This paper presents the seismicity and seismotectonic characteristics of Esfahan and adjoining regions, an area bounded between 49.5-54o E and 31-34.1o N. According to Mirzaei et al. (1998) this region is placed between two major seismotectonic provinces: Zagros and Central-East Iran. The boundary between these two provinces is known to be the Main Zagros Reverse Fault. Geological maps with scales of 1:100000 and 1:250000 were used to provide the faults map of this region. We determined several probable faults in the region that can help us to introduce the potential seismic source more precisely. Among a total of eighteen focal mechanism solutions of earthquakes with Mw ≥ 4.3 in the region of interest, one event is normal, six events show dominant strike-slip components, and eleven events have dominant reverse components, which confirms the dominance of reverse/thrust faulting. Using global database of earthquakes provided by USGS/NEIC and ISC, as well as catalog of historical (pre-1900) and early- instrumental (1900-1963) earthquakes provided by Ambraseys and Melville (1982), a uniform catalog of earthquakes for the interest region has been provided that involves 170 instrumental and 10 historical earthquakes. To unify the scale of earthquake magnitude for each seismotectonic province, we established empirical relations to convert mb to Ms. Because historical earthquake in Iran have been defined in Ms and lack of events with magnitudes above the saturation level of Ms in the interest region, surface wave magnitude is the most appropriate magnitude scale for the region, and on a broader scale, is appropriate for Iran. In order to establish empirical relations between mb and Ms we applied the orthogonal regression (OR) method that takes into accounts the errors of measurements in both variables. Based on this uniform catalog, seismicity of the interest region was studied, and seismicity parameters were calculated utilizing the method proposed by Kijko and Sellevoll (1992), in which one can consider magnitude uncertainty and completeness of data in calculations. In order to achieve more reliable results, the completeness of catalog and uncertainty of magnitudes have been estimated and considered in our calculations. In this method, dataset of the catalog should be divided into extreme and complete part, and each complete part can be subdivided again into several complete parts that have their own completeness threshold. In this work, the whole data was separated into six complete parts with threshold magnitudes between Mc=3.2 (for events after 1996) to Mc=5 (for events after 1939). To estimate the magnitude threshold (Mc) combination of two methods were used. The first, is maximum curvature (MAXC) and the second, is goodness of fit test (GFT). In cases where lack of data does not allow using GFT, only MAXC method is used. Magnitude uncertainty of each event is considered to be 0.2 and 0.3 for modern-instrumental and early-instrumental earthquakes, respectively, when Ms has been assigned directly based on seismogram readings. For the events that their surface-wave magnitudes have been obtained by empirical conversion relations, magnitude uncertainty is considered to be 0.4. In the case of historical events, uncertainties are considered to be in the order of 0.4 to 0.8 (Mirzaei et al., 1997a), based on their quality (a, b, c, d) that was assigned by Ambraseys and Melville (1982). For Central-East Iran part of the interest region, the results show that b-value is equal to 0.81 ± 0.12, λ is equal to 0.48 ± 0.129 and Mmax=7.8 ± 1.74. For Zagros part, b-value is equal to 0.95 ± 0.05, λ is equal to 4.976 ± 0.413 and Mmax=7.4 ± 0.67. The impact of classification of data on seismicity parameters has been investigated, and the results show that it has a significant impact on the b-value (or β) and recurrence rate of the seismic events (λ); while its effect on Mmax is negligible. Furthermore, classification of complete part of the catalog significantly decreases the uncertainty in the evaluated parameters of Mmax, λ and b (or β). The minimum impact on uncertainty of parameters appears in b, and the maximum appears in Mmax.

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.

Temporal variations of radon concentration in soil and groundwater might be one of the few promising precursors for earthquake prediction. In this study, the relation between radon concentration and aftershocks of Bam Earthquake (26/12/2003, Ms=6.8) has been investigated.The radon monitoring station was located at 29N and 58.4E, precisely on Bam Fault where there have been high occurrences of seismic activities. The study was carried out using an active method involving an Alpha Gurad PQR2000, Alpha Pump and relative accessories which is a device capable of accurately measuring radon concentrations every 10 minutes. Air was being pumped from ground to the measuring system with a flux of 1 L/min. Forced air suction was chosen in order to avoid stratification effects, very common for radon, due to its elevated weight. Radon-monitoring sites are usually chosen in the areas where higher concentrations of radon in the surface soil layer can be expected. For this propose, the radon monitoring site was placed exactly on Bam Fault, which was placed between Bam and Baravat Cities. Radon concentration monitoring data was collected in soil at 90 cm depth exposed for a period of 90 days, every 10 minutes. Radon concentration changes are not only controlled by an earthquake, but they are also controlled by meteorological parameters at the radon monitoring site such as rainfall, soil moisture, temperature and atmospheric pressure. Therefore, in order to use radon variations as a reliable earthquake precursor, we must be able to differentiate changes that are due to earthquake from those which are not.In recent years, artificial neural networks have become very powerful, intelligent tools, used widely in ignal processing, pattern recognition and other applications. The main advantages of the method are the earning capability for developing new solutions to problems that are not well defined, an ability to deal with computational complexities, a facility of carrying out quick interpolative reasoning, and finding functional relationships between sets of data.We have used a modified Adaline structure to estimate the temporal variation of radon concentration related to environmental parameters. This enables us to differentiate the changes due to phenomena in the earth such as earthquakes from those of environmental parameters. Radon concentration data obtained from our site and meteorological parameters measured in meteorological station of Bam were processed by the adaptive linear neural network, Adaline. It was indicated that the linear neural network was able to differentiate linear variations of radon concentration caused by the meteorological parameters from those arose from anomaly phenomena due to the aftershocks.

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.

Seismic hazard assessment like many other issues in seismology is a complicated problem, which is due to variety of parameters affecting the occurrence of earthquake. Uncertainty, which is a result of vagueness and incompleteness of the data, should be considered in a rationale way. Herein, fuzzy set theory is used to take into account the uncertainty existed in the seismic hazard analysis. The Fuzzy Set Theory (FST) is an attractive methodology when vague and subjective judgments of a unique phenomenon enter probabilistic or mathematical models. Fuzzy sets are sets whose elements have degrees of membership. In classical set theory, the membership of elements in a set is assessed in binary terms according to a bivalent condition; an element either belongs or does not belong to the set. By contrast, fuzzy set theory permits the gradual assessment of the membership of elements in a set; this is described with the aid of a membership function valued in the real unit interval [0, 1]. Fuzzy sets generalize classical sets, since the indicator functions of classical sets are special cases of the membership functions of fuzzy sets, if the latter only take values 0 or 1. In fuzzy set theory, classical bivalent sets are usually called crisp sets. Tehran is a densely populated metropolitan in which more than 10 million people live. Many destructive earthquakes happened in Iran in the last centuries. It comes from historical references that at least 6 times, Tehran has been destroyed by catastrophic earthquakes. The oldest one happened in the 4th century BC. In this study, seismic hazard assessment of Tehran region, capital city of Iran, is conducted using a combination of probabilistic approach and fuzzy sets theory. The earthquake catalog used in the current study contains occurrence times and hypocentre locations of Iranian earthquakes and is derived from the seismic catalog published by Mirzaei et al (2002) for earthquakes during 1975 to 2000. The International Seismological Centre catalog (www.isc.ac.uk) was used to update the catalog data up until the year 2007. In order to calculate seismic hazard for different return periods in probabilistic procedure for the Tehran region, an area encompassed by the 49.5°–54°E longitudes and 34°–37°N latitudes has been divided by 0.1° intervals generating 1350 grid points. Seismicity parameters are evaluated using the method in which magnitude uncertainty and incompleteness of earthquake data are considered. A total of 20 area potential seismic sources are introduced, and several seismicity rates and b-values, maximum expected magnitudes are assigned to each of seismotectonic province and potential seismic sources. To carry out seismic hazard analysis in the framework of fuzzy sets theory, all of the variables converted into triangular fuzzy sets with -cut method. Eventually, the fuzzy response is defuzzified using the surface center method. Two maps are developed to indicate the earthquake hazard of the region in the form of iso-acceleration contours. They display a fuzzy-probabilistic estimate of peak ground acceleration (PGA) over bedrock for the return periods of 475 and 50 years. PGA values for this region are estimated to be 0.35g-0.38g and 0.12g-0.15g for 475- and 50-years return periods, respectively. Outcomes of this study would contribute for the quick and better estimation of the seismic design of structures.

Based on the standards for design of structures, any structure should be designated for seismic loads and any combinations containing seismic loads. For spectral analysis of a structure, the site effect is taken into account by considering its effects on the design spectra. Because of different lateral and vertical stiffness of the soil layers underneath the structure, the design spectra are different from soil to soil. The stiffer the soil the higher the velocity results in. The shear and compressive wave velocities of the soil as continuum are criteria for categorizing the soil in stiffness point of view. Since, the structures are more affected by lateral forces than vertical ones, the shear wave velocity is more important than the compressive wave velocity. Moreover, the soil nearer to the structure affects the structure more than the soil far from it. Thus, in the standards for design of structures, the mean shear wave velocity of the upper 30 m of the soil layer is used for categorizing the soil underneath the structure in stiffness point of view. In the Iranian code of practice for seismic resistant design of buildings, the standard number is 2800, the sites have been categorized into four different types, which are rock (with the mean shear wave velocity of the upper 30 m denoted as larger than 750 m/sec), medium alluvium (where m/sec), soft alluvium (where m/sec), and very soft alluvium (with m/sec). To analyze the structures using design response spectra, specified horizontal and vertical spectra are needed for each category. Because of the increase in the number of strong motion accelerograms in recent years, in this research the horizontal and vertical design spectra for the first category (the rock site) are prepared based on Iranian data. To do so, all the existing horizontal and vertical acceleration time histories in different stations fixed on rock sites are gathered. The data are filtered and baseline corrected by Seismosignal software to remove the noise frequency components and to modify the magnitude of both the displacement and velocity. In addition, all the data are normalized for the Peak Ground Acceleration (PGA). According to the data, the quality of 60 vertical time histories and 71 horizontal time histories were acceptable. With this data, both the vertical and the horizontal response spectra are prepared for each time history and for four different damping ratios, which are %2, %5, %10 and %20. Averaging the response spectra, the unsmoothed design spectra are obtained. The smoothed design spectra are plotted in tripartite coordinate system and spectral acceleration-time system, as well. These procedures are done for the average plus one standard deviation of vertical and horizontal response spectra. Finally, the smoothed design spectra from the data of this research are compared with that of the Iranian code of 2800 regulation and also the Mohraz design spectra. It is shown that the results are in good agreement with the Mohraz design spectra except that in long periods, the spectral acceleration obtained in this study is smaller. Comparing the result of this research with that of 2800 regulation, it is seen that in short periods, the spectral acceleration in this study is higher than that in the 2800 regulation, while for long periods, the spectral accelerations in this study is much less than that given in 2800 regulation. It means that in the category of short period structures more strengthen structures may be needed.

S e v e r a l a s p e c t s o f a n e a r t h q u a k e, i n c l u d i n g t h o s e o f a s e i s m o l o g i c a l, e n g i n e e r i n g, a n d s o c i o-e c o n o m i c a l n a t u r e, m u s t b e u n d e r s t o o d a n d i n c o r p o r a t e d e f f e c t i v e l y b e f o r e t h e i m p a c t o f a n e a r t h q u a k e c a n b e p r e d i c t e d. E a r t h q u a k e d a m a g e a n d l o s s m o d e l i n g r e q u i r e s c o m p r e h e n s i v e a s s e s s m e n t o f e a r t h q u a k e h a z a r d, t h e s e i s m i c v u l n e r a b i l i t y o f t h e b u i l t e n v i r o n m e n t a n d a s s o c i a t e d e x p o s u r e. O n e o f t h e s t e p s i n h a z a r d m i t i g a t i o n a n d t h e r e a l i s t i c m o d e l i n g o f d a m a g e, f a t a l i t i e s a n d c a u s a l i t y d u e t o f u t u r e e a r t h q u a k e s, i s t o a p p l y a p p r o p r i a t e d a m a g e a n d f a t a l i t y f u n c t i o n s. A m o n g d i f f e r e n t m e t h o d s f o r e v a l u a t i n g d a m a g e f u n c t i o n s, t h e m o s t r o b u s t m e t h o d i s t h e e m p i r i c a l m e t h o d, w h i c h i s b a s e d o n t h e d a m a g e i n f o r m a t i o n o f d i f f e r e n t b u i l d i n g t y p e s f r o m p a s t e a r t h q u a k e s. T h e a p p l i c a t i o n o f a p p r o p r i a t e d a m a g e f u n c t i o n s c o u l d p r o v i d e a n e s s e n t i a l i n p u t f o r t h e s e i s m i c h a z a r d m i t i g a t i o n p l a n b e f o r e a n e a r t h q u a k e, a s w e l l a s p r e p a r e d n e s s a n d r e s p o n s e p l a n s a f t e r a n e a r t h q u a k e. D a m a g e f u n c t i o n s v a r y f o r e a c h r e g i o n o r c o u n t r y, b a s e d o n t h e t y p e s o f s t r u c t u r e, m a t e r i a l s a n d c o n s t r u c t i o n m e t h o d s. T h e r e f o r e, t h e d a m a g e d a t a f r o m a p a s t e a r t h q u a k e s h o u l d b e c a r e f u l l y g a t h e r e d a n d a n a l y z e d. T h e n, t h e g r o u n d m o t i o n l e v e l, i n t h e f o r m o f s e i s m i c i n t e n s i t y, p e a k g r o u n d a c c e l e r a t i o n o r v e l o c i t y, n e e d s t o b e e s t i m a t e d i n d a m a g e d a r e a s a n d t h e i r r e l a t i o n t o t h e o b s e r v e d d a m a g e o f s t r u c t u r e s s h o u l d b e i n v e s t i g a t e d. T o t h i s e n d, t h e r e c e n t e a r t h q u a k e s i n I r a n p r o v i d e a u n i q u e o p p o r t u n i t y t o d e v e l o p d a m a g e f u n c t i o n s, a n d t o s t u d y t h e i r r e l a t i o n s w i t h d i f f e r e n t l e v e l s o f d a m a g e e x p e r i e n c e d b y d i f f e r e n t t y p e s o f s t r u c t u r e.I n t h i s p a p e r, t h e e m p i r i c a l d a m a g e f u n c t i o n s f o r n o n-e n g i n e e r i n g b u i l d i n g s (a d o b e a n d m a s o n r y s t r u c t u r e s) w e r e e v a l u a t e d i n t e r m s o f i n t e n s i t y a n d p e a k g r o u n d a c c e l e r a t i o n f o r t h e Z a r a n d r e g i o n. T h e b u i l d i n g d a m a g e d a t a w e r e c o l l e c t e d f r o m t h e d a m a g e s u r v e y o f t h e 2005 D a h o o i y e h-Z a r a n d e a r t h q u a k e, b y t h e f i r s t a u t h o r, a n d t h o s e g a t h e r e d f r o m t h e d a m a g e r e p o r t p r o v i d e d b y d i f f e r e n t o r g a n i z a t i o n s. T h e e a r t h q u a k e i n t e n s i t y w a s e v a l u a t e d i n t h e d a m a g e d a r e a s c o n s i d e r i n g t h e v u l n e r a b i l i t y c l a s s a n d t h e d a m a g e g r a d e o f b u i l d i n g s u s i n g t h e E u r o p e a n M a c r o s e i s m i c S c a l e (E M S-98). F u r t h e r m o r e, p e a k g r o u n d a c c e l e r a t i o n w a s e s t i m a t e d f r o m t h e r e c o r d e d d a t a a n d b y a p p l i c a t i o n o f a n i n t e r p o l a t i o n t e c h n i q u e t o t h o s e e s t i m a t e d b y a t t e n u a t i o n r e l a t i o n s h i p s i n d a m a g e d a r e a s, w h e r e n o r e c o r d e d d a t a w e r e a v a i l a b l e. T h e n, t h e d a m a g e f u n c t i o n s f o r a h e a v y d a m a g e l e v e l (G4+G5) w e r e e v a l u a t e d f o r a d o b e a n d m a s o n r y s t r u c t u r e s, w h i c h w e r e t h e m a j o r t y p e s o f s t r u c t u r e i n t h e Z a r a n d e g i o n. T h e i n t r o d u c e d d a m a g e f u n c t i o n s r e v e a l e d t h a t t h e h e a v y d a m a g e t h r e s h o l d f o r n o n-e n g i n e e r i n g b u i l d i n g s i n t h e Z a r a n d r e g i o n i s a b o u t 100 c m/s$^2$ o r V I I o n t h e E M S-98 i n t e n s i t y s c a l e.

Recordings of ambient noise or microtremors are increasingly used to find valuable information on soil in one dimension at a given site. Ambient vibrations, which are assumed to be mainly composed of surface waves, can be used to determine the surface wave dispersion curve in order to retrieve shear wave velocity profile. In this regard, microtremors are usually recorded simultaneously in an array of stations and they are processed in two steps; finding the dispersion (autocorrelation) curve and then inversing it to estimate the shear wave velocity profile. Microtremors are usually recorded in various apertures in order to get the spectral curves over a wide frequency band, and different methods also exist for processing the raw signals. The two most popular microtremor processing techniques are frequency-wave number (F-K) and spatial autocorrelation (SPAC). The SPAC method, which generally employs a circular array of stations and one central station, permits an in-depth understanding of the temporal and spatial spectra of seismic waves. Nowadays, it is widely used to estimate the structure of sub-surface layers and the shear wave velocities of sediments. In the SPAC method, the dispersion curves (phase velocity versus frequency) of surface waves are deduced by analyzing the normalized correlations between microtremors recorded at different stations. The dispersion curves are then used to characterize the structure of the medium. The method is based on a statistical analysis of the observed signal, which is assumed to be stationary and ergodic in time and space. In this paper to find reliable results in the processing of microtremors in shallow structures, the spatial autocorrelation coefficients are calculated for the vertical components of recorded signals using the MSPAC method and a new one (the revised SPAC method). Both methods are based on considering all possible autocorrelation pairs among the circumference stations. Their difference is that the new model considers all possible autocorrelation pairs among the circumference stations and makes an average on the calculated autocorrelations, on the other hand in the MSPAC model the pairs are put in different rings according to the distance between each pair. The deduced autocorrelation coefficients are then inverted. The results of applying the two models on real data are presented and compared. This comparison reveals that the results of both models are in good agreements with the site geology, although the new method expresses the Vs profile at depths smaller than 10 meters more successfully than the MSPAC method.

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.

The Makran subduction zone in southeastern Iran and southern Pakistan is where the oceanic crust of the Arabian plate (Oman Sea) is subducting beneath Eurasia. Compared to other subduction zones in the world, the Makran subduction zone has some unusual features, including different seismicity patterns in its eastern and western parts. Also, the Quaternary volcanoes in the eastern part of Makran are located far from its foreland comparing to the western part of Makran. The very low seismicity of western Makran causes two different viewpoints about its current situation; i.e., whether the subducted plate is undergoing aseismicity or has been locked strongly. The Partitioned Waveform Inversion (PWI) method is used here to image the S-velocity structure of the upper-mantle and Moho-depth variations of Makran subduction zone and explore the relationship of the Makran seismic structure with the seismicity and the volcanic arc in the region. For this purpose, we used the vertical components of the seismograms recorded by the National Iranian Seismic Network with high signal to noise ratio from the earthquakes with magnitudes of 5.5 to 7.7. Despite the limited number of stations around the Makran region, choosing proper earthquakes enables us to improve the azimuthal and path coverage and apply the PWI method in the region. Our tomography data show that the Moho depth across the Makran subduction zone is increasing from the Oman seafloor and Makran forearc setting to the volcanic arc. Generally, the crust in the western Makran is thicker than its eastern part and the maximum crustal thickness in the Makran region reaches to 50±2 km below the Taftan volcano. The Moho map clearly depicts the western edge of the Makran subduction zone, where the Minab fault (representing the eastern edge of the Hormuz Straits) marks the boundary between the thick continental crust of the Arabian plate and the thin oceanic crust of the Oman Sea. Our results show clearly that the high-velocity slab of the Arabian plate subducts northwards beneath the low-velocity overriding lithosphere of Lut block in the western Makran and Helmand block in the eastern Makran. We found that the slab in the western Makran starts with a gentle dip (about 8?) and increases to about 55?, where it plunges into the asthenosphere beneath the volcanic arc. In eastern Makran, the slab is subducting with a low dip angle of about 8? and reaches approximately 20?below the volcanic arc. We found that the bending of the subducted plate occurs with relatively low dip and much farther beneath eastern Makran than in the western part which may explain the different volcanic arc offsets across the Makran subduction zone.

Seismic gaps as a method of earthquake prediction, initially, were used mainly for long-term prediction. Nowadays, seismic gaps are one of the most important precursory phenomena for intermediate-term earthquake prediction. These are part of the tectonic regions that are quiescent at the moment, but might cause damaging earthquakes in the future. Based on the study of strong earthquakes in mainland of China it is suggested that intraplate gaps due to the activity of moderate and small earthquakes may be divided into "background gaps" and “preparation gaps”. A background gap is a gap surrounded by larger earthquakes in a larger area and with a longer duration before the main shock. A preparation gap is a gap surrounded by small earthquakes in a smaller area and with a shorter duration before the main shock(Lu and Song, 1989). Background gaps are of critical importance and are a clue to relatively high Ms earthquakes (Ms > 5). Preparation gaps build up in a region inside a background gap or its surroundings in a short time interval (afew years) before the main earthquake. The preparation gap is usually surrounded by small precursory earthquakes, even though one or few relatively large earthquakes (still smaller than the main earthquake) may occur in regions on the edge of the gap. Such smaller magnitude earthquake activity has been considered as premonitory phenomena useful for intermediate-term and even short-term earthquake prediction. Three criteria are proposed for the identification of these gaps (Lu and Song, 1989): (1) with the formation of a preparation gap the seismic strain release should accelerate both in the gap and in its vicinity, (2) the ratio of earthquake frequency outside the gap to that within it should reach a maximum value during the formation of the gap, and (3) some moderate earthquakes often occur in the forthcoming seismic source area before formation of the background gap. The former two are the main criteria for identification of the gap, and the third is a subsidiary criterion for determining the location of the forthcoming earthquake. In this study, based on the history of earthquakes in the Central-East Iran seismotectonic province, we have found a background gap and three preparation gaps. One of these gaps is related to the destructive earthquake of Ms =6.8, which occurred on 26 December2003 in the Bam region of southern Central-East Iran. This earthquake occurred in the edge of the recognized preparation gap in the Bam region. The other gap is related to a large earthquake of Ms =6.0, on February in the Sefidabeh region of south-eastern Iran. This earthquake also occurred in the edge of the recognized preparation gap in the Sefidabeh region.The strain release curve, the ratio of the earthquake frequency outside the gap to that within it, and the cumulative number-time curve, have good correlation with the earthquakes happened. In addition, recognition of a preparation gap in the Dasht-e-Bayaz region, eastern Central-East Iran, implies accumulating seismic strain and a large earthquake may occur in that region in the future. The well- known Dasht-e-Bayaz and Abiz earthquake faults are located in this preparation gap.