BULLETIN OF EARTHQUAKE SCIENCE AND ENGINEERING
Year:2019 | Volume:6 | Issue:1 |Papers Count:10

This work aims at the assessment of the occurrence probability of future earthquakes, taking into account Coulomb stress changing based on the time-dependent models. The influence of Coulomb stress changing on the occurrence probability of characteristic earthquakes is computed, taking into account both permanent (clock advance) and transient (rate-and-state) Coulomb perturbations. Calculations are based to the time elapsed since the last characteristic earthquake on a fault and to the history of the following events. For this purpose, earthquakes with magnitude Mw≥ 5. 8 are applied. Then, by using the BPT and the Weibull models, the occurrence probability of characteristic earthquakes for the 10, 30 and 50 year periods are estimated. The Zagros region included in the rectangle of coordinates 27-31. 2 N° and 49. 6-53. 4 E° and faults such as Kazerun, Borazjan, Sabzpushan, Qir, Karebas and parts of MFF and ZFF were selected. For calculating coulomb stress, Coulomb 3. 3 software was used. Time-dependent models called renewal models, have been applied to investigate shocks on single faults [1-2] or in seismic sources that include, in addition to the main fault where the characteristic earthquake is generated [3-4]. In the renewal processes, the conditional probability of the next large earthquake, given that it has not happened yet, varies with time and is small shortly after the last one and then increases with time. In recent years, many models for earthquake occurrence probability were proposed. This study used BPT and Weibull models. Weibull distribution is one of the most widely used lifetime distributions in a wide range of engineering applications [5-6]. The Weibull distribution has also been widely used for specifying the distribution of earthquake recurrence times [7] and follows from both damage mechanics and statistical physics. For computing probabilities with Weibull distribution, γ parameter is needed that is the shape parameter of the distribution, defined as the inverse of the coefficient of variation [8]. Adding Brownian perturbations to steady tectonic loading produces a stochastic load-state process. Rupture is assumed to occur when this process reaches a critical-failure threshold. More recently, the Brownian Passage Time (BPT) model, assumed to adequately represent the earthquake recurrence time distribution, has been proposed to describe the probability distribution of inter-event times [9]. One of the important properties of this model is that with increasing time since the last event, the BPT hazard rate decreases toward a non-zero constant asymptote [9]. The expected recurrence time Tr is the necessary piece of information. Besides, a parameter as the coefficient of variation (also known as aperiodicity) α , defined as the ratio between the standard deviation and the average of the recurrence times, is required. In this study, Cv values 0. 5 and 0. 75 were used for individual faults as Yakovlev et al. [10]. As we are dealing mainly with events, for which details as fault shape and slip heterogeneity are not known, rectangular faults with uniform stress shop distribution are assumed [11]. For modeling faults and calculating stress changes due to earthquakes, fault parameters like strike, dip, rake, rupture dimensions and receiver fault mechanism are necessary for all the triggering sources. Moreover, the rupture length and rupture width are required. In most cases in this study, these two parameters are indistinctive, so Wells and Coppersmith [12] empirically relations were used for computing rupture length and width. Characteristic earthquake yearly rate was computed by using the relation given by Field et al. [13]. Then by inversing obtained amounts, the mean recurrence time of earthquakes could be computed. The effect of Coulomb stress change on the probability for the future characteristic event can be considered from two viewpoints [14]. The first idea is that the stress change can be equivalent to a modification of the expected mean recurrence time, Tr to the T'r, the second view point works on the idea that the time elapsed since the previous earthquake is modified t to the t'r by a shift proportional to Δ CFF. According to Stein et al. [14], both methods yield similar results nearly. In this study, the alternative between the first and the second view has been decided in favor of the second one. By substitution of t' into the hazard function, the probability modified by the permanent effect (P-mod) of the subsequent earthquakes were calculated. Khodaverdian et al. [15] calculated shear strain rate for the most of the faults in the Iranian Plateau. These values have been used for the calculation of tectonic stressing rate 𝜏 ̇ . For computing the probability obtained from the sum of the permanent and the transient effect (P-trans), we would have aftershock duration (ta) and Aσ parameters. The obtained amount of aftershock duration by using window algorithm for aftershocks according to Gardner and Knopoff method is 1. 4 year. Accordingly, by using ta and tectonic stressing rate, Aσ parameter was obtained for each fault. Taking into account the effects of earthquakes stress change, caused changing the results of conditional probabilities that obtained from both models, so that in some of the seismogenic sources increased probability result and in others decreased. The result shows that the probabilities obtained from the sum of the permanent and transient effect are generally smaller than the conditional probabilities obtained from the permanent effect only. This is due to the assumption of constant background rate made for the application of the rate-and-state model. The maximum obtained probability is related to the Kazerun fault that shows the high seismic activity of Kazerun fault. The uncertainties are treated in the parameters of each examined fault source, such as focal mechanism, mean recurrence time, magnitudes of earthquakes, epicenter coordinates and coefficient of variation in the statistical model. Taking into account these uncertainties by Monte Carlo technique will lead to more accurate results.

The reliable estimation of seismic loads on a structure is required in order to earthquakes resistant design of the structure. The difference in seismic loading in different support points of the structure is important in large and long structures. In general, the lack of access to the reliable time histories in different support points of the structure is the main problem of performing non-uniform excitation analyses. Numerical analyses and calculation of ground motion at different points of the foundation of the structure is one of the ways to achieve the non-uniform support excitation. This paper aims to evaluate the seismic response of Pacoima dam site by performing three-dimensional boundary element analyses in the time domain. The pattern of displacement and amplification due to seismic waves scattering in the dam site are evaluated, and calculated results are compared to the recorded ground motions. The numerical modeling has been executed using the time-domain boundary element that is based on the boundary integral equation of the wave motion. To transform the governing integral equation into the ideal form, it has been discretized in both time and spatial domains. Finally, the obtained equations have been expressed in the matrix form and have been implemented in a computer code named as BEMSA. Earlier, several different examples of wave scattering have been solved in order to show the accuracy and efficiency of the implemented BE algorithm in carrying out the site response analysis of topographic structures. Pacoima dam is a concrete arch dam located in the San Gabriel Mountains in Los Angeles County. The height and the length of the crest of the dam are 113 m and 180 m, respectively. The dam is instrumented by use of 17 accelerometers at different elevations on the dam body and its abutments. For site response analyses, the dam site has been subjected to vertically propagating recorded motions of the Pacoima dam 2001 earthquake with a magnitude of 4. 3, depth of about 9 and epicentral distance of about 6 km south of the dam. The medium assumed to be homogeneous linear elastic with density of 2. 64 ton/m3, shear wave velocity of 2000 m/s and Poisson's ratio of 0. 25. The 3D topographic model has been generated up to a radius of 5000 meters, using 1218 eight-node quadrilateral isoparametric elements with the average effective element size of 25 m in the center part of the model. In order to investigate the seismic response of the canyon, a couple of points at four levels have been considered on both sides of the canyon and the results analyzed in time and the frequency domains. Despite the actual record earthquake motions, which includes the effects of the interaction between the foundation and the dam structure as well as the lake behind the dam, the calculated motions include only the wave scattering by the topography of the canyon. Therefore, although the exact matching of the recorded and calculated motions are not expected, comparison of the motions show that the patterns of the displacements are close together. This phenomenon indicates the importance of valley shape and its important contribution to the dynamic response of the dam site. Assessment of the displacement time histories in various points at both sides of the canyon indicates that the amplitude of the motions decreases when the height of the point increase. Besides, the comparison between the motions of the left and right sides points show have a higher frequency content and a higher shear-wave velocity. 4) In all ten soil groups, the shear wave velocity that due to the non-symmetricity of the canyon, displacement amplitudes in the left side are larger than the right side. Based on the calculated displacements on the various points, the maximum amplitude along the canyon would be changed up to three times. In the frequency domain, different points of the canyon surface have generally the similar amplification patterns. There are two main peaks of amplification in the frequency range of 3-5 Hz and the frequency range of 6-8 Hz. In both sides of the canyon by increasing the height of the points amplification is increased, especially in the frequency range of 6-8 Hz. Moreover, at the same elevation points, the amplification value in the left side is higher than the right side. Comparison of amplification curves of recorded and calculated motions, show the appearance of new peaks of amplification in higher frequency, which could be related to the real conditions of the dam site. Finally, although the motion amplitude in time domain decreases by height increasing on both sides of the canyon, as expected, the amplification in the frequency domain, especially in high frequencies, increases. This insists that the amplification characteristics of a site should be considered and interpreted as a frequency dependence phenomenon. Moreover, the results indicate the spatial variation of the motion due to the topography effect along the canyon, in which the amplitude of peak ground displacements along the canyon has been changed up to three times.

Under small strains (ε ≤ 10− 3%), the shear-wave velocity (Vs) and its resultant maximum shear modulus (Gmax) are important parameters in geotechnical engineering calculations and soil dynamics analyses. At present, the shear wave velocity of sand is typically determined using measurement and theoretical analysis methods. The measurement methods include in-situ and laboratory tests. In-situ tests are commonly conducted using a borehole method or a surface wave dispersion analysis method. Laboratory tests include bender element tests, resonant column tests, ultrasonic tests, and dynamic triaxial tests. In this regard, the evaluation of the influences of soil particle size on the dynamic behaviour of soils during wave propagation has been an important issue in geotechnical engineering. Heretofore, the effects of particle size on shear-wave velocities in soils have been examined using various experimental techniques. Most of this research was carried out over a limited range of particle sizes, and the results indicated various effects of particle size on shear-wave velocity: there has been no comprehensive and unambiguous outcome describing the influences of particle size on shear-wave velocity in soils. This research focused on the influences of particle size on shear-wave parameters in a particular type of sandy soil. A digitally controlled triaxial testing machine equipped with bender elements was used. A significant advantage of bender element test is that it can be incorporated in standard soil mechanics apparatuses such as triaxial and oedometer devices, and the approaches for data interpretation are relatively simple. This research aims to experimentally examine the effects of a wider range of particle sizes on shearwave velocity and other shear-wave parameters, transmitted in dry sandy soils, using a bender element apparatus embedded in a triaxial testing machine under confining pressures of 50-500 kpa. In this research, the sandy soil was initially categorized into 10 different groups using ASTM standard sieves, and all triaxial samples were prepared with an identical void ratio. The void ratio plays a vital role in the determination of the maximum shear modulus of soil. For all ranges of particle size, the maximum and minimum void ratios were determined, in order to provide an acceptable level of comparison among the results, all samples were prepared with a single void ratio of 0. 80. In this study, homogeneously identical samples were assumed as a prerequisite for all experiments. Therefore, it was necessary to take practical measures to ensure this crucial prerequisite in all specimens. In this regard, various experimental methods may be used to achieve a desirable void ratio, including the wet and dry tamping method, dry pouring technique, and water precipitation methods. In this study, the dry tamping method was carried out to prepare similar specimens with an identical void ratio. To measure the shear-wave travel time, the frequencies between 5 and 12 kHz were used. The significant results obtained in this study were as follows. 1) With reference to different methods of determining the shear-wave travel time, the results of this research showed that the cross-correlation and peak-to-peak methods gave the most reasonable values of the shear-wave velocity. 2) The outcomes revealed that, in a particular soil sample, as the excitation frequency increases, the received signals possess significant amounts of higher frequency components, and surprisingly, these signals are similar in shape. 3) Particle size influences the shape of the received signals, such that the frequency content of received signals in both fine and coarse grained soils are quite similar, but medium-sized soils increased with increasing confining pressure. 5) The results showed that the increasing size of soil grains leads to increased shear-wave velocity in a particular range of particle sizes, and decreased shear-wave velocity in the other range. 6) Although the effects of particle size on shear-wave velocity were the main subject of this study, it seems that this factor alone cannot dominate, and other factors must also be considered, such as the type and shape of particles and the surface roughness.

Isolating the earth structures such as retaining walls, bridge abutments and buried pipes using the compressible materials is a novel solution to reduce the lateral earth pressure. In this technique, a layer of the compressible material with relatively small stiffness and limited thickness is implemented between the retaining wall and the backfill. This material acts as a seismic buffer due to its high compressibility, which absorbs the excess dynamic earth pressure significantly and attenuates the transmitted force to the retaining structure. Choosing the appropriate materials for construction of seismic buffers is based on their physical and mechanical properties as well as cost-effective considerations. Most of the previous studies were focused on some specific materials such as expanded polystyrene (EPS) foam blocks and tire chips. This paper investigated the performance of polymeric seismic buffers made from Polyurethane (PU) foam on seismic response of non-yielding retaining walls. PU foam has appropriate properties and eliminates some of limitations on materials used in previous studies. The purpose of current study was to evaluate the applicability of PU foam as a new option for construction of seismic buffers with regard to its benefits. Hence, the behavior of non-yielding retaining walls was investigated in two conditions of with and without presence of the seismic buffers by conducting of a series of 1g shaking table tests. Seismic buffers included PU foam blocks, which were prepared by injecting foam into the cubic molds and spraying a certain amount of water on the specimens. A total of 13 tests were carried out on two models (retaining wall with and without seismic buffer) with changing the input base acceleration from 0. 07g to 0. 46g. The input motion was a horizontal sinusoidal excitation with a constant frequency of 3. 6 Hz, which was applied for 10 seconds to the longitude direction of the model. The model responses including wall force and backfill soil displacement were measured during the excitation in each test. The results showed that the implementing seismic buffers made from PU foam reduce the total and dynamic horizontal wall forces on average of 30% and 45%, respectively. The force attenuation and backfill soil displacement have an inverse relationship to each other. For an equal Normalized compressible inclusion stiffness, this type of foam has a better performance in comparison with similar materials such as expanded polystyrene foam (EPS). Moreover, it is identifying that the force attenuation is not uniform along the height and the maximum attenuation occurs at the top of the retaining wall. The force distribution is triangular for static conditions. As the peak base acceleration is increased and the contribution of dynamic loads on upper elevations is increased, the force distribution becomes nonlinear. Therefore, at earthquakes with moderate to high intensity, the point of application of total horizontal force is transferred to the upper elevations of the retaining wall. Moreover, it is revealed that the efficiency of this technique increases for moderate to high-intensity earthquakes (acceleration amplitude more than 0. 24g).

A series of 1 g shaking table tests were performed to investigate the response of Tabriz subway tunnel, a circle-type tunnel embedded in dry sand, under sinusoidal excitations. In prototype, the subway tunnel with 9. 2 m diameter and 0. 35 m thickness was embedded in a soil layer. Two reduced-scale 1 g shaking table models, designated as FF and SF, were constructed in 1/45 scale. The FF was constructed to study the seismic response of the soil layer in free field condition, while the SF model includes a subway tunnel to study its seismic response during different excitations. The shaking table of Tabriz University with a platform of 3m×2m and one-degree of freedom was used to induce the desired excitations to models. The table can carry up to 6 tones and can reach acceleration levels up to 1. 5 g with peak displacements of ± 100 mm. A laminar shear box was designed in Tabriz University that includes 20 aluminum frames with dimensions of 1320×814×860 mm (L×H×W). In order to reduce the friction between the layers and simulate the displacement of soil layers, ball bearings were used between two adjacent frames. In this box type, the lateral boundary effect on the seismic response of the soil layer is reduced. The simulation laws for 1 g shaking table tests were utilized in the current study. Based on the simulation laws and the size of the laminar shear box, the prototype to model scale factor was considered to be 45. Therefore, the tunnel model was constructed by aluminum alloy with a diameter of 195. 5 mm and thickness of 1. 5 mm. Uniform dry sand provided from Qomtapeh was used in this study. During the construction, the tunnel and all the embedded instruments were placed in the model. To avoid any interaction of the tunnel with the laminar shear box, the tunnel wasselected shorter than the box width. Two PVC circular plates were placed at both the tunnel ends to avoid the sand entrance into the tunnel model. To simulate the effects of friction on the soil– tunnel interaction, the outside surface of tunnel was covered by sand particles using epoxy coating. For reaching the same target relative density (Dr=65%) during the construction of models, the bulk unit weight was controlled to be constant for all layers. Seven strain gauges were installed on the tunnel surface to monitor the behavior of the tunnel. Five accelerometers were placed in different levels of the model to record the acceleration in the soil. Besides, two LVDTs were placed on the top of the model to measure the soil surface settlement. A 32-channel dynamic data logger was used to record and transfer all the measured data to a personal computer. Two types of excitation were applied to the models by shaking table: I) irregular waves with high frequency content and low amplitude to determine the natural frequency of the models, and II) harmonic waves with low frequency content and high amplitude to study the seismic response of the tunnel. Two peak ground accelerations of 0. 35 g and 0. 50 g with frequencies of 1, 3, 5 and 8 were applied to the models at this stage. The recorded data highlighted significant aspects of the dynamic response for the above type of underground structures: -The results show that the ground response of the free field model is different from the tunnel-soil model and the natural frequency of the free field is slightly larger than soil-tunnel model. This indicatesthe effect of the tunnel on the applied frequency to the system.-The recorded horizontal accelerations at different levels indicate that accelerations are amplified towards the soil surface and the tunnel acts as an obstacle against the propagation of shear waves upward.-According to the results, the dynamic response of circular tunnels can be split into two stages: transient stage and steady-state cycles. During the transient stage, which lasts for the first few cycles, the tunnel reaches a dynamic equilibrium configuration. The transient stage is followed by the steady-state cycles, during which the forces in the tunnel lining oscillate around a mean value.-For all tests, bending moments and lining deformations increase by increasing in maximum base acceleration, but the location of the highest and the lowest amounts stays the same.-According to the results, for A=0. 35 g, maximum bending moment is constant or reduces a little by increasing frequency; however, for A=0. 50 g, maximum bending moment reduces sharply by increasing of the loading frequency. The results show that in the earthquakes with high PGA, the dynamic bending moments caused in the tunnel lining are larger than cracking moment that can lead to a serious damage to the lining in combination with other loads.

This research investigates the trend of changes in seismic response of tall hybrid framed tube skeletons according to the obtained analytical resultsthrough conducting nonlinear dynamic response history analyses(NL-RH analyses) under three components near-field earthquake records. For this purpose, three 30-story structural models with framed tube resistant skeletons were selected and designed. The first resistant skeleton is classified as the basic model with a framed tube structural system. The second and third models are introduced by embedding of multi-level configurations of large scale zipper elements on the basic model, which connected to one or two columns in the first story. The existence of a designed multi-level arrangement of large scale zipper elements prevents the formation of expanded plastic mechanism and also relatively blocks the occurrence of any possible buckling in the lower-stories columns. The connection of the large scale zipper elements to the columns was defined rigid. The studied structures were loaded and designed in accordance with the notified provisions recommended by the Iranian national building codes (divisions six and ten) as well as the standard 2800 (fourth edition) [1-3]. The assumed hysteresis loops related to thepossible formation of plastic hinges in structural elements have been adapted from the FEMA 356 [4]. These notifications were described to clarify the assigned non-linear behavior of the elements of each studied structure. All of the analyses were conducted through SAP 2000 software [5]. To perform nonlinear dynamic response history analyses, an ensemble of five earthquake records including one far-field and four near-field ground motions contain forward-directivity effects, were selected and scaled according to the fourth edition of the Standard 2800. The main criterion in choosing near-field records is the existence of distinct coherent pulses caused by the strong rupture directivity effects, which are emerged in the ground velocity time history [6-7]. In this research, a comprehensive numerical assessment was accomplished on the seismic response parameters of the studied structural models. The analytical evaluations are focused on the maximum inter-story drift ratios, the maximum relative velocity and absolute acceleration of the floors (defined at the center of mass CM), maximum axial and shear force resultants, the upper bound of flexural and torsional moment of the columns, and also the maximum rotation of the formed plastic hinges. By comparing the configuration of the plastic hinges formed in columns and beams, it is resulted thatthe presence of the large-scale zipper elements in the lower four storiesof the structure relatively causes less damages as well as a greater time domain of dynamic stability. The use of these elements in the perimeter bays of tall framed tube structures results a more uniform distribution of the axial and shear forces, as well as bending and torsion moments in the peripheral columns. It is also resulted a noticeable reduction for the maximum inter story drift ratio of floors, the maximum relative velocity and absolute acceleration of floor levels. Moreover, by comparing the total weight of studied models, it is clear that the architectural embedding of the large-scale zipper elements would cause a slight increase for this factor while reducing the average relative displacement near to 15% as well.