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

Northeastern part of Iran islocated in the Kopeh Dagh seismotectonic province. This province is part of the Alpine-Himalayan orogenic belt. In the last two decades, a number of earthquakes occurred in this region. Therefore, investigating and relocating these earthquakes with adequate precise play a great role in seismicity research of the study area. Adequate crustal structure velocity has a key role in relocating the earthquakes. Since the earthquakes in this region are shallow earthquakes, the upper part of crustal structure can be acquired. So in this study, Mashhad and Quchan seismological networks data were used to determine the crustal structure velocity. In order to determine the velocity structure, Vp/Vs was first determined using the methods of Wadati and travel time of1.72 .Then by applying various filters and selection of several earthquakes 1D velocity structure of the arrival times was determined. Results reveal two velocity discontinuities approximately at 4 and 14 km depths. The resulting model consists of a 4-km-thick layer with VP = 5.9 kms−1 overlying a 10-km-thick layer with VP = 6.17 kms−1. Then the initial 1D velocity model was used to determine the three-dimensional velocity structure using LOTOS code. The results show significant changes in the vicinity of Mashhad. Two zones with higher velocity are observed in the north and south of Mashhad due to Kashaf Roud and Binaloud Faults. In the north of Mashhad, a change in velocity is observed, which might be due to the Tous fault. In the south of Mashhad, a sharp velocity discontinuity at the depth of 4 km is probably due to an alluvium cover above a bedrock of varying thickness. Also, the tomography results indicate that the area of Shandiz fault system in the south of Mashhad is associated with a lower velocity zone compared to its adjacent areas.

NW Iran is part of the complex tectonic system caused by the interaction between Arabian plate, Anatolia and Eurasia. The North Tabriz Fault (NTF) is one of the main structural features of the region and is considered to be the eastern termination of the Gailatu-Siah-Chesmeh-Khoy fault (Karakhanian et al., 2004), which merges with the Maku and the Nakhichevan dextral strike-slip faults and continues to move farther east. Part of the northward motion of Arabia is transferred to Anatolia by this complex system of faults and, the oblique orientation of the motion relative to the Zagros mountain range,results in the partitioning of the motion between shortening and thickening in the Caucasus and right-lateral strike-slip motion along the NTF.
In this research, we investigated the laterally two-dimensional velocity structure in the upper crust of NW Iran (mainly around the NTF) using local earthquake P-waves tomography. Several data sets were utilized, including Pg phase pickings of the Tabriz Network permanent stations governed by Institute of Geophysics, University of Tehran (1996 to 2013), temporary seismic stations installed around the North Tabriz Fault by International Institute of Earthquake Engineering and Seismology (IIEES) (April to July 2004) and temporary seismic stations installed by Institute for Advanced Studies in Basic Sciences (IASBS) (2009 to 2011) that merged with our data set so as to improve ray coverage in the eastern parts of the study area. The merged data set, recorded by 72 stations, consisted of more than 20,000 local earthquakes out of which, only 940 earthquakes were good enough to be selected for the local earthquake tomography. The velocity structures were resolved via a simultaneous solution of the coupled hypocenter and velocity model programmed in SIMULPS14. The tomographic images obtained from the linearized inversion are dependent on the initial velocity models and hypocenter locations. We primarily calculated the initial velocity model through the use of the 940 earthquake selected datasets. The time difference between the observed phase arrival time and predicted arrival time was then calculated and called travel time residuals. The residuals were further used as inputs for SIMULPS14 simulator to be converted into velocity model, which would in turn be used to adjust earthquake location parameters. Following four iterations for our inversion process, we obtained a 2D velocity tomogram that clearly showed different velocity structures on the two sides of the NTF. The velocity contrast across the NTF might have been caused by existence of different kinds of rocks on the two side of the fault trace. The North Tabriz Fault is an active and steep strike-slip fault generating strong structural differences around its surficial trace. It is a WNW–ESE trending fault in which the motion is concentrated on the fault at a rate of 7 mm/year. Such a strong rate of sliding explains the clear structure difference on the two sides of the fault. An anomalous low velocity feature can be seen in the central part of NTF. Comparing the velocity tomogram with the geological map of the region, one can observe that there exists a thick sediment basin in the same area. The low velocity anomaly is probably related to the thick, low velocity sediments deposited in that area.

Nearly one-third of the shortening in Iran plateau due to Arabia-Eurasia convergence is accommodated in the Zagros fold and thrust belt, which is seismically one of the most active regions among the continental collision zones in the world. Deploying a temporary seismic network in the north of Bandar Abbas in Hormozgan Province, which consisted of 32 three-component seismograph stations operating for a duration of about 3.5 months, provided appropriate data for studying the upper crustal structure of the easternmost Zagros by usingLocal Earthquake Tomography. Three dimensional velocity model obtained the conversion of P-wave arrival times indicates southwestward overthrusting of the lower crust of northern parts, supporting active underthrusting of the Arabian plate beneath Central Iran in the easternmost Zagros. A very low velocity zone is also observed in the eastern part of the study area that could be associated with the Zendan-Minab-Palami fault system.