پژوهشنامه زلزله شناسی و مهندسی زلزله
سال شانزدهم شماره 2 (پیاپی 60، تابستان 1392)

In this article, we try to investigate the ambient noise characteristics based on amplitude, frequency content and its origin that were recorded on the broad-band seismic stations. To do this, we reviewed the technical literature from middle of 20th century noise studies until recent years. Moreover, cross correlation function of noise that is equivalent with the experimental green function is explained as the background study. For the first time, Campillo and Paul (2003) have calculated group velocity of Rayleigh and Love surface waves from waveforms of 101 teleseismic earthquakes recorded in the national Mexican seismic network. After that, the investigation into ambient noise for Green function analysis have been continued by Shapiro and Campillo (2004, 2005), Schuster et al. 2004), Snieder (2004), Bensen et al. (2007) and Wapenaar et al. (2013). They showed that it is possible to get the Green function between stations through calculating Cross Correlation Function of recorded ambient noise. Ambient noise is important since it does not dependent on occurred an earthquake. That’s why ambient noise is used widely in order to study inner structure of the ground between two seismic stations. It should be noted that the dynamic range and frequency content of the seismometers has important effect on the wide band of group velocity dispersion curves. In other words, the ambient noise recorded by means of short period sensors is useful to derive low period dispersion group velocity curves only. Therefore, we strongly suggest using broadband seismic stations to study surface wave analysis based on ambient noise. The next key factor to derive sharp experimental green function is optimum distance between two seismic stations. The shorter distance generates a high frequency green function which is due to shallow depth penetration of the energy. Therefore, the depth of target will define the distance between two seismic stations. However, the type of sensor can be important. Based on seismic wave propagation theory, the Rayleigh wave can be seen on vertical component and the Love wave can be seen on the horizontal components of a seismic station. Therefore, if one like to study dispersion curves of Rayleigh wave need to process vertical component of seismograms. Of course, in practice, using rotated horizontal component can generate sharp Love wave. The seven seismic broad-band stations located in the south-southeastern part of the country are selected for this research. The cross correlation method was used to calculate the experimental Green’s functions for pairs of seismic stations (BNDS CHBR, BNDSZHSF, BNDS-KRBR, BNDS-NASN, BNDS-TABS, and BNDS-SHRT). Dispersion curves of group velocities belonging to fundamental mode Rayleigh waves determined by frequency - time analysis. Thegeneral form of calculated dispersion curves is very similar to the earthquake dispersion curves. Besides, results of dispersion curves are comparable with the average global model. Our study, show the ability of ambient noise analysis for determining the earth's velocity structure. The results may prepare the basic curves for the seismic structure modeling like surface wave velocity structure and/or seismic surface wave tomography.

Investigation of seismicity pattern before large earthquakes usually reveals the precursory activities. The seismic precursors have some advantage with respect to the non-seismic ones, including the wide range of their spatial and temporal domains. Two phases in seismicity change before a forthcoming, major earthquake are widely observed, which are the seismic quiescence and activation. The RTL (Region- Time-Length) analysis is a statistical method developed by SOBOLEV and TYUPKIN (199, 1997) to detect seismic anomalies preceding isolated large earthquakes. The RTL algorithm has been successfully used to detect precursory seismic activation and quiescence in different seismotectonic region around the world. We, therefore, consider the RTL algorithm a helpful technique to reveal the signi cantly precursory activity occurred before large earthquakes of Iranian plateau. The precursory seismicity in and around the epicentral zone of the 2012 Ahar-Varzaghan (Iran) dual earthquakes, by applying the RTL algorithm to earthquake catalogues derived from the Iranian Seismological Center for the period 2006 2012. First, we have determined the completeness magnitude for the selected region around the desired events. In order to avoid any bias in the results due to different magnitude conversion formulas, the original magnitudes in the catalogue (i.e. Mn) have retained. The RTL algorithm codes have been written using the Matlab software. By applying the RTL prognostic parameter, a clear quiescence stage followed by a period of foreshock activation is observed before two events of Ahar-Varzaghan, 2012. RTL performances are sensitive to the choice of spatial and temporal parameters. Following the method for automatic parameters selection developed by Chen and Wu, we calculated many sets of the RTL functions with various combinations of r0 and t0, then computed the correlation coefficients over pairs of the RTL functions. It is expected that high correlation between two RTL functions could be found when their values of r0 and t0 approach the optimal values mostly stabilizing the pattern of the RTL function. The best performance was obtained using r0 = 30 km and t0 = 1 year. However, the results are robust as we change r0 and t0 values. This implies that the RTL changes are not artifacts and could stem from some real and physical preparedness stage.

In order to approach general guidelines to improve existing conditions of buildings, it is necessary to predict and evaluate the seismic vulnerability of existing buildings due to future earthquakes. Most constructed buildings in cities like Bandar Abbas/ Iran are seismically vulnerable because of shortcomings of previous codes, changing application, adding new stories, using weak materials, insufficient supervision, lacking expertise and experience of technicians and so on. Due to aggressive corrosion environment in Persian Gulf, existing steel and reinforced concrete structures have more critical conditions in terms of seismic vulnerability. In this study after reviewing the various typical methods for qualitative seismic vulnerability evaluation of existing buildings, four selected methods are compared to investigate seismic vulnerability of 30 sample buildings. Then, the Arya method has been selected as the proper qualitative evaluation method due to most typical building systems in the area, large volume of data and the need for fast and easy evaluation. Considering masonry buildings as the majority of existing structures in Bandar Abbas and compatibility of the Arya method with these buildings and its success in other provinces of Iran, using this method with some relevant modifications based on past experiences was considered the best alternative. Finally, about 550 different buildings in Bandar Abbas have been seismically evaluated by this method and the average damage index values for each kind of structures were illustrated in charts. Also the results were shown graphically on the city map for different building structures subjected to earthquakes with MSK7 intensity. Based on the obtained conclusions, over 98% unreinforced masonry buildings without required bands will be destroyed due to MSK8 intensity. The result for 60% of concrete moment frame structures is moderate damage in MSK8 and for 75% of them is low damage in MSK7. Steel braced structures do almost the same as concrete frames. The concrete moment frame structures with concrete shear walls show the best result which is low damage in MSK8 intensity.

Selection of structural systems among the allowed ones of ISIR2800 is very important in designing phase and may affect the building costs. It is worth noting that designing of buildings has two phases; in the first one element sections are selected based on the working loads, including the seismic ones. In the second phase, the obtained structure should be controlled for the story drifts. In this phase the story drift should be less than 0.02 and 0.025 for the structures with natural period of vibration more and less than 0.7 second, respectively. The story drift is calculated by the reduction factor multiplied by 70 percent of the calculated drift in linear modeling of the structure for the seismic load (by considering pdelta effects). Therefore, despite the first phase, higher reduction factors intensify the second phase, in such a way that a lot of structural elements are practically determined by the drift controlling criteria. In low rise buildings, structural element sections are normally determined by the working loads and the drift criteria is passed automatically. However in medium to high rise buildings, the drift controlling criteria is dominated and most element sections are determined by these criteria. This research studies and compares costs of four structural systems, with different reduction factors, among ISIR2800’s accepted systems, including ordinary, intermediate and special moment frames as well as dual systems for regular steel buildings (with 4, 6 and 8 stories and 3 and 4 bays in the plan). In this study combination of intermediate moment resisting frame and concentric bracing is assumed for the dual system. All buildings are designed based on ISIR2800 and Iranian national building regulations (No. 10). The structural weight is calculated for each structural system as a representative of the structural cost, regarding that the cost is a linear function of the structural weight. The obtained results show that dual systems are more economical specially for higher buildings. In the assumed 4 story building the working loads determines the element's sections, therefore, the system with less reduction factor leads to smaller sections for beams. However the column dimensions are determined by compact section criteria. In the assumed 6 to 8 story building with moment frame systems, the sections are mostly determined by drift controlling criteria. In contrast, in similar building with dual system, the obtained sections for service loads could pass the drift criteria automatically and that's why it is more economical than the others.

Steel shear wall system in terms of research and development in recent decades has been considered by researchers and industrialists, as a resisting system against seismic loads. The steel shear wall performance in structures is like concrete shear walls but the main feature of this system is that; it is more affordable and also more accurate in implementing. Therefore, the use of steel shear wall systems in recent years has shown very suitable behavior of the system in structures, especially for seismic energy dissipation as well as deformation control. In the steel shear walls, firstly, by the buckling of the steel plate within the allowable limits and then formation of the diagonal tension, this generated post-buckling strength does play a role in improving the structural behavior. As a result, it can be concluded that monitoring and controlling the out-of-plane buckling of the steel shear walls would lead to optimizing the use of this system. In this regard, steel shear walls are often stiffened and implemented by utilizing steel straps. On the other hand, with regard to the opening sin steel shear walls, the main concern has been the stress concentration generated in the vicinity of the opening and thus, it can be stated, the use of FRP material sand FRP stiffeners placement on obtaining the perfect solution for controlling the steel plate buckling and other structural weaknesses used by the presence of the openings, is a very good remedy. To this end, in this study, the behavior of steel shear walls with openings reinforced with FRP fibers has been scrutinized. Accordingly, 83 models of steel shear walls with openings are generated and analyzed by employing the well-known ABAQUS software via a nonlinear finite element analysis. The results show that, considering different arrangements of the FRP stiffeners, a much more uniform distribution of the stresses in the steel plate as well as the boundary elements would be achieved and the stresses near the opening decreases. Moreover, according to the critical buckling occurs at the edges of the opening, the use of combined horizontal- vertical stiffeners, have an important role in controlling theout-of-plane buckling of the plate while the diagonal stiffeners have proved to be the best arrangement in increasing the structure shear capacity, stiffness and energy absorption. Thus, the effect of changes in dimensions of the openings for a fixed area of the openings as a percentage of the steel shear wall area on parameters such as the shear capacity, the out-of-plane buckling, energy absorption and the in-plane stresses have been studied. Due to the obtained results, the influence of the height is more than the width of the openings. Furthermore, the shear force correction factor for the opening are equals to 2 percent of wall area, is variable from 59% to 88%, depending on the shape of the opening. Finally, the behavior factor for the steel shear wall with the opening and reinforced with FRP, is obtained 15 for average of 20 random samples via the Young’s method.

Sufficient knowledge about the method of analyzing and reliability of network is essential to estimate the damage of systems and networks after earthquake. So, the method of reliability analysis networks is important and it is one of the aims of this study. A special characteristic of the current organization of the human life is the growing level of the interconnectivity. A tendency to increase in dimension and to aggregate in complex interconnected systems exists in economical, social, and technological structures. One important property of networks is that the connection between any two nodes of the networks can be typically reached through a number of redundant paths, thus making the connection basically reliable, and it makes them an advantaged structure both in natural and technological systems. Therefore, the reliability of networks has always been a main alarm since the earliest times of reliability engineering. Since the early 60s the exponential growth of the size and number of networks of all types has led many researchers to study the various characteristics, the topology, and the reliability of the networks. Now, the analysis of the network reliability is a major concern for the formation, validation and maintenance of many real systems such as communication, computer or electrical networks. The reliability of these complex systems is of increasing importance since the failure of some components may lead to catastrophic results.The failure of definite sets of components in the worst case may cause the failure of the network itself. In many real-life states, components and/or systems can be in one of more than two states (up and down). Systems that are characterized by different levels of performance are known as multi-state system (MSS).Power systems and computer systems are examples of MSS where the performance of the elements is characterized by generating capacity and data processing speed respectively. Most of reliability analysis and optimization models suppose that system is made of binarystate components, where the states are functioning and failure. Binary-state reliability models don’t allow to satisfactorily describing some aspects of the MSS as MSS operation processes, remaining resources, system state evolution, reasons of system failures and mechanisms of their prevention.The main difficulty in the MSS reliability analysis is the “dimension damnation” since each system element can have many different states(not only two states as the binary-state system). In this study, graphs, systems and binary decision diagram are introduced. Complex networks are described. Also, the specific properties of random networks and scale free networks are illustrated. Finally, weighted multi-state networks have been explained.