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

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

Iran is located between two lithospheric plates (Euroasia, Arabian) and these two plates converge toward each other with a velocity of 25 mm/yr rate. Shortening that produced from this convergence with Makran subductin shows faulting and folding in Zagros orogenic belt, Alborz and Kopedagh in north and sliding in sum strike slip faults with north-south trend in central Iran. The NW-SE trending Zagros fold and thrust belt, extends for about 1800 km from a southeast of the East Anatolian Fault in northeastern of Turkey to the Strait of Hormuz, where the north-south trending Zendan-Minab-Palami fault system separates the Zagros belt from the Makran accretionary prism. The Zagros range currently accommodates almost half of NS shortening between the Arabia and Eurasia. This active fold and thrust belt is subdivided into five morphotectonic units: the High Zagros Thrust Belt, the simple Fold Belt, the Zagros Fordeep, the Zagros Coastal Plain and the Persian Gulf-Mesopotamian lowland. The studied region in this paper located in Fars province, in the High Zagros Thrust Belt, north Eastern part of this region is Abarkuh desert that located in central Iran is less active than the other border region around studied region. The west and southeastern part of this region is located in Zagros seismotectonic province that a lot of earthquakes were seen. For study of velocity of seismic waves and upper crustal velocity model in Shiraz region the recorded data by Shiraz seismic network during 2002 to 2009 were used and for seismicity, International Seismological Centre (ISC) catalog, Harvard Centroid Moment Tensor (CMT) catalog and historical earthquakes catalog (Ambraseys and Melville, 1982) were used. Crustal structure of this region is a particular issue that is not yet resolved in the studied region. In order to study the crustal structure of studied region, we use Shiraz network that have 5 stations, To assesse the velocity model, subset of 78 events was selected that recorded by minimum of 4 stations, with an azimuthal gap less than 270º, residual RMS less than 0.3s and uncertainties in epicenter less than 6 km and depth less than 10 km. Consequently using these events, a crustal velocity model obtained with VELEST software for the upper crustal velocity model beneath studied region. The calculated velocity model for the studied region showed three discontinuities in 6, 10 and 14 kilometer depths. P wave velocity has been obtained 5.68 km/s, 5.88 km/s, 6.54 km/s and 6/66 for the first layer, second layer third layer and half space, respectively. Plotting Tsj-Tsi (S arrival time to stations i and j respectively for same event) versus Tpj-Tpi (P arrival time to stations I and j respectively for same event) for all events and all stations, Vp /Vs ratio was computed about 1.77 with 908 arrival times. Comparison of obtained VP/VS value with other research results shows that there is not a significant difference between them. Also, local velocity curves for Pg, Pn, Sg, and Sn phases are obtained in the study area by using the data base 2002 through 2009. The slopes of these curves give crustal P and S velocities of 6.16±0.02 and 3.71±0.02 kms-1, and Moho P and S velocities of 7.8±0.1 and 4.78±0.04 kms-1, respectively.

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

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

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.

In current study, earthquakes occurred in northwest of Iran in the past 20 years were relocated using JHD algorithm. A dense seismic network is usually required to produce a stable and precise earthquake location, i.e., improved quantitative and qualitative earthquake data are achieved due to various parameters including low amount of azimuthal gap. Combining data from different seismic networks in the study area can be effective and useful to improve earthquake locations. Thus, a list of earthquakes in the northwest region of Iran was developed using data available from different seismic networks. The recorded earthquakes data by the seismic network stations of Iran and neighboring countries were collected and a complete data set of earthquakes in the region was prepared. Then, the information from 30,000 recorded and reported earthquakes in the area was compiled. This information includes more than 400,000 seismic phases from databases of International Seismological Center (ISC), Iranian Seismological Center (IRSC), Broadband Iranian National Seismic Network Center (IIEES) and National Seismic Network of Turkey (DDA). This dataset was sorted and the outliers were taken off based on analyzing the travel-time residuals. In the next step, the recorded phases of an earthquake from different stations were combined and a homogeneous catalog was established. This catalog is more complete than the original catalog, e.g., because of increasing the number of phases, azimuthal gap of earthquakes has decreased. In the next step, after the preparation of the data, the hypocenter relocation was done using single event and the JHD algorithms in VELEST computer program. Comparison of RMS of earthquakes indicates a decrease in the RMS due to relocation process. The average value of RMS for the entire earthquakes reduced from 0.69 s to 0.41 s at the final stage, and this shows a reduction in difference between calculated and real earthquake location. The RMS value for more than 90% of earthquakes is less than one second. Thus, a homogeneous catalog of earthquakes in the region was prepared and new results were evaluated and compared with the initial information. Finally, the seismicity patterns in the region were investigated by drawing the hypocenter depth sections in the active seismic areas. Based on the obtained results in many sections, the removal of unrealistic structures due to systematic errors in earthquake location was evident, and in some sections, scattering of the earthquake hypocenter locations reduced and the existing structures were more obvious.

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 northward motion of the Arabian Shield related to the Eurasia at a rate of ~22 mma−1 is primarily accommodated across the Iranian Plateau and results in different styles of deformation in various parts of this continental collision zone. The deformation is concentrated in the Zagros, Alborz, and Kopeh Dagh mountains and shear zones surrounding Central Iran that behaves more or less as a relatively rigid block. The deformation in northeastern Iran is concentrated in the Kopeh Dagh and Binalud mountain ranges and the boundary between Alborz–Binalud and the Kopeh Dagh mountain ranges run along the Atrak River. In this study, we determine an optimum one-dimensional (1-D) velocity model of upper crust for northeastern Iran from local earthquakes travel time inversion.