فهرست مطالب ایوب کاویانی

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.

Crustal structure of the Iranian plateau which is located between two convergent Arabian and Eurasian plates is studied. Teleseismic earthquakes recorded by broad band stations of Iranian National Seismic Network (INSN) are used to compute the receiver functions for each station. Rayleigh wave phase velocity dispersion curves were estimated employing two-station methods for all possible station pair of the above mentioned seismic network. A combined inversion of Rayleigh wave phase velocities and body wave receiver functions increases the uniqueness of the solution over separate inversions and also facilitates explicit parameterization of layer thickness in the model space. Our result indicates the crustal thickness differs from a minimum of 40 ±2 km in southeast of Iran, (ZHSF) to a maximum of 56 ±2km beneath the Sanandaj-Sirjan zone (SNGE). We observe a crustal thickness of 47 ±2km beneath the central Zagros (GHIR) to 52 ±2km below the eastern most of Zagros (BNDS), then to 47 ±2km beneath the northwestern part of the Zagros (SHGR). Crust of the Central Iran (KRBR) has a thickness of 48 ±2 km while the average Moho depth in southern parts of the Central Alborz (DAMV and THKV stations) is 54±2km. Our analysis shows a thinning of the crust to 43 ±2 km beneath the northwest of Iran (MAKO) and western part of the Caspian basin (GRMI).

We compute p receiver functions (RF) to investigate the crustal thickness and Vp/Vs ratio beneath the northwest of Iran and map out the lateral variation of Moho depth under this region. We selected data from teleseismic events (Mb≥5.5, 30˚>r > 95˚) recorded since 2000 to present at 8 three component short period stations from Tabriz teleseismic network. RF method is now a well-known tool for studying crustal thickness when such a complete data set is available. As the p-to-s conversion points at the Moho is laterally close to the stations, the Moho depth estimation is less affected by lateral velocity variations and thus provides a good point measurement. First of all, we calculated RFs for each station and then the Moho depth will be estimated only from the delay time of the Moho p-to-s conversion phases. Then we used an H-Vp/Vs stacking algorithm (Zhu & Kanamori, 2000) to estimate crustal thickness and Vp/Vs ratio under each station from P-to-S converted waves in receiver functions. The best value for H and Vp/Vs ratio is found when the three phases (Ps and crustal multiples) are stacked coherently. The average Moho depth is approximately 48 km and varies from 38.5 1 to 53.5 1 km. Deeper and shallower Moho is found under the western and eastern stations beneath SHB and SRB stations respectively. The northwest Iran's crust has an average Vp/Vs ratio of 1.76, with higher ratio of 1.82 0.03 in Tabriz station and lower ratio of 1.73 0.03 in Azarshahr station.

Characterization of the detailed structure of the crust and upper mantel is an important continuing goal of geophysical studies. Teleseismic body waveforms have been used to infer crust and upper mantel structure. In this study we use teleseismic receiver function method to determine the crustal thickness and VP/VS ratio under Kermanshah network in north-west of Zagros using teleseismic data (30°