مجله علوم و مهندسی زلزله
سال دوم شماره 4 (پیاپی 5، زمستان 1394)

In recent years, many models for earthquake recurrence were proposed. The methodology adopted in this study is based on the fusion of the statistical renewal model called Brownian passage time (BPT) with a physical model. The later taking into account the instantaneous change of the static Coulomb stress (CFF) for the computation of both the permanent and the transient effects of earthquakes occurring on the surrounding sources. Renewal models are frequently used to estimate the long-term time-dependent probability of the next large earthquake on specific faults or fault segments where large shocks occur repeatedly at approximately regular intervals. In this approach, it is assumed that the times between consecutive large earthquakes (inter-event times or recurrence intervals) follow a certain statistical distribution. East of Iran experienced more than ten destructive historical earthquakes and 14 instrumental earthquakes with magnitudes larger than 5.5 (Mw). Two of these events by Mw>7.1 occurred in east and west of the Dasht e Bayaz fault. This fault by east-west trend in west segment is perpendicular to the Abiz fault that experienced one of the most destructive earthquakes (Mw=6.2) in this region. Both mentioned faults have strike-slip mechanism, but the largest event (Mw=7.3) occurred on the Tabas fault that showed a reverse mechanism. Because of the high potential of a large earthquake occurrence, we decided to use the BPT model in this area and calculate the probability of future events on the Tabas, and Ferdows faults, and east and west segments of the Dasht e Bayaz fault. The analysis has been carried out on East (Dasht e Bayaz-Ferdows region) of Iran (32–35◦ N; 56–61◦ E), containing four seismogenetic sources. In this study, we adopt the BPT distribution to represent the inter-event time probability distribution for earthquakes on individual sources. Unlike all the other renewal models, which are based only on an arbitrary choice of the probability density distribution, the BPT is associated to considerations on the fault physical properties [1]. For this model, in addition to the expected mean recurrence time, the coefficient of variation (also known as aperiodicity) α of the inter-event times is required. If α > 1, the time series will exhibit clustering properties. Values below the unit indicate the possible presence of periodicity, with increasing regularity for decreasing α. In this study, we considered 0.5 and 0.75 for α. By using the probability density function and the time of the last event, we will be able to compute the probability of an event that occurs between time t andtt  . This renewal process assumes that the occurrence of a characteristic event is independent of any external perturbation, but in real circumstances, earthquake sources may interact. Therefore, we consider fault interaction by the computation of the Coulomb stress change (CFF) caused by Birjand-Qaen earthquake (May 10, 1997) on the investigated fault. For this purpose, we used the Coulomb stress Change function that used by King et al [2]. For calculation of imposed stress, we used an amount of 0.4 as an effective coefficient of friction because for faults in the continental region, it is the best choice [3]. The permanent effect of Coulomb stress changes on the probability of an impending characteristic event can be approached from two points of view [4]: (1) Modification of the elapsed time since the previous earthquake, and (2) Modification of the expected mean recurrence time. Both of these approaches have the same effect, and in this study, we used the first approach. We need to consider the transient effect, due to rheological properties of the slipping faults. The application of the Dieterich [4] constitutive friction law to an infinite population of faults leads to the expression of the seismicity rate as a function of time after a sudden stress change. The transient effect of the stress change is expressed as a change in expected rate of the segment events [4]. By considering East of Iran, as a study area that had been limited by the rectangle of coordinates 32–35◦ N and 56–61◦ E, and assuming an earth model such as a half space characterized by uniform elastic properties, we calculated the maximum amount of imposed Coulomb stress changes due to Birjand-Qaen on the East and west Dasht e Bayaz, Tabas and Ferdows faults and the obtained amounts are 5.076, - 0.0014, 0.0015 and 0.0051 Mpa, respectively. The computation of the occurrence conditional probability of future earthquakes would require the knowledge of the mean recurrence time of characteristic earthquakes (Tr), aperiodicity, the time elapsed since the previous earthquake, and tectonic stressing rate. By using the relation given by Field et al [5], we computed Tr. In this study, Mw≥6 has been considered as the characteristic magnitude. Khodaverdian et al [6] calculated shear strain rate for the Iranian Plateau. The values of the average shear strain rate have been used for the computation of  multiplying it by μ. The computation of the probability connected to the transient effect, combined with the permanent effect would require the knowledge of the aftershock duration (ta) and A. The values of the aftershock duration has been obtained 11.32 year. By using the relation given by Dieterich [4] we computed A. Positive coulomb stress changes indicate loading on receiver fault and brings it to fracture and negative Coulomb stress changes unloads the receiver fault, and there will be a delay time on the future event on that fault. Obtained results showed that East Dasht e Bayaz, Tabas and Ferdows faults received positive coulomb stress changes, and West Dasht e Bayaz fault received negative amount of imposed Coulomb stress changes. For the Ferdows and Tabas faults, the received Coulomb stress changes are less because of the large distance from the epicenter of the BirjandQaen earthquake, and thus the obtained amount from probability model is less and near to zero. On the other hand, we obtained high-imposed Coulomb stress changes for the East Dasht e Bayaz fault because the occurred BirjandQaen earthquake had a little distance from this fault. Imposed negative Coulomb stress changes on the West Dasht e Bayaz fault caused delay on the future event on this fault and reduced the probability on this fault a little.

With growing of civilization and population in urban areas, the seismic vulnerability of man-made structures is also growing. This is more significant in regions with high seismic hazards. Iran Plateau is located at the second most seismic belts of the earth, and many cities and urban area are located in high seismicity regions. On the other side, an accurate seismic analysis and design of structures requires realistic and representative strong motion records of the building area. However, such a strong motion record is not available for any region, or there are not sufficient numbers to accurately represent random and stochastic nature of the earthquakes. Putting all these together, the need for generating synthetic strong motion records or accelerograms that are coherent with the source and site characteristics of any area is highlighted. Amongst different approaches to generate synthetic accelerograms, one is empirical simulation using stochastic models based on the statistical characteristics of ground motions of the area. This approach was recently utilized to generate synthetic accelerograms for a seismic source, which are different in seismic parameters [1]. The present study is using this approach and accelerograms of two horizontal components of about 258 events in Iran Plateau recorded by Iran Strong Motion Network (ISMN) with the moment magnitude greater than 5.5. Seismic parameters of these events are extracted with regression analysis and used to develop empirical relation for generating synthetic accelerograms for two site conditions of shear wave velocity smaller and greater than 600 m/s in two components of parallel or perpendicular to the fault. The performance of the relations is verified by comparing the synthetic accelerograms with the actual records of Avaj 2002 earthquake with the moment magnitude of 6.4, Figure (1). Furthermore, the response spectrum of the synthetic records of this event is compared with the standard response spectrum of the recent version of Iran seismic code 2800 for the Avaj area, Figure (2).

Earthquake-induced cyclic shear stresses may lead to a remarkable loss of shear strength, accumulation of pore water pressure, and permanent large amplitude deformations in granular soils. The technical term liquefaction is commonly attributed to the family of phenomena named above. Liquefaction of clean sands has been studied extensively in the laboratory, and in past, it used to be believed that the presence of non-plastic fines in coarse granular soils definitely eventuates in strengthening the soil structure against liquefaction. Nevertheless, Yamamuro and Lade [1] revealed that the majority of the catastrophic liquefaction case histories have occurred in natural and man-made silty sand alluvia. Surprisingly, the latest experimental studies have pointed out that the silty sands are very prone to flow liquefaction instability under both monotonic and cyclic shear loading scenarios. In this subject, adding non-plastic fines up to a transitional threshold within the range of 30 to 40% by weight of the total solid phase leads to a gradual decrease in both shear strength and tendency towards dilation. More recent experimental studies have reported the gradual downward relocation of the Critical State Line (CSL) with fines content in void ratio vs. mean principal effective stress (i.e., e vs. p) plane for fines contents lower than the threshold value [2, 3]. Downward relocation of the CSL within the context of the critical state soil mechanics can explain the continuing decrease in shear strength and the tendency towards the contraction observed in silty sands. Recently, Discrete Element Method (DEM) has received considerable attention as a state-of-the-art versatile tool to study the macro- as well as micro-mechanical behavior of granular media. Herein, the drained behavior of coarse granular samples over a wide range of void ratio and confining stress values is simulated using DEM. It is shown that all samples approach towards an asymptotic ultimate state at which soil deforms continuously without any further changes in shear stress, mean principal effective stress, and volumetric strain, a certain state known as the critical state in soil mechanics. Consequently, samples with 2.5, 5.0, 7.5, 10, 15, and 30% by total weight of homogeneously distributed fines are made. Diameter of particles in the coarse phase is five times greater than that of the fines phase. At least, ten numerical simulations are performed for each fines content. In the shear stress vs. mean principal effective (i.e., q vs. p) plane, it is observed that the slope of CSL is not affected by the fines content indicating that the slope of CSL in the q vs. p plane is mainly influenced by the particles shape, Figure 1(a). In an opposite fashion, it is observed that CSL in the e vs. p plane relocates remarkably with the increase of fines content, Figure 1(b). To explain the downward relocation of CSL in the e vs. p plane, Thevanayagam et al. [3] concluded that fine particles fill the voids between coarse particles and reduce the void ratio; however, they do not participate actively in soil load carrying microstructure. As a direct outcome, the conventional void ratio expressed in term of voids between all particles may not be considered as a legitimate index of compactness in sand with varying fines content. Thevanayagam et al. [3] suggested that the concept of intergranular void ratio, e*, in terms of the voids between active particles in the load carrying structure must be used instead of the conventional void ratio in silty sands:b FC e b FC
e
1 (1 ) (1 )*
where FC is fines content and b [0,1]  is fines participation factor. For b=0, fine particles act as filler without any participation in loading the bearing structure. On the other hand, fine particles contribute as actively as coarse particles when b=1. Golchin and Lashkari [4] and Lashkari and Golchin [5] suggested a critical state compatible bounding surface plasticity model for clean sands with the following distinctive features: Elastic strains are obtained from a Gibbs free energy function to guarantee the conservation of elastic energy in any arbitrary closed loop. Proper constitutive equations enable the model to consider elastic-plastic coupling in medium-large shear strains. As a direct consequence, the model can take into account the impact of shear stress-induced anisotropy on the elastic ingredients of the model. In this study, constitutive equations of the model of Golchin and Lashkari [4] and Lashkari and Golchin [5] is modified in such a way that intergranular void ratio is used instead of the conventional void ratio. This modification enables the model to consider the relocation of CSL in the e vs. p plane with evolving fines content. It is shown that the model can reasonably simulate the mechanical behavior of clean and silty sands with varying fines content using a single set of parameters, Figure (2).     

The landslides occurred due to an earthquake have ever caused great losses of life and property. Earth slope stability is influenced by many factors that should be taken into account for a comprehensive assessment of it. However, slope seismic analyses are complicated due to the necessity of concerning the dynamic stresses caused by an earthquake and the soil resistance changes in seismic conditions. Seismic slope instability can arise from two significant factors including the increase of inertia forces and the decrease of soil shear strength. Moreover, the earthquake motions can produce considerable dynamic shear and vertical stresses in earth slopes. If these stresses are added to the static stresses existing in the soil mass, they may lead to slope instability [1]. The methods of analyzing the earth slopes stability can be pseudo-static and stress-strain analyses or simplified integration method (Newmark rigid-block method). In the pseudo-static analysis, slope seismic safety factor is determined in a very similar way as what is done in static equilibrium analysis. In the stress-strain analysis, first of all, soil mass is divided into the finite elements. Afterwards, by using numerical methods, the stresses and strains caused by external and internal forces in soil mass is calculated. Although this method is accurate and gives a realistic model of the earth slopes, it requires the special expertise and precise input information. The rigid-block model assumes that permanent deformation initiates when the earthquake-induced accelerations acting on a slide mass exceed the yield resistance on the slip surface. The resistance is quantified by the seismic yield coefficient (ky). At this point, the slide mass breaks away from the rest of the underlying slope and sliding occurs at a constant rate of acceleration equal to ky. During this time, the velocity of the ground is greater than the velocity of the slide mass. Sliding continues until the following conditions are met: (1) accelerations fall below ky, and (2) velocity of the slide mass and the underlying ground coincide [2]. In deterministic computation methods, input data such as viscosity, inner friction angle, inclination of slope, etc. is considered as fixed values; therefore, by using these methods, the probabilistic variation of a parameter cannot be considered. In other words, deterministic methods do not include the uncertainty related to the effective design parameters. In the probabilistic method, instead of using a fixed value for a parameter, the probability distribution function (Probability Density Function) of it is used. Due to the possibility of performing fast extensive numerical calculations by computer, the numerical simulation methods such as Monte Carlo simulation have been employed in slope stability analysis. Assuming homogeneity of materials, along with, absence of underground water on one hand, and considering that the slope seismic yield coefficient is a function of the angle of slope β, height of slope h, soil bulk density γ, the coefficient of soil cohesion c and internal friction angle φ, on the other hand, a closed-form relationship between the above-mentioned parameters can be achieved using the extensive results of limit equilibrium analysis and artificial intelligence methods such as genetic algorithm [4]. Any change in each of the stated parameters can lead to a change in slope seismic yield coefficient. The uncertainty in calculation of slope seismic yield coefficient can be taken into account by utilizing genetic algorithm and probability distribution functions of input parameters.
Due to the fact that coseismic slope deformation is a function of seismic yield coefficient (ky) and other effective parameters, using the statistical analysis, the statistical distribution and the standard deviation of ky can be assessed. In the present study, the following steps were taken for studying the variation of ky: Implementing a pseudo-static analysis of earth slopes having various physical and mechanical parameters, determining the relationship between slope seismic yield coefficient and other given parameters using curve fitting and genetic algorithm method (evolutionary strategy method), simulating the effective parameters on ky by means of Monte Carlo method based on a specified statistical distribution [9]. The Monte Carlo simulation procedure is as follows: - Providing a deterministic model for solving the problem. - Selecting the random variables including soil characteristics, slope geometry and proper probability density functions. - Generating the input parameters by taking into account the density functions and random variables dependency (parameters’ dependency) to each other. - Using simulated numbers related to all independent parameters, along with defined deterministic model, the value of slope seismic yield coefficient and probability density function of it are obtained. Consequently, the following results were concluded: 1. If normal distribution is selected for the parameters affecting ky, slope seismic yield coefficient will be normally distributed. 2. The coefficient of variation of ky has a direct relationship with coefficient of variation of parameters affecting it. If slope seismic yield coefficient is increased, the acceleration coefficient will be decreased. 3. With selecting the minimum amount of coefficient of variation for effective factors, the range of coefficient of variation of slope seismic yield coefficient is between 5 and 11 percent. 4. With considering the maximum amount of coefficient of variation for effective factors, the range of coefficient of variation of slope seismic yield coefficient is between 25 and 59 percent. 5. With taking into consideration the minimum amounts of the coefficient of variation for effective parameters, and increasing mean value of slope yield acceleration from 0.1 to 0.35, its variation coefficient (COV%) decreases from 11 percent to 5 percent. 6. With taking into account the maximum values of the coefficient of variation for the effective parameters, and increasing mean value of slope yield acceleration from 0.1 to 0.35, its variation coefficient (COV%) decreases from 59 percent to 25 percent. In this study, coefficient of variation of ky is calculated assuming that the properties of a soil layer changes similarly in all points of it. If these properties vary from point to point, there is a need to employ random field method (stochastic finite element) in order to simulate the spatial variation of soil properties.

Pile driving is one of the pile construction methods to install prefabricated concrete piles using special tools. Driving method and its process is one of the most important aspects of implementation of this pile, so that the lack of sufficient accuracy to this problem, in addition to reducing operation efficiency, affects the surrounding environment and soil. In this paper, the behavior of cylindrical and tapered hollow piles were investigated with real tests and numerical analyses using FLAC3D software. The efficacy of pile geometry such as cylindrical and tapered hollow piles is analyzed in pile driving under hammer impact. Pile T is fully tapered hollow pile, and pile C is cylindrical one. All piles made by steel have the same length (Lp = 250 cm) and volume. Their properties are in Table (1). The single acting hammer with a 300 kg weight falling through a distance of 1 m is used to install all piles to the embedded length of 0.5 m to 2 m. The pile driver is made by different parts such as mounting frame, hammer and electric motor for lifting the ram to the selected falling height, and the hammerhead falling to induce an impact on the hammer components. The sand deposit was located in Vardavard-Tehran with maximum specific weight (gd,max), 2.16 kg/cm3 and relative density of about 93%.
The pile driving analyser (PDA) with two accelerometers and strain transducers was used to record the induced pile velocity and strain. The PDA takes results from every impact velocity and force signals obtained by sensors attached to the pile. To measure forces and velocities at the pile, they were attached externally along the pile shaft at 32.4 cm from the pile head in two opposite directions. The driving records for all piles with different toe condition in terms of cumulative hammer blow count versus the penetration depth are illustrated in Figure (1). It can be seen that for achieving the same final penetration, the cylindrical hollow pile required more number of blows than tapered ones. Besides, velocity and displacement were measured as shown in Figure (2). By comparing the obtained results from experimental and numerical analyses, it has been found that the behavior of hollow piles was changed by varying of its geometry. It is also concluded that the hollow piles offer better drivability performance with used energy reduction from 25 to 60% than the filled ones of the same volume and length.  
The open or close-ended pile has also an effect on pile driving process. In the case of applying the same energy during pile driving, the open-ended pile has better performance than close-ended one, and tapered shape pile is also performed better than the ones with other different geometries. In this case, tapered pile has also performed better than cylindrical one by an increase in pile velocity and displacement.

Liquefaction phenomena may cause two major concerns: 1- Ground deformation or ground failure due to the excess pore water pressure, and 2- Changing in ground site response caused by softening of liquefiable soil [1]. Liquefaction assessment and its consequences have been investigated by many researchers [2]. Although, there are not enough studies to determine the effect of soil softening and liquefaction on the seismic ground response. The main purpose of this study is to evaluate the ground response for liquefiable sites. Hence, Specific Cumulative Strain Energy (SCSE) parameter is defined to consider the effects of earthquake duration and thickness of liquefiable layer. The results of amplification pattern were presented in both of the effective and total stress analyses.
Numerical modeling: Numerical prediction of the liquefiable soil response requires a nonlinear, effective stress analysis with a relatively sophisticated constitutive model. In order to use compatible constitutive model in numerical analysis, multi-yield surfaces models applied in CYCLIC1D software [3]. The centrifuge model of VELACS project (model number 1) is examined for the verification of numerical modeling, Figure (1). Note that, this program models the nonlinear, inelastic behavior of soils and can represent phase transformation behavior of the potentially liquefiable soils.
Results and Discussion The cumulative enclosed area of the earthquake-induced shear stress–strain loops is referred to as dissipated strain energy density (or unit energy). Cumulative strain energy is an internal response of soil body to the external loading, and thus, it can be employed as a useful measure for the analysis of soil behavior. Hence, Specific Cumulative Strain Energy (SCSE) parameter was defined to consider the effects of earthquake duration and thickness of liquefiable layer. In order to study the effect of excess pore pressure and strong motion characteristics on the seismic response of ground surface, the following equation was presented:( ) ( )/ ( ) RSR T S T S T tot a eff a (1) where, Saeff and Satot are the response of effective and total analyses, and RSR is response spectra ratio, respectively. The earthquakes were classified as strong (SCSE≥0.66), moderate (0.34≤SCSESummary and Conclusion :In this paper, the effects of strain energy and excess pore pressure on the seismic response amplification of liquefiable soil were studied based on the effective and total stress nonlinear analyses approaches. The main important conclusions drawn from present study are as follows: 1. Regarding the liquefaction potential throughout the soil profile, the results of total and effective stress analyses are in compliance with each other for weak ground motions. However, for stronger motions, effective stress analyses method is essentially recommended. 2. Strong ground motions that generated the excess pore water pressure and consequent increasing of the cumulative strain energy in depth of soil, reduce ground response spectra. 3. The value of RSR for earthquake with weak and mediocre strain energy variety from 0.8 to 1.1, that indicate the similarity of effective and total stress methods results for weak and mediocre strain energies. 4. The average values of spectra ratio (RSR) for high strain energy for all periods of input ground motions are lower than one. In other words, by growing strain energy, the soil nonlinear behavior and absorption of strain energy increases. Thus, the response that calculated by effective stress method (Saeff) reduces compared to the result of the total stress method (Satot).