جستجوی مقالات مرتبط با کلیدواژه "group velocity" در نشریات گروه "زمین شناسی"

To obtain the distribution of the Love wave group velocities in northwest Iran and its surrounding areas for 7, 10, 20 and 25 second periods, the waveform of 241 local and regional events used in a dispersion curve tomography of the Love waves. The evnets occurred within NW Iran and surrounding areas between 2005 and 2015 and were recorded by 39 permanent and temporary medium and broad band seismic stations belonging to national and international seismic networks. In order to reduce the effect of non-uniform distribution of events, we selected the more uniform list of events out of an initial number of 1734 non-uniform distributed events. We applied the Frequency-Time Analysis method to each event-station path for estimation of Love wave group velocities, then we used a tomographic method to compute the distribution of local group velocities throughout the region. From the analysis of the surface wave tomography, we concluded that crustal structure in the South Caspian Basin and the Kura Depression is almost the same, but it is significantly different from that of the northwest Iran. In the presence of thick and low shear velocity sediments in the South Caspian Basin, we observed low velocity anomalies at periods less than 10 seconds that are surrounded by relatively higher velocities along Alborz, Talesh and Lesser Caucasus. In the “Eastern Anatolian Accretionary Complex” where volcanic activities are much higher than in rest of the region, less group velocities were observed for periods less than 25 seconds. The main reason for this observation can be related to the presence of partial melting zones inside the crust as a result of intensive volcanic activities in this region. In Zagros, for the periods of 7 and 10 seconds, a relatively high velocity anomaly along the “Sanandaj-Sirjan” metamorphic zone was observed, which was trapped by two low velocities along the “Zagros Folding and Thrust Belt” in one side and “Urmieh-Dokhtar Magmatic Arc” in another side. The active and broken crust, reverse, active and shallow thrust faults and high seismicity of Zagros Folding and Thrust Belt is characterized by low group velocities in our results for periods of less than 25 seconds. In most areas of central Iran and Urmieh-Dokhtar Magmatic Arc, local sedimentary basins are characterized by low velocities for periods of 7 and 10 seconds. Our results indicate that crustal structure in the northwest of Iran is almost uniform and has the minimum changes, but it is completely different from the crustal structure in the South Caspian Basin, eastern Turkey, and the Zagros mountains.

In this study, using single-station dispersion curves, two-dimensional tomographic maps of group velocity have been estimated for Kopeh Dagh region in the northeast of Iran. An investigation of the structure of the lithosphere and the asthenosphere of the Kopeh Dagh region is of great interest because of the complex tectonics of this area. The Kopeh Dagh forms an NW–SE range of mountains separating the Turkmen (Turan) shield from central Iran. The belt is up to 3000 m in elevation, rising 2000 m above the Turkmen plains. It is highest and narrowest in its center and the east, becoming broader and lower towards the west, where it merges with the lowlands of the SE Caspian shore. The range is asymmetric, with a steep, linear and narrow NE side on which short, straight streams run directly from the high ground to the Turkmen plain. Its SW flank contains gentler slopes, is less regular, and contains a broader, more developed drainage network. The NE flank of the Kopeh Dagh is assumed to be underlain by ‘Hercynian’ basement of the Turan shield, while the range itself contains thick Jurassic–Cretaceous marine sediments overlain by Eocene marls with some andesitic volcanoclastic horizons. These, in turn, are overlain by thick marine Pliocene units in the west (which continue into the South Caspian Basin) merging eastward into a continental sequence of equivalent age, reflecting the diachronous emergence of the range. Although there is some evidence for local structural detachments in Jurassic evaporites, most of the late Cenozoic and active folds in the Kopeh Dagh are thought to have developed in the hanging-walls of basement-cored thrusts. There is no evidence for major salt thicknesses that can produce large-scale regional decollement levels, as in the Zagros. Earthquakes in the Kopeh Dagh involve mostly right-lateral strike-slip faulting trending N to NNW or reverse faulting parallel to the NW regional strike. The abrupt linear topographic front forming the NE margin of the Kopeh Dagh follows a fault zone referred to as the Main Kopeh Dagh or Ashkabad Fault. The analysis of the lateral variations in group velocities in low, intermediate and long period is an optimal tool for the identification of the main different tectonic features present in the complex Kopeh Dagh region. These parameters are sampling crust and upper mantle structures as period increases from 3 s to 35 s. Generally, shorter periods bring information about velocity properties at shallow depths, whereas longer periods sample deeper into the Earth’s upper mantle. To explore two-dimensional tomographic maps, in the time period of 2006 to 2013, local earthquakes with high signal-to-noise ratio recorded in six broad-band stations operated by International Institute of Earthquake Engineering and Seismology (IIEES) are selected. In the first step, initial corrections were performed and surface wave fundamental modes for 1075 earthquakes are isolated. Frequency- time analysis software (FTAN) is used to isolate fundamental mode from higher modes. By using this approach, single-station dispersion curves are calculated. The estimated group velocity dispersion curves are used to explore two-dimensional tomographic maps. Yanovskaya–Ditmar linear inversion method was used to achieve this goal. According to the obtained two-dimensional tomography maps, in low periods of 3 s and 5 s (shallow depth), we find a high-velocity anomaly in the south of Kopeh Dagh region that has sharply separated Alborz–Binalud zone from the Kopeh Dagh zone; so we can consider a separation boundary between Alborz–Binalud zone and Kopeh Dagh zone. In the period of 20 s, we have a high-velocity anomaly from north to south. At the west of this high-velocity anomaly, also there is a low velocity that makes a sharp boundary. In the period of 35 s, the tomographic model obtained reveals high- and low-velocity anomalies in the near distances of the uppermost mantle which have been separated from each other with a sharp boundary.

The delineation of the elastic, or velocity, structures of the Earth has long been a goal of the world's seismologists. In the first few decades of seismological research, the investigation of velocity structures was restricted to the determination of one-dimensional models of the solid Earth and various regions within it. Seismologists are currently working on three-dimensional velocity models, and they are trying to resolve finer and finer features in the Earth. The knowledge of seismic velocity structure of the crust and the upper mantle is important for several reasons: It includes the accurate location of the earthquakes, it is used in the determination of the composition and origin of the outer layers of the Earth, it improves our ability to discriminate nuclear explosions from earthquakes, it helps to interpret the large-scale tectonics as well as a reliable assessment of earthquake hazards. In this study, we used highfrequency dispersion curves to estimate the elastic properties of Tehran and the suburbs. Tehran city is located in the Alborz major seismic tectonic zone. The Alborz is an arcuate chain of mountains in the Northern Iran that wraps around the Southern side of the South Caspian basin; the boundary is roughly the present shoreline of the Caspian Sea. The range is actively deforming on range-parallel thrusts and left lateral strike-slip faults. The thrusts dip inward toward the interior of the range from both its northern and southern sides, and the GPS-derived shortening across the range is 5 ± 2 mm/yr at the longitude of Tehran (Vernant et al., 2004b). Most are parallel to the range and accommodate the present-day oblique convergence across the mountain belt. Recent large earthquakes occurring in this region suggest that the seismicity is connected with major faults of the recent age that cut across the regional Quaternary Lineaments. In this study, in order to estimate the upper crust elastic structure, we conducted a tomographic inversion of the Love wave dispersion to obtain two-dimensional Love wave group velocity tomographic images in a period range from 2 s to 5 s for the city of Tehran and the suburbs. We used two databases to derive dispersion curves for different paths. In the first dataset, continuous ambient noise in ten stations located in and around the city of Tehran and installed by the Tehran Disaster Mitigation and Management Organization network was used to explore the inter-station Green’s Functions. In the second database, forty seven earthquakes recoded by the Parsian stations were prepressed and rotated to be used as single station records to estimate the Love wave group velocity. In the next step, the inter-station Green functions and single-station records were used to estimate the Love wave dispersion curves by applying a multiple filtering technique. All dispersion curves were used to estimate the two-dimensional Love wave group velocity models. For this purpose, Tehran region was divided into 0.1° × 0.1° cells. According to the ray coverage, the minimum dimension of distinct heterogeneity was 6 km. In our study, topographical features and near-surface known geological structures were two main criteria to assess the credibility of the estimated Love wave group velocity variations. There was a strong correlation between the estimated group velocities and topographical features. The prominent surface geology units at the mountain range consisted of varying structures including sand-stone, siltstone, claystone, and massive limestone.