جستجوی مقالات مرتبط با کلیدواژه « سازوکار کانونی » در نشریات گروه « زمین شناسی »

In this research, to understand the present-day tectonic situation of Gorgan-plain in the east of the South Caspian basin, the tectonic stress regime of this area and its adjacent areas was estimated using the inversion of the data of the earthquake focal mechanisms. For this aim, in addition to analyzing focal mechanisms solved by different sources, the focal mechanisms were solved for several earthquakes with appropriate and sufficient information. The results indicate the presence of various mechanisms, including thrust, normal, and strike-slip, and as a result, various orientations of kinematic P and T axes, which can indicate the complexity of the region. By analyzing the earthquakes with a minimum magnitude of 4 in the Gorgan-plain, the Kopeh-Dagh in the east of the plain, and the entire eastern region of the South Caspian, a compression regime with a NNW-SSE trend was obtained. However, by analyzing all of the earthquakes (smaller and larger earthquakes) in the Gorgan-plain, a strike-slip regime was obtained, in which the trend of the P and T axes were calculated NE-SW and NW-SE, respectively. The results indicate a local strike-slip regime in addition to the regional compressional regime in the region.

Recording, processing and identifying the parameters of independent earthquakes larger than Mw = 4 using advanced equipment, software and professional experts, can be calculated nowadays in less than 10 minutes automatically in the Iranian plateau. Double earthquake of November 14, 2021 with a small time difference (less than 90 seconds) occurred at a very close distance with magnitudes of 6.1 and 6.4, respectively. The calculation of the parameters of location, time and magnitude of the first event, in addition to the calculation of its focal mechanism parameters, was made possible with the help of semi-automatic or automatic software. However, the interference of the seismic phases of the first event with the second one in farther stations poses a serious challenge to the automatic and semi-automatic calculation of location, origin time, magnitude, as well as the parameters of the focal mechanism of the second event. In this study, by merging recorded seismograms and accelerograms in seismic stations within the country, the parameters of this double earthquake and their two main aftershocks with magnitudes of 5.1 and 5.4 have been calculated. In addition, the calculation of the focal mechanism of these four events is carried out based on the polarization method of the first P-wave polarity, the amplitude ratio of P and S waves and the modeling of seismic moment tensor. The results of this study show that according to the locations and focal mechanisms calculated for the double earthquake and both major aftershocks, the activity of the Handun basement fault, which is probably related to the formation of the Handun salt dome, caused double earthquakes of November 14, 2021 and its two main aftershocks in the northwest of Bandar-Abbas, south of Iran.

The Kopeh Dagh and Binalud-Alla Dagh mountains are important structural elements located in the northeastern boundary of Arabia-Eurasia collision zone. Due to existence of large cities with a long history of civilization, there is a relatively rich body of data on historical seismicity in this area. Nonetheless, little adequate instrumental seismic data were available prior to this study. In this paper, we utilized the temporary China-Iran local seismic network data deployed in the area for 13 months. Based on this data, we determined 37 focal solutions through first motion polarity approach suggesting a combination of strike-slip, reverse and normal mechanisms. There is a significant concentration of epicenters and focal mechanisms around the eastern Alborz - Kopeh Dagh boundary along the Atrak River. The southern and western boundaries of the Binalud Mountains also show relatively high seismic activities. The Local magnitude (ML) of the events ranges from 3.5 to 4.9 with depths of up to 20 km, mostly concentrated at ~10 km. The principal stress axes and slip vectors obtained from the focal mechanisms agree well with kinematic state of main faults and tectonic regime of the area, confirming the seismogenic nature of these faults. In addition, they are consistent with right-lateral component of slip along thrust faults in eastern Kopeh Dagh and Binalud and both laft- and right-lateral motions in the central Kopeh Dagh.

Knowing about stress variations in the Zagros and Makran transition zone in the southwest of Iran is necessary to study the deformation resulting from the oblique collision between the Eurasian and the Arabian plates and gain insight into the complicated tectonics of this crucial region. The stress tensor inversion of earthquake focal mechanisms is one of the methods used to study tectonic stresses. In this study, the direction of maximum horizontal stress in the transition zone between the Zagros and Makran was obtained using this method. The results indicate significant changes in the principal axes of stress in this region and show the stress field's complicated pattern. The rotation in the right of the Oman Line takes place in a clockwise manner from west to east around this imaginary line. Still, on the other side, especially where Qeshm Island is located, the opposing direction of the maximum horizontal stress directions indicates the stress field's complexity. The axis of maximum horizontal stress in this region is compared with the single earthquake focal mechanism's P axis. Since almost all earthquake faulting mechanisms are of reverse and strike-slip type, the comparison shows a good agreement between the resulting directions. The maximum horizontal stress directions are also investigated concerning the trend of the active faults consistent with the fault mechanism in the studied areas. Focal mechanism data were used to obtain information on the state of stress in 5 subdivisions of the data, including teleseismic and local events in the Zagros region. The investigation shows acceptable agreement between the observed faulting mechanisms and what can be predicted based on the fault plane orientations and stress directions in the area. These alignments also correlate with the data from other geophysical methods that exhibit a similar rotation around the Strait of Hormuz. The plate motion velocity vectors were estimated using the NOVEL-1A model. On the right side of the Oman Line in the SE Zagros, the stress and plate motion directions are similar, while in the western part of the Zagros, they differ by about 35 degrees. This angular difference between the stress and plate motion velocity vectors is obverse between the Qeshm Island and the area in the Strait of Hormuz's left-hand side in the Zagros. The oceanic and continental crust differences in the area may be responsible for the variation of these directions in the region. By providing a new perspective, this study can help in understanding the tectonic structure and processes in this complex region.

When an earthquake occurs, a part of the released energy is propagated in the form of elastic waves at a speed that depends on the nature of the propagation environment. The radiated energy from earthquake focus in all directions is called radiation pattern and depends on the fault mechanism. The environment response to longitudinal waves appears as compression and expansion of the environment. This can affect on water levels in aquifers, wells and springs before, during and after the event. The changes of the groundwater levels caused by an earthquake are important with respect to other induced parameters; they may generate crustal deformations, affect on water supplies and production of oil wells, initiate liquefaction, control the distribution of aftershocks, trigger earthquakes and mud volcano eruptions. In this study, the effect of April 2013 Kaki earthquake (Mw6.3) on water levels in wells located in a range of 50 km radius around the earthquake epicenter was compared in successive months in the preceding years, during and after the event. Regarding the earthquake focal mechanism and using the estimated tensile and compressive zones in the study area, the correlation of these areas with changes in water levels and discharge of wells and springs was examined. The results of evaluations indicate that the water levels in wells rose and the discharge of the wells and springs located within the radiation range of the pressure waves increased after the earthquake. The wells located in the direction of compressive radiation showed a level of elevation. In contrast, wells located along the shear strain either experienced a drop in water level or had no significant changes. The results show that there is compatibility between the pattern of seismic waves and the changes in the water levels in wells and springs. Because of the difference in characteristics of geological formations, the response of the wells located in the same seismic pattern may not be the same. On the other hand, since the earthquake occurred in the low rainfall time, changes in water levels cannot be caused by rainfall. Field data shows other recorded effects such as the recovery of the springs of Kale and Seyyed Ali which indicates the increase in discharge of water in the region. The liquefaction created in the area is a reason for the passage of a compressive wave that has led to increase of water levels in the wells. The amount of decrease or increase in water level depends on the magnitude and distance to hypocenter of the earthquake and geological characteristics of the well. The water level in the well located in Shonbe village (28.34°N, 51.76°E) increased 1.6 m after the earthquake.

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.

Calculating the focal mechanism of earthquakes, helps us to investigate the direction of rupture propagation, the style of faulting, stress field and the seismicity potential of an area. Focal mechanisms classically are determined by using the polarity of P- wave first motion. This method depends on the number of P wave polarities per event and the data set dispersion distribution Significant improvement of long period instruments caused advantages in using waveform inversion method for determining focal mechanism of earthquakes. In this method outspread outlier data can be used.
Extensive availability of small earthquakes makes them useful in determining focal mechanisms. The current study accomplished to determine focal mechanisms of 4204 events with magnitude 2≤ Mn ≤5. The earthquakes were recorded in 41 stations of Iranian Seismological Center (IRSC) in the Alborz region. The focal mechanisms were prepared by using the polarity of P wave first motion and S/P amplitude ratios.
In this study, waveforms of three-component seismograms were filtered with bandpass filter in a 0/1–10 Hz frequency range. Seismograms with velocity components were integrated to the corresponding displacement components. P-wave first motion polarities are revised by reviewing the waveforms carefully. For calculating the S/P ratios, P wave, S wave and background noise amplitudes were measured by selecting 2 second time windows from waveforms. Maximum amount of S/P ratio is near the nodal planes and the minimum amount is far from the nodal planes. Using P wave polarities and ratios, the final focal mechanisms were obtained. With the progressive improvement of IRSC networks since 2012, more P wave first motion polarities can be recorded and the number of the calculated S/P ratios for each event has increased.
In this study, data set of 1997 to May 2015 was investigated. Because of weak qualities of small earthquakes waveforms, small events were omitted and a focal mechanism catalogue containd strike, dip and rake angels for earthquakes with Mn greater than 3.5 was produced. The method also was tested with 12 synthetic earthquakes in the Alborz rigion. Most of the focal mechanisms at north of the region have thrust-faulting components and at south they are strike-slip. Along the left side of the Talaghan fault, most of the focal mechanisms are left-lateral strike-slip which shows the left-lateral motion along the Mosha fault. The Indes fault has north west-south east strike. Along the south east side of this fault, the focal mechanisms of 18/6/2007 event and its aftershocks show the inverse faulting. The results of this study are compatible with most of the previous studies in this region

An earthquake of local magnitude 5.2 which occurred in Malard recently, was accompanied by four earthquakes with local magnitudes largerthan 4.0 These events can be considered as a manifestation of the state of Tehran, a mega-city with more than 13 million inhabitants, in last decade. For this reason, we tried to do a comprehensive seismic study on Tehran and surrounded area using all available data. In order to do this, we used three independent datasets belonged to Iranian Seismological Center (IRSC), International Institute of Earthquake Engineering and Seismology (IIEES) and Tehran Disaster and Mitigation Organization (TDMMO). We merged these three datasets to get a uniform one, using 3D cells gridding technique and then explosions or outliers were removed. The final dataset included 2000 events. Using 310 selected events based on horizontal and depth errors, azimuthal gap, RMS and minimum number of recording stations, we calculated 1D velocity model by utilizing Particle Swarm Optimization (PSO) method. The calculated model consists of three layers. The thickness and P velocity of layers are 4.0 km and 5.4 km/s, 6.0 km and 5.8 km/s and 5.0 km and 6.05 km/s for the first, second and third layer, respectively. The final layer is a half-space with a P velocity of 6.5 km/s. The computed Vp/Vs is 1.71. Then all events were relocated using our new velocity model utilizing fully non-linear probabilistic method to get as much as possible accurate locations. The results show that 40% of all relocated events have uncertainties less than 2.5 km and 5.0 km in horizontal and vertical direction, respectively. The final calculated mean RMS is ~ 0.24 s. In order to define the geometry of the active faults, three different subsets of events were selected based on their location uncertainties, azimuthal gap, RMS and minimum number of recording stations. This helped us to use well-located events for better defining of the fault traces on map view and in depth cross-sections. We plotted four cross-sections perpendicular to the strike of the main faults in the region. The focal mechanism solutions for 38 selected events were also computed based on P-wave first polarity method. The final results show that the eastern part of the study region is more active than the western part, at least in the last decade, and surprisingly, the Malard earthquake occurred in a region without any major activity from three months before the main shock. Stress field study also reveals that the maximum stress axis is N36E and the main seismotectonic regime is left lateral structures. These results are very consistent with the main trends of the North Tehran fault

One of the most seismically active parts of Iran is Zagros area. The basement-involved active fold-thrust belt of the Zagros in southwest Iran is underlain by numerous seismogenic blind basements thrust covered by the folded Phanerozoic sedimentary rocks. The present morphology of the Zagros active fold-thrust belt is the result of its structural evolution and depositional history: a platform phase during the Paleozoic; rifting in the Permian Triassic; passive continental margin (with sea-floor spreading to the northeast) in the Jurassic-Early Cretaceous; subduction to the north­east and ophiolite-radiolarite emplacement in the Late Cretaceous; and collision-shortening during the Neogene.Besides, there are a lot of different faults in Zagros, for example, the Main Zagros Reverse Fault (MZRF), the Main Recent Fault (MRF), the High Zagros thrust belt, the High Zagros Fault (HZF), and Mountain Front Fault (MFF). This study is focused on the last-mentioned one. The MFF flexure is introduced for the first time by Falcon (1961) and then is presented as the mountain front fault by Berberian and Tchalenko (1976)], which delimits the Zagros simple fold belt and the Eocene-Oligocene Asmari limestone outcrops to the south and southwest. The Mountain front fault (MFF), is a segmented master blind thrust fault with important structural topographic, geomorphic and seismotectonic characteristics. Therefore, the study and recognition of seismic parts of Iran are important. The aim of this study is to determine the focal mechanisms of Mountain Front Fault (MFF) at a latitude of 46 to 48.5 degree in Zagros. Due to the salt layers, large earthquakes rarely reach the surface. In such cases, the seismic method is an appropriate tool to understand the faulting mechanisms. By means of focal mechanisms, it is possible to gain information about the fault geometry and its related mechanism. The data used in this study are from International Institute of Earthquake Engineering, and Seismology (IIEES) and Institute of Geophysics of the University of Tehran (IGUT). Because of some wrong relocation, during this study relocated them to reach a well-relocated data base and better results. Getting the focal mechanism of an earthquake can occur in various ways. In this study, first, the waveform modeling by Isola software was used to find the focal mechanisms. To determine the accuracy of focal mechanism solutions obtained by waveform modeling, the polarity method was used to solve focal mechanisms. Besides, some of these earthquakes have also been reported by CMT. After determining focal mechanism solutions with the stated method, they were compared with CMT, and all the focal mechanism were mapped in the area so that the trend of this part of MFF can be recognized better. Because there are many earthquakes in this area, a reliable decision can be made. By looking at the maps, it is easily understandable that the trend in this part is obviously EW. Finally, the prevailing trend that obtained in the study area is found. Most of these earthquakes are trending EW. The study of 31 focal mechanisms in the area has permitted to constrain the faulting mechanism of MFF.

Northwestern Iran is one of the seismically active regions with a high seismic risk in the world as confirmed by the historical background and instrumental earthquakes. The Ahar–Varzeghan double earthquakes of August 11th, 2012 with magnitudes higher than 6 are the examples of high seismic activities in this area. Our knowledge of stress state in a region is useful for a better understanding of different rupture mechanisms. The accumulation of stress is the main cause of earthquake, and its analysis is a crucial task considering the dense population of the region. The focal mechanism of an earthquake is one of the important source parameters which is practical for studying and analyzing the stress field. This parameter is affected by fault geometry and principal stress directions. One of the big advantages of the focal mechanism solutions is the ability to study the stress regime depp within the lithosphere. In this study, we determine the focal mechanisms of 15 earthquakes using the moment tensor inversion method of ISOLA program (Sokos and Zahradnik, 2008) in Ahar–Varzeghan region. This method was first proposed in order to calculate the source parameters at teleseismic distances (Kikuchi and Kanamori, 1991). It was developed later for regional and local distances by Zahradnik et al. (2005). In this method, Green’s functions are calculated by the discrete wavenumber method (Bouchon, 1981). The events used have moment magnitudes higher than 4 and encompass latitudes between 37° N and 40° N, and longitudes between 44° E and 49° E, during the period 2012–2014 for the focal mechanisms determined in this study and the period 1997–2014 for the Global Centroid Moment Tensor (GCMT) ones. We used the broadband stations of Iranian Seismological Center (IRSC), International Institute of Earthquake Engineering and Seismology (IIEES) and also stations of several other countries bordering northwestern of Iran. The focal mechanisms determined are often strike-slip and reverse which show a correspondence with the tectonic of this region. Then we analyze the stress state using these focal mechanisms and also by the focal mechanisms of other large and moderate earthquakes determined by GCMT in the region. We calculate the principal orientations of stresses by multiple inverse method that was originally proposed by Yamaji (2000). The method is a numerical technique to separate stresses from heterogeneous fault slip and focal mechanism data. Using the information of the acquired and GCMT focal mechanisms containing strike, dip and rake angles for the main shocks, we study the state of stress in the northwest of Iran. The result shows the average stress model with 1σ and 3σ equal to 141 and 50.2 degrees with a stress ratio of 0.6 in the region between Urmia Lake and Talesh in the northwest of Iran. This stress ratio shows that most of the motions are strike-slip. Using focal mechanisms of aftershocks, the same values are respectively 132.5, 42.4 degrees and 0.3 in Ahar–Varzeghan region. The value 0.3 shows that the motions are reverse in this part of the northwest of Iran. The small difference between these values in the northwest of Iran and Ahar–Varzeghan region shows that the values of the acquired principal stress directions using aftershocks are close to those of the main shocks. The faults directions of right-lateral strike-slip motion are in accordance with the stress direction determined.

Earthquake relocation has an important role in the investigation of tectonic settings of a region. An increase in accuracy of a relocation problem can enhance the calculation of the local velocity model as well as associated studies of a risk analysis. There are some important procedures to make a reliable catalog of earthquakes. Verifying the seismic waveform to correct the picked phases, utilizing other seismic networks and using an appropriate local or regional velocity model, we can improve the final results. In a case with a well-conditioned network geometry, using an accurate local velocity model has a significant effect on increasing the depth accuracy. In this study, we have tried to use all available information from seismic networks in NW Iran and south Azerbayjan. We relocated the double main-shock of Varzaghan-Ahar Mw 6.5, Mw 6.3 and more than 1800 aftershocks with Ml > 2.0 over a 10-month period after the main shocks. To increase the accuracy of the earthquake location, we merged the recorded information of eight short periods of the Iranian Seismological Center (IRSC), one broad band station of the International Institute of Engineering and Earthquake Seismology (IIEES) and five broad band stations of the Azerbaijan National Seismic Network (ANSN). We also calculated a local velocity model using a P arrival time inversion scheme (using VELEST code) not only to obtain a more reliable earthquake location especially in depth, but also to decrease the hypocentral error. We have used all the data between 2006 and 2013 after applying a band-pass filter to get a uniform dataset of the earthquake within 250 km around the main shocks. The final velocity model indicated two velocity layers in the upper-crust with p-velocities of 5.87, 6.01 km/s and 6, 18 km thicknesses, respectively. These layers lay on a half-space with a p-velocity of 6.40 km/s. To minimize the effect of the initial velocity model on the final result, we implemented 50 random depth-increasing velocity models. Furthermore, making use of a non-linear probabilistic approach for relocating the earthquake leads to more accurate results compared to linear location programs. A comparison of the results of hypocenters between the IRSC catalog and those calculated by this study shows better line-alignment in the direction of the infer fault. Epicenter and depth error reduction due to making use of an accurate local velocity model were clearly obvious. Plotting the five depth cross sections along and perpendicular to after-shock sequence, shows a more clear geometry compared to the fixed-depth results from the IRSC catalog. According to some statistical parameters such as the hypocentral error, RMS and also preliminary location conditions such as the azimuthal gap and the number of stations, we defined two classes of events and plotted them for both IRSC and this study in map view and cross sections. This was a better way to show accuracy of our dataset (relocated events) than what we obtained by IRSC. Using the focal mechanism of the two main shocks obtained by the IRSC, along with the event distribution especially in depth, showed that there was more consistency between the results of this study and the fault orientation. This showed that the infer fault had a near vertical plane with an east-west direction and this suggested that it would be a strike-slip fault.

We have analyzed the state of stress in Fars Province, Iran, based on systematic inversion of available focal mechanisms of the earthquake method of Angelier (2002). Fars Province makes up a significant portion of the “Fars salient” in the Zagros fold-thrust belt. The main purpose of this study was to evaluate the uniformity of the seismotectonic stress regimes and to investigate the dominant and possible kinetic mechanism of the faults that cause earthquakes in different parts of the study area. We analysed the data using two main methods. First, the application of the right dihedral method quickly provided a robust graphical expression of the mechanical compatibility within a set of focal mechanisms. Second, a direct inversion gave an accurate quantitative account of the stress state. This method is based on consideration of the SSSC (Slip Shear Stress Component) criterion. The SSSC is the component of stress acting in the slip direction of a fault. The intrinsic characteristics of the adopted criterion results in two main technical properties of the method. First, no choice between the nodal planes is needed prior to or during the inversion. Second, the numerical aspects are reduced to a minimum so that the runtime is negligible regardless of the size of the data set. The major advantage of this method is that, after the inversion, three main estimators enable one to evaluate the mechanical consistency of a data set in terms of both the individual and the average misfit levels obtained from the best-fitting stress tensor. These estimators are the estimator of the stability of the stress regime, the estimator to determine the accuracy of the analysis level and the estimator of mechanical stability stress tensors. The focal mechanism of the earthquake data was collected from 1935 to 2013 from a variety of sources. Some of these sources were online moment tensor catalogs and other sources were extracted from the literature. Due to the very different characteristics of tectonics in Fars Province, we deemed it necessary to divide the area into zones with relatively similar stress regimes, and then follow these methods for each zone. Therefore, in view of the tectonic features and earthquake characteristics, the best linear unbiased geostatistical estimator method (Kriging) was used which provided the ability to predict under study variables in each unsampled coordinates based on a spatial correlation function. Therefore, using the spatial prediction method of Kriging, we could predict the spatial axes of the stress variables P and T to pre-identify the zones. This pre-identification of zones improves and facilitates using the method of systematic inversion of focal mechanisms. To choose the best spatial model, the Leave-one-out cross validation method was used. In addition, prediction residuals were evaluated to select the best spatial Kriging method. Evaluating and cross validation results showed that the Ordinary Kriging method presented the best spatial prediction method to pre identify the zones. According to the Ordinary Kriging results, the five relatively distinct zones of stress characteristics in Fars Province were identified. The results of the systematic inversion of focal mechanisms indicated that the mean principal stress axes optimized zones do not change significantly, and the common trend of NNE SSW (N21E to N34E) was determined. The uniformity of the stress regime of the area (Fars Province) increased as we moved from the northwest (the range of Kazeroun shear fault zone) to the southeast and the southwest areas. The nonuniformity of the stress regimes implied heterogeneity and heterogeneous kinetic mechanisms for seismic faults and the stress distribution in the mentioned areas. The kinetic mechanism of the northwest and southeast fault zones have a tendency to strike slip and general mechanism of southwest area is revers for many faults.

The V shape kink of the Alborz Mountains at its southern end reaches to the Garmsar city located 100 km southeast of Tehran metropolis. We investigated seismicity and seismotectonic features of the Garmsar area by precisely locating of microearthquakes recorded by our local dense seismological network and by the Iranian Seismological Center (IRSC). Our results indicate high seismic activity at the central and western parts of the Garmsar fault. Three computed focal mechanisms revealed compressional movements of the central part of this fault. Very little seismic activity is observed on the Eyvanekey and the Pishva faults. The only computed focal mechanism for the northern hills of the Garmsar fault shows tensional movements in this area, which refers to strain release among the Garmsar and Sorkheh reverse faults. Most of the calculated focal mechanisms in the Garmsar area indicate compressional and strike slip motions with overall P axis direction of 10° to 35°. The calculated P axis with NW-SE trend, close to the Sorkheh fault, is different from the other calculated P axes that show NNE-SSW direction. This is probably due to rotating of structures in this area, as revealed by recent GPS measurements in this region.

Having knowledge of stress variations in the Zagros region, southwest Iran is necessary to study the deformation resulting from oblique collision between the Eurasian & the Arabian plates and to obtain insight into the complicated tectonics of the region. In this study, earthquakesfocal mechanism data were used to collect information on the state of stress in 12 subdivisions of the data including teleseismic and local events in the Zagros region. The stress axis show noticeable variations in the Zagros region, especially around the Oman Line. The angular difference between the stress & strain axis increases from the southeast to the northwest of the Zagros Mountain. The deformation partitioning due to pre-existing faults and fractures and introducing a weak zone in the NW Zagros under the influence of the Main Recent Fault activity may explain this increasing.

Tehran, Iran’s capital with more than 10 million population is located in the southern foothills of the Alborz collision zone. The Alborz active mountain range consists of several sedimentary and volcanic layers, EW trending mountain belt 100-km wide and 600-km long, is bounded by Talesh Mountains to the West and by the Kopet Dagh Mountains to the East. 5±2 mm/yr shortening and 4±2 mm/yr left-lateral strike-slip motion in central Alborz implies a slip partitioning between strike-slip and reverse faults across Alborz. The city is bounded by active faults. Several of these faults have been mapped but their geometry at depth, their seismicity and kinematics are not precisely known. Historical earthquakes are associated with Mosha, Taleqan, Parchin and Garmsar faults, with the largest events on the Garmsar (Ms ~ 7.6) and Taleqan (Ms ~ 7.7) faults during the third and tenth centuries BC, respectively. Obtained information until now reveal that better understanding of the Alborz region needs more detailed studies in longer time intervals. Several questions about faults geometry, associated seismicity, their interactions and the mechanism of deformation in this region are remained unanswered. Considering the weak geological evidence of fault activity in some parts of Tehran, and rare calculated focal mechanisms for large earthquakes, moment tensor solution of small ones can help us with better understanding of fault behavior in this region.Combining the data recorded by 29 local seismic and accelerograph stations, October 17, 2009 Ray Earthquake, Mw 4.3, was located in the westernmost part of the Parchin fault in the south of Bibi Shahrbanoo Mountain, 35.57° latitude, 51.51° longitude and 15 km depth in south-east edge of Tehran mega city. Using first motion data, a reverse mechanism with a small component of the strike-slip motion was determined.Deviatoric moment tensor was inverted by using broadband data recorded by seven Iranian stations from National Seismic Network, INSN. We used ISOLA program (Sokos and Zahradnik, 2008) that is based on the multiple point-source representation and iterative deconvoloution method, similar to Kikuchi and Kanamori (1991) for teleseismic records, but here the full wavefield is considered, and Green functions are calculated by discrete wavenumber method of Bouchon (1981). Doing many tests, we selected the 0.06-0.095 Hz frequency range that resulted in the highest variance reduction. Besides, we examined the centroid-depth range between 5 and 23 km to find the best correlation. To calculate Green functions, we used the velocity model by Abbasi et. al. (2010) for the Southern flank of Alborz. Inversion with different data subsets verified the stability of the solution.The deviatoric moment tensor inversion for this earthquake by waveform modeling shows almost a pure reverse mechanism, 97% DC component, in northwest-southeast direction along Parchin fault and a centroid depth of 11 km. It is another evidence of dominant reverse mechanism in the southern edge of the Alborz region that implies the accommodation of deformation in Alborz by the slip partitioning. The estimated seismic moment for this earthquake was 3.096e15 Newton meter resulting in a 4.3 moment magnitude using Kanamori (1977) relation.

Mosha is one of the most important and threatening faults in TehranMegacity (Capital of Iran). Instrumental seismology in the region was limited to insufficient data along with location errors especially in depths as well as the small number of available focal mechanisms in bound with the trends in Hedayati et al. (1976) and Ashtari et al. (2005). In the study ahead, by installing 48 local and temporary seismological stations during June to November 2006, micro earthquakes around the eastern part of the southern flank of central Alborz particularly the Mosha fault zone were recorded and processed. The local temporary network consisted of 24 one-vertical-component TAD-2 Hz, 11 3-components MiniTitan- 5 S and 13 3-components Guralp 6TD 0.02-10 S sensors. Sampling rates were 100 samples/sec the for Guralp sensors and 125 samples/sec for the others in continuous and triggering threshold modes. 115 well recorded micro earthquakes with an appropriate azimuthal gap (Gap ≤ 180°), a trivial residual timing and location errors (RMS ≤ 0.3 sec, Erh ≤ 2 km and Erz ≤ 3 km) were selected and applied for the wave velocity ratio (Vp/Vs) calculation based on Wadati (1933) and Chatelain (1978) approaches (1549 P-wave and 1495 S-wave arrival times). A 1-D model of the upper crustal velocity structure was determined as well. SEISAN software (Havskov and Ottemöller, 2005) for phase readings, Hypo71 (Lee and Lahr, 1975) Hypocenter (Barry, 1994.) for seismic event locations, VELEST (Kissling, 1988) for a crustal velocity layers model and FOCMEC program (Snoke, 2003) for focal mechanism solutions were used. Four layers at the depths 3, 7, 16 and 24 km of the crust were determined with P-wave velocities of 5.4, 5.8, 6.1 and 6.25 km/sec, respectively. Accurate locations of 553 micro earthquakes and 15 A and 31 B (excellent for A and good for B groups in red and blue colors in related figures respectively) classes of focal mechanism solutions of the reliable micro earthquakes with a high quality of P-wave first arrival polarities (more than 8 Pg onset’s signs), were provided for the possible analyses of seismicity, the fault geometries-movements and seismotectonic interpretations. We have found that the Eastern part of Mosha fault, longitudinally located from 51.7° to 52.5°, has a northward high dip angle and complex focal mechanisms. The fault mechanisms varied from thrust, strike slip with a small reverse component to reverse with a small normal component from the West to the East. From grouping analysis of the focal mechanism P (or T) axes, the strikes, N 40 (or N 130) were derived for the compression (or tension) stress direction approximately. The focal mechanisms accompanying with the geodynamic analyses from GPS measurements in the studied area reveal a slip partitioning in the local and regional scale compatible with some conclusions from the previous studies (Ritz et al., 2006 and Tatar et al., 2007). Although micro and large earthquakes nonlinearity relation in stress axes orients as true or not proved is important (Mercier et al., 1991 and Hatzfeld et al., 1999), seismotectonics strain analyses of the micro earthquakes in the studied area show the same results of the large earthquakes stress analyses in stress inversion method by Gillard and Wyss (1995). In addition, this study has demonstrated a seismic active trend as mentioned by Jajrood-Pardis-Absard in the South of Mosha fault. Concentrated seismic activities around Mosha fault in the time of data recording have shown that it is a potential hazard for the studied area.

The December 20, 2007 earthquake has occurred three months after the September 16, 2007 earthquake near the Tabriz city in East Azarbaijan province. We have used SH- waves accelerographs data and Brune model to estimate the causative fault plane parameters. The strike, dip and rake have been estimated as 310o, 85o and 170o, respectively. The focal mechanism shows right- lateral strike slip, which is consistent with the North Tabriz Fault. This is the first focal mechanis for the North Tabriz fault based on the strong ground motion data.

From June 2004 to December 2008 low seismic activity was recorded near North-Tehran، Taleghan and Kahrizak faults and inside of Tehran city. In contrast، seismic activity along Mosha، Garmsar and North-Alborz faults is considerable. Generally seismic activity decreases from 51 degrees longitude to west. Two earthquakes with 15 and 17 km depth were located in the west of Tehran city. The calculated focal mechanism for one of them is pure strike-slip. High seismic activity is observed along Mosha fault close to Damavand، Boumehen cities and Lavasant-e-Bozorg region. Calculated focal mechanisms along this fault includes both strike-slip، and reverse mechanisms that implies transpression motion، dominantly left-lateral slip along this fault that continued to Lavasanat region in south of the eastern end of the North Tehran fault. Precise location of some events shows depth range of 4-32 km. Generally، calculated focal mechanisms in studied region include both strike-slip and reverse mechanisms and seems that in southern part، approaching Central Iran، reverse mechanisms are dominant. It implies slip partitioning in southern margin of Central Alborz.

A moderate earthquake (MW= 6.0) struck the Qeshm Island in the Hormozgan province on November 27, 2005 (17:22 GMT) and resulted in a severe damage and about 10 casualties. The main-shock was followed by an aftershock with magnitude MW=5.5 with different focal mechanisms from main-shock. A dense local network including 17 stations was installed in the region for aftershock study. Analysis of aftershock data shows diffuse distribution of the aftershocks; however, an alignment trending NW consistent with main shock focal mechanism is clear at depth. Two types of focal mechanisms can be observed: strike-slip and reverse. Diffused pattern of aftershock seismicity and focal mechanisms do not allow us to make a distinction between two possible explanations: occurrence of the second event in NW oriented strike-slip fault or partitioning of the deformation in the western border of the Hormoz Straight by reverse and left-lateral strike-slip faults.

Iran is located in a very complex tectonic area, where continental shortening takes place due to the collision of the Arabian and Eurasian plates. This Arabia-Eurasia Convergence occasionally causes a distractive earthquake such as the Silakhor Earthquake to occur in Iran. The March 31 2006 Silakhor Earthquake, with a magnitude of Mn=6.0, occurred on Friday at 4:47:03 local time near the village of Chalan-Cholan in Lorestan province. It was preceded by two large foreshocks with magnitudes of Mn ‌‌=‌‌‌‌ 4.7, 5.2 and followed by two relatively large aftershocks of Mn=4.9 and 5.3. The Silakhor plain was seriously affected in earthquake: about 70 people were killed and more than 2000 were injured (Mirzaei Alavijeh et al. 1385). Mahdavifar and Tajik (1385) reported a macroseismic intensityof I0 = VIII on the MSK98 scale for the Silakhor Earthquake. More than 30 foreshocks and many aftershocks were recorded in the Silakhor Earthquake. Such extensive foreshock-mainshock-aftershock sequences for an earthquake of moderate. magnitude (Mn = 6.0) is unusual The earthquake sequence occurred along the Main Recent Fault (MRF) in the northern Zagros. The right-lateral strike-slip displacement along the MRF fault is about 3 ± 2 millimeter per year (Vernant et al. 2004). All seismic stations which recorded Silakhor earthquake are located at a distance of at least 100 kilometers from the epicenter. No attempts were made to record micro events by deploying a temporary seismic network in the source region. To analyze this earthquake, an approach was made to relocate the recorded sequence by gathering all available data and using a proper velocity model. The region in this study enclosed between 48.4º to 49.2º east longitudes, and 33.3º to 34º north latitudes. The Silakhor earthquake sequence was recorded in many permanent seismic stations. These stations are operated by the University of Tehran's Institute of Geophysics (IGUT), the International Institute of Earthquake Engineering and Seismology (IIEES), and the Karkeh Dam. All picked phases data were compiled from the bulletins of organizations listed above. Additionally, in order to plot a PGA contour map of the source region, the acceleration data recorded in stations which are run by the Building and Housing Research Center (BHRC) were used. For this research, recorded events were relocated by using different location programs and a proper velocity model. The results show that applying the Hypoinverse program (Klein, 1984) gives less errors for hypocenter parameters in comparison with other location programs, such as Hypo71 (Lee and Lahr, 1975) and Hypocenter (Lienert et al. 1986). The focal mechanism of the main shock was determined by using the polarity data of the first arrival waves in the seismic stations. The mechanism for the mainshock was obtained as Strike/Dip/Rake = 310º, 46º, 171º. The obtained focal mechanism shows that the activated fault segment in this earthquake has a right lateral mechanism with a dip toward the north-east. The focal mechanism of mainshock and iso-acceleration curves show that the mainshock rupture in the Silakhor Earthquake was a unilateral rupture and that it initiated near the southeastern end of the rupture zone and propagated toward the northwest. The distribution of relocated aftershocks shows that in the area of this earthquake two fault segments were active. The first segment, which includes the main shock, is located between Dorud and Silakhor. The second segment is located between Silakhor and Borujerd. It seems that there is a gap of about 10 km length between them.