جستجوی مقالات مرتبط با کلیدواژه « زمین لرزه های محلی » در نشریات گروه « زمین شناسی »

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.

In this study, 23 focal mechanisms were calculated by moment tensor inversion of small regionalـlocal earthquakes in the Western Alborz and surrounding areas using wave-form inversion method. Calculated reverse-fault focal mechanisms around the Khazar and Alborz Faults in the Central Alborz, considering southward-dipping nodal plane as the fault plane, are consistent with relatively low-angle thrusts. It also implies dominant compressionsl regime in the north of the Central Alborz. Focal mechanisms in 1990 Rudbar-Tarom Earthquake region show a combination of strike-slip mechanisms and a complex fault system in the middle of the Western Alborz. A major region of dominant strike-slip mechanisms is observed in the Talesh area, located in the west of the South Caspian Basin, and around the Masuleh, Sangavar and Bozqush faults. The only calculated focal mechanism close to the southern margin of the Western Alborz, considering the western part of the North Tehran fault as the fault nodal plane, implies leftـlateral motion in this area. In the south of the western Alborz Mountains approaching the Central Iran, two calculated mechanisms indicate dominant reverse movement, similar to the 2002 Changoreh-Avaj Earthquake. Five focal solutions close to the Kushk-e Nosrat and Soltanieh Faults, considering these faults as the fault nodal planes, are consistent with right-lateral motion along them. Depth of the earthquakes in the studied region is in the range of 2 and 20 km, indicating the brittle upper crust in the region.

Summary: In this study we used seismic data available from the permanent stations in the NW Iran to study seismic activities and crustal structure beneath the Tabriz region. The region of study is situated in the seismically active NW tectonic province of Iran، where detailed seismic information on the crustal structure and seismicity is lacking. The availability of seismic data collected since 1996 by Tabriz permanent short-period seismological network provided us with an unprecedented opportunity to study the seismicity and crustal seismic structure of this region. Our seismic data consisted of waveforms of local events collected by Tabriz seismological network operated by the Institute of Geophysics of Tehran University (IGTU) and two broad-band permanent seismic stations from the Iranian National Seismic Network operated by the International Institute of Earthquake Engineering and Seismology (IIEES). We manually picked the P and S arrival times on all collected waveforms. The events were initially located using a 1-D model used by the Tabriz network for routine daily event location. Of a total number of 728 events of local magnitudes greater than 2 recorded between 1996 and 2006 by the seismic stations، 361events of azimuthal gaps of less than 180 degrees and time residual of less than 1. 0 second were selected for the subsequent analyses. We then simultaneously inverted the arrival times of the local events for one dimensional velocity-depth models and event relocation. By considering the nonuniqueness of the inversion process، we inverted the data using a sufficient number of initial models. Finally، the models showing consistency with each other were selected. Some standard tests were performed to make sure that the final models were in agreement with the observed data. These tests revealed that the results of inversion were stable. The final best 1-D model is a 5-layer model to a depth of 35 km. Since we used local shallow events recorded at a small-aperture local network، the models are not valid for the depths deeper than 35 km. The final 1-D model gave P and S velocities smaller than the global average values for the orogens. This was because the study area was an active orogenic belt. On the other hand، the Vp/Vs ratio (which was found to be equal to 1. 745 using the Wadati Method) of our 1-D models showed values comparable to the average values for a typical continental crust. This presumably indicates that sources of partial melts، if exist beneath the region، are most likely residing in the lower crust. We also examined the event distribution in depth in order to delineate the geometry of the active faults. The majority of events occurred at the north of the North Tabriz fault. Therefore، it seems to be a vertical fault with a gentle northward dip and the events frequently occur in a depth range of 10 to 25 km (within the crystalline basement). And also some hidden faults seem to exist، which are not traced on the surface geological maps.

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.