مجله علوم و مهندسی زلزله
سال چهارم شماره 3 (پیاپی 12، پاییز 1396)

Seismicity pattern studies are one of the effective tools in the interpretation of variation in seismic sequence. The study of variations of seismicity parameters as a function of time indicates that the temporal distribution of events is not uniform, and these parameters can give quantitative information about the seismic patterns of different regions. In this research, to investigate temporal variations of seismicity pattern in the Zagros fold and thrust belt, the Schreider algorithm is applied. This algorithm that introduced by Schreider (1990) to detect seismic quiescence has been used in different parts of the world. For this purpose, four earthquakes with Mw≥6 that have recently been occurred in Zagros have been studied. At first, a complete catalogue from the period of 2000 to 2017 within a circular area has been selected. Then, the catalogues are homogenized to ML and the Minimum magnitude of completeness are computed (Mc=3.4). To perform Schreider algorithm, the time between consecutive earthquakes ( ) should be calculated. The smoothness procedure is used to evaluate a convolution function of . The smoothness of this function is done by Gaussian function. In the R radius, smoothness parameter (s) has controlled the extent of surrounding earthquakes to detect smooth values. The kth seismic event is related with the temporal convolution T(k) that decrease and increase indicate seismic activity or low seismic activity, respectively. The numbers of earthquakes that are located in the nearest distance to main shock determine the l parameter. The value of l is determined when the function f(n,s) is approximately zero. Therefore, the function T(k) depends on the s and l parameters. To investigate temporal variations of seismicity, in addition to the temporal convolution (T(k)) plot the magnitude-time, number-time and space-time plots have drawn. The results show that before the 2006 Silakhor and 2014 Mormori earthquakes, both of which occurred in the north part of Zagros, the precursory doughnut pattern is seen. Several years before the 2013 Dashti and 2005 Qeshm earthquakes, both of which occurred in the south part of Zagros, the seismic quiescence is seen for which ended with the sudden occurrence of these earthquakes. The result of sensitivity analysis showed that smoothing parameters of Schreider algorithm have a significant influence on the algorithm outcomes, and these parameters should be selected with more accuracy. The results of this paper show that Schreider algorithm can demonstrate precursory seismic quiescence before the occurrence of large earthquakes due to the intelligent usage of t parameter.

Several large earthquakes have influenced the settlements in the Lut block in the past, causing heavy damages in the region. The Dasht-e Bayaz area is located in the north of the active right-lateral faults of the Sistan suture zone, which may accommodate a large proportion of the about 20 mmyr-1 right-lateral shear between Iran and Afghanistan. The numerous Dasht-e Bayaz destructive earthquakes were outstanding clusters of interactive earthquakes that include earthquakes on left-lateral, right-lateral, and reverse faults.
New studies have been discussed recently by several authors in which a strong correlation between Coulomb stress changes and the spatial distribution of earthquakes have been observed. The stress change maps would be useful for earthquake hazard to foresee the most likely locations of the upcoming aftershocks. Generally, the changes in Coulomb stress range from 0.1 to 1 bar, which are considered enough to trigger the future earthquakes. It is perceived that the successive events on the Dasht-e Bayaz areas have characteristic spatial distribution patterns, and the seismicity after the main shocks is consistent with the Coulomb stress change. For all the computations, we used the shear modulus (G) of 3.2 × 105 bars, in a uniform elastic half-space with Poisson’s ratio (ν) of 0.25, Young modulus (E) 8 × 105 bar and an effective coefficient of friction that includes the effect of pore fluid pressure and has been postulated to vary in the range 0.2 – 0.8 with an average value of μ'= 0.4 often used. Coulomb 3.3 software is used to calculate Coulomb failure stress changes. The rupture parameters (strike, dip, rake and depth) and source faults of the earthquakes after the 1968 west Dasht-e Bayaz earthquakes are defined from the published studies. The empirical relationships proposed by Leonard have been used to calculate the width, length and slip of the source faults and assuming homogeneous fault slip. The calculated Coulomb stress change due to the 1968 W; Dasht-e Bayaz event indicates that this main shock can "trigger" the second one or move the Ferdows thrust fault (1968.09.01, Ferdows earthquake) because it was located on the high stress zone of Coulomb stress changes pattern. Besides, the cumulative effects of Coulomb stress changes due to the 1968 Dasht-e Bayaz and Ferdows events on the Avash fault as a receiver (1976 Vondic event) revealed that the central section of Dasht-e Bayaz and hypocenter of Vondic earthquake is located in the negative lobe of Coulomb stress changes with 0.2 – 0.5 bar.
The influence of the earlier earthquakes (1968-1976) on the 1979 Koli-Buniabad earthquakes indicates that the eastern section of the Dasht-e Bayaz segment prompted (1979.11.27, Koli-Buniabad earthquake) into the failure regime by showing a positive increase (0.2 – 0.4 bar) in coulomb stress.
To validate the Coulomb stress calculations, we plotted the earthquakes after the main shocks on each stress changes map, and good correlations between the main shock stress changes and the spatial pattern of the seismicity were observed at the terminate part of strike slip faults. Moreover, geomorphic evidences such as, eroded earthquake scarps, displacement of channel, castle trace on ancient hills and displacement of Qanate show several seismic activities of these faults.

In order to develop the underground structures such as subways, metro tunnels and other buried facilities in big cities, the dynamic interaction of these structures and their environment should be considered. In this study, all metro lines in Tehran are investigated and the 7th line with 28 km long and its environmental conditions in all length have been used as a case study. This line is selected because it passes through all kinds of geotechnical alluvium and also the plan and profile of this line are suitable for interaction with the buildings. A numerical method in this study was analyzed with finite difference code, FLAC, in dynamic cases. The models are performed for 8, 10,12,13,15 and 20 meter overburden and three different alluviums are used.
In dynamic interactions, previous studies showed high amplification of displacement in the vicinity of the tunnel in the surface. The parameter of acceleration is also investigated in some physical model tests. Both acceleration and attenuation of acceleration are reported in different locations and under the different seismic motions. For choosing the critical sections in selected line, the buildings with overburden less than 20 m and the maximum surface distance up to 5D are selected. In this study, two conditions of problem such as free-field, and tunnel field are analyzed. In tunnel-filed models, results show that the presence of tunnel can increase the surface acceleration up to 33% in initial part of line with 8 m overburden. This result is accrued under Chi Chi earthquake with the dominant frequency of 1.68. This increment of acceleration in critical cases can change the design acceleration of the building to 0.46 g instead of 0.35 g that is proposed by Iranian Code. By considering the results of all cases, it can be seen that the most effective case happened when the dominant frequency of motion is so close to the natural frequency of medium. The frequency content of medium is predicted by 1D propagation of wave with equivalent linear method in frequency content. The region between 0.5D to 1.5D from the center of the tunnel on the surface is most affected by the interaction. The effect of overburden of tunnel is also studied that shows that the deeper tunnel has less effect on surface but the influence zone on surface is greater. Both amplification and attenuation happened in the models for acceleration of the surface. The output data show that the variation of acceleration in surface depends strongly on the frequency content of input motion, the surface distance from axis of the tunnel and overburden of the tunnel. The most important factor that can describe the behavior of the surface is the variation of wave propagation around the tunnel. The presence of the tunnel under SH wave based on the mode of deformation can amplify the motions around the tunnel and not top of it in the surface. The tunnel in the medium can also prevent the propagation of waves to top of it and this may cause attenuation of motion. The color contour of the acceleration distribution around the tunnel and near of the surface during the dynamic analysis of model can better describe these phenomena.

In common practices, the simplified beam-spring model is applied for modeling the pipe behavior against fault displacement. On the other hand, due to the ease of modeling, most simulations have been focused on strike-slip faults and very rare studies have paid attention to the simulation of pipes crossing reverse faults. In the present study, the behavior of the buried pipes subjected to reverse fault motions have been investigated by using three-dimensional continuum finite element modelings. The ABAQUS software has been utilized in the simulations. By this software, the analyses have been performed by using the explicit method. To provide better adaptation between simulation and the behavioral properties of pipe and soil, shell elements and solid elements have been used for the modeling of pipe and soil, respectively. The material non-linearities associated with pipe-material and soil is modeled by considering elasto-plastic behavioral model for soil and pipe. In addition, interface elements have been considered between the soil and the pipe elements. As for the first stage of numerical modeling, the numerical simulation procedure was validated by simulating a large-scale physical model of a pipe crossing a reverse fault. Comparison of the results (in terms of axial compression strains of the pipe) obtained from the simulations with those of the physical model indicates a good match. In the next stage, the behavior of a pipe with a reverse fault motion is investigated from two different approaches. To this aim, the current approach, i.e. three-dimensional continuum modeling was compared with conventional beam-spring model, and the results of the simulations are compared. The results show that the beam-spring model gives logical answers only for small amount of fault displacements while for large fault motions, the model cannot consider correctly the justified behavior of the pipe. The reason is because of the governing local buckling of the pipe at large fault displacement, which cannot be well considered in the beam-spring model. In other words, the beam-spring model can only take the global buckling into consideration; however, this approach is not suitable to study the pipe behavior for large fault displacement, and thus, the problem should be studied by considering the continuum body of the soil as well as the pipe body. In this study, the effect of the diameter to pipe thickness ratio was investigated by using the 3D simulations. The results show that as the diameter to thickness ratio is varied, the failure mechanism of the pipe is changed too. As the diameter/thickness ratio increases, a local buckling is generated at small level of fault displacement, and hence, the resistance of the pipe against the local buckling decreases. In addition, the pipe deformation pattern is different. For the thicker pipe, the pipe deforms in a longer distance around the fault; however, the thinner pipe is crushed at the location of the differential fault displacement. As the other parameter that is effective on the pipe deformation pattern is the soil dilatancy. The numerical modeling indicates that as the soil dilatancy increases, the axial strains of the pipe augments too. The increase in dilatancy from zero to 30 degrees causes a double increase in the pipe strain level. The effect of fault dip angle on the pipe deformation is also investigated numerically. To do this, two faults with different dip angles of 40 and 70 degrees were considered in the modelings. It was found that as the dip angle of the fault is smaller, the level of the axial compression strains increases too. The rate of increase
in the axial strain to the fault displacement is higher too. The deformation pattern of the pipe is investigated, which released that the pipe is much more deformed and damaged for smaller fault dip angle (40 degree). As a conclusion, it can be briefly deduced that: 1) in order to study the deformation of pipe crossing reverse faults, 3D numerical modeling approach are more justified than the simplified beam-spring approach; 2) To reduce the pipe damage, the fill around the pipe should be filled with fines-grained soils, which have low values of dilatancy; 3) As the dip angle of the fault increases, the pipe should be selected as to be thicker in order to prevent local buckling of the pipe.

Urban fabrics in developing countries are growing rapidly, without sufficient attention to sustainable development criteria. Nowadays, there are many big cities that accommodate millions of residents, while most of them are exposed to natural hazards. Accordingly, the growth rate of big cities has been considerable in Iran especially during the last decades. More than 70 percentages of population are now living in urban areas, while 60 years ago this proportion was vice versa. This has caused further exposure of human lives to natural disasters, including strong earthquakes. Therefore, in the occurrence of an earthquake in such fabrics, heavy destruction and considerable socio-economic consequences can be expectable.
In order to reduce the potential impacts of earthquakes in such cities, different measures could be taken into account to mitigate vulnerability and increase coping capacities. However, due to the shortages of technical and financial resources, especially in developing countries like Iran, priority activities should be selected based on the acceptable risk level and available capacities to determine the most feasible and effective interventions in different urban zones. Therefore, it is necessary to develop appropriate methodologies to be used for prioritization of applicable risk reduction and disaster management measures.
In this paper, a new approach is developed to assess the existing earthquake risk in urban fabrics. For this purpose, different physical as well as socio-economic parameters affecting the safety level of urban areas have been identified and classified to understand their impacts on earthquake risk level. The parameters in the model and their weights have been determined based on expert judgments, using AHP (Analytic Hierarchy Process). For this purpose, a questionnaire based survey has been implemented through city managers, engineers, planners and disaster management experts. According to the results of the survey, most important parameters and indicators have been selected and weighted. Additionally, the effects of different measures on improving response capacities and the efficiency of other applicable interventions on risk level have been evaluated.
By applying this model, the effectiveness of developing community-based activities as well as the establishment of additional search and rescue bases for risk reduction have been evaluated and compared. The results revealed that besides the importance of capacity building in relevant search, rescue and relief organizations (such as fire-fighting stations or medical care centers), promoting public participation in disaster risk management activities can also be considered as an important priority in short-term planning to reduce earthquake risk and to improve response capacity.
The proposed methodology can be applied for prioritizing risk reduction and management measures in any places based on local conditions, if the relevant parameters could be estimated locally. By using this decision support measure, the local managers might be able to evaluate the effectiveness of any different interventions on risk and safety level of urban fabrics, before approving intervention plans. This can help them to allocate the limited available budget to the most critical projects.

One of the situations that distorts the safety of concrete moment frame structures is the rare seismic events. Bam earthquake in 2003 was one of the rare seismic events that caused damages to many newly built structures. In this paper, the capacity of structures was evaluated according to the standard record sets of FEMA P695 and maximum considered earthquake (MCE). A comprehensive method should be used to express the seismic behavior of structures for assessing the collapse capacity. Incremental dynamic analyses proposed by Vamvatsikos and Cornell in 2002. This method is used for assessing the collapse capacity of structures in this study. The proposed methodology is used for collapse assessing of an individual as well as a group of buildings with due attention to rare seismic events and incremental dynamic analyses method. Illustrative results show that, if structures provide minimum acceptable requirements of FEMA P695, they would have been secured against rare seismic events.
Development of nonlinear models for collapse stimulation is the first step of collapse assessing methodology. All of the structures have been designed according to ASCE 7-05 code, and for expressing of nonlinear behavior of materials, Mander and Menegotto-Pinto model has been considered. Selection of ground motion record sets for collapse assessment of building structures is very important. Both far-field and near-field records have been considered in FEMA P695, but in this paper, the far-field records were used. Three analyses have been considered in assessing the collapse capacity. Eigenvalue analyses, incremental dynamic analyses and static pushover analyses are required for assessing the collapse capacity. Incremental dynamic analyses is one the suitable methods for expressing of seismic behavior of structures. The basic idea of this analysis was described by Bertero in 1997. In 2002, this method was accompanied with big progress by Vamvatsikos and Cornell. Illustrative results show where the incremental dynamic analyses curve slope is equal to 20% of the elastic while the point also belongs to softening branch defined as collapse point. Additionally, another candidate point is displacement ratio of 10%. Illustrative results show that where the incremental dynamic analyses curve lining to infinity is being defined as collapse point. The incremental dynamic analyses curves show record to record variability, thus it is essential to summarize such data. The fragility fitting approach has been used widely for defining the median collapse acceleration. Adjusted collapse margin ratio is the most important parameter for assessing the collapse capacity of structures. According to FEMA P695, the acceptable value of the adjusted collapse margin ratio for each individual model within a performance group should exceed ACMR (20%). Additionally, the average value of adjusted collapse margin ratio for each performance group should exceed ACMR (10%).
Finally, collapse capacity of 5 and 10 story concrete moment frame structures are defined. Both structures have acceptable adjusted collapse margin ratio and both of them have acceptable safety according to rare seismic events. Structures that could not satisfy the FEMA’s conditions must increase their lateral strength and re-evaluate.