جستجوی مقالات مرتبط با کلیدواژه « velest » در نشریات گروه « فیزیک »

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.

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.