Effects Of Aftershocks Research Articles

Fragility assessment of existing low-rise steel moment-resisting frames with masonry infills under mainshock-aftershock earthquake sequences

This paper presents the fragility assessment of non-seismically designed steel moment frames with masonry infills. The assessment considered the effects of multiple earthquakes on the damage accumulation of steel frames, which is an essential part of modern performance-based earthquake engineering. Effects of aftershocks are particularly important when examining damaged buildings and making post-quake decisions, such as tagging and retrofit strategy. The procedure proposed in the present work includes two phase assessment, which is based on incremental dynamic analyses of two refined numerical models of the case-study steel frame, i.e. with and without masonry infills, and utilises mainshock-aftershock sequences of natural earthquake records. The first phase focuses on the undamaged structure subjected to single and multiple earthquakes; the effects of masonry infills on the seismic vulnerability of the steel frame were also considered. In the second phase, aftershock fragility curves were derived to investigate the seismic vulnerability of infilled steel frames with post-mainshock damage caused by mainshocks. Comparative analyses were conducted among the mainshock-damaged structures considering three post-mainshock damage levels, including no damage. The impact of aftershocks was then discussed for each mainshock-damage level in terms of the breakpoint that marks the onset of exceeding post-mainshock damage level, as well as the probability of exceeding of superior damage level due to more significant aftershocks. The evaluation of the efficiency of commonly used intensity measures of aftershocks was also carried out as part of the second phase of assessment.

Lessons for loss assessment from the Canterbury earthquakes: a 22-storey building

By comparing damage and loss predictions with those observed for a 22-storey steel building with eccentrically based frames (EBF) that underwent the Canterbury earthquakes, this paper gauges the performance and explores the benefits of applying the PEER performance-based earthquake engineering assessment procedure. The study shows that steel EBF repair costs are significantly affected by the work required to remove and reinstate non-structural elements in order to access the steel structure. It is therefore concluded that the location of non-structural elements relative to structural elements should be considered in the seismic design of new buildings and within loss assessment procedures. Two retrofit and/or rehabilitation options are considered for the building; improving the deformation capacity of drywall partitions, and re-detailing the exterior cladding connections to better accommodate structural deformations. The findings show that, for the specific building, improving the drywall partitions should lead to a reduction in expected annual losses whereas the use of improved detailing for the precast concrete cladding would not. This suggests a general strategy for cost-effective decisions might be to focus on increasing the performance of the most vulnerable components (plasterboard partition walls, in this case). An examination of the potential effects of aftershocks to the selection of cost-effective options adds support to this conclusion. Aftershocks are found to increase the predicted losses by around 10%, emphasizing the need to consider aftershocks when assessing expected annual losses due to earthquakes.

An Analysis of the Effects of Seismic Events on The Turkish Stock Market1

Purpose – This study aims to elaborate on the mechanisms of interaction between seismic events and the Turkish stock markets. Design/methodology/approach – To model this relationship, the properties of earthquakes has been modeled using Mw and ML parameters. Earthquakes’ distance to the surface and the distance to the closest city center are also added as independent variables. Dataset consists of 7333 observations of daily frequency between 01.01.2000 – 28.01.2020 covering longer than a 20-year period. The index of BIST TUM is used to represent the response of the Turkish stock market. Preliminary analysis on the dataset suggested a threshold effect and therefore, the threshold VAR model has been used to model the series. Findings – Findings validate the existence of a significant threshold effect at 4.3 magnitude, which points out to the conclusion that earthquakes below a certain magnitude do not have a significant relationship with the stock markets in Turkey. Additionally, as seismic events occur closer to the surface, their negative effects on the market seem to amplify. This effect is also observed as seismic events get closer to city centers. Additionally, non-damaging seismic events seem to cause substantial market responses. Discussion – The effects of nondestructive earthquakes along with the effects of aftershocks following a destructive earthquake indicate that the financial consequences of earthquakes in Turkey are likely to be more related to the perception of risk, rather than the actual destructiveness of the earthquake. Findings also indicate that the aftershocks following a big earthquake hasten the recovery of the BIST TUM index.

Portfolio Seismic Loss Estimation and Risk-based Critical Scenarios for Residential Wooden Houses in Victoria, British Columbia, and Canada.

This study presents a city-wide seismic risk assessment of single-family wooden houses in Victoria, British Columbia, and Canada. The novelty and uniqueness of this study include considerations of detailed building-by-building exposure model for residential houses, current national seismic hazard model for Canada, and rigorous seismic fragility modeling of wooden houses based on nonlinear dynamic analysis of structures subjected to mainshock-aftershock sequences. A full consideration of stochastic event scenarios in probabilistic seismic risk analysis allows the identification of critical scenarios from overall regional seismic risk perspectives and provides valuable insights in informing earthquake disaster risk management actions. Outputs from the developed catastrophe model for Victoria are compared with the empirical model that was developed based on insurance claim data from the 1994 Northridge earthquake. Results of the seismic loss calculations highlight the importance of seismic resistance of the existing houses and of aftershock effects. The integrated use of the outputs from the advanced catastrophe model facilitates risk-based identification of critical earthquake scenarios, which are useful for different stakeholders for earthquake risk management purposes.

Vulnerability assessment of a high‐rise building subjected to mainshock–aftershock sequences

SummaryStrong aftershocks have the potential to further aggravate the damage state of structures, and much less attention has been given to the seismic vulnerability of high‐rise buildings than that of low‐ to medium‐rise buildings. This study assesses the seismic vulnerability of a 32‐storey frame–core tube building by performing the incremental dynamic analysis on the material‐based three‐dimensional numerical model. A storey damage model based on the material damage is developed using the weighted average method. Eighteen recorded mainshock–aftershock sequences, whose mainshock records match the target spectrum, are selected. The results indicate that the developed stroey damage model can effectively reflect the additional damage induced by aftershocks. Strong aftershocks have high potential to change the location of weak storeys. Notably, shifts of weak storeys are observed in more than 30% of aftershocks with relative spectral acceleration of 0.8. As the mainshock‐induced damage state becomes more severe, the mainshock‐damaged building becomes increasingly fragile to the aftershock excitation and more sensitive to aftershock intensities. The probability of exceeding severe damage state increases from 35.3% to 62.1% due to the effects of strong aftershocks. The results in this study can provide supports to the seismic resilience assessment of this high‐rise building.

Effects of Aftershocks on the Potential Damage of FRP-Retrofitted Reinforced Concrete Structures

This study investigates the effects of aftershocks on the potential damage of reinforced concrete (RC) structures retrofitted by fibre-reinforced polymer (FRP). An eight-storey RC frame that was poorly confined due to deficient transverse reinforcement was selected and then retrofitted by FRP wraps to provide external confinement. Inelastic time history and damage analyses of the frame under single earthquakes and mainshock–aftershock sequences were performed. FRP retrofit did not ensure a damage-free structure, and it minimised the damage though. The results indicated different contributions of aftershocks to the damage of the retrofitted frame, and this finding illustrates the limitations of current seismic codes. The damage caused by single earthquakes is proposed to be used as a criterion for dealing with the effects of aftershocks as follows. If the mainshocks cause minor or light damage, the effect of aftershocks is negligible, whereas the effect of aftershocks needs to be considered if the mainshocks cause moderate or severe damage. The outcomes of this study can be used in making decisions about whether the effects of aftershocks should be considered.

Cumulative Damage of Structures under the Mainshock-aftershock Sequences in the Near-fault Region

ABSTRACT This manuscript quantifies the effects of aftershocks with pulse characteristics on the cumulative damage of structures. The revised randomization method is developed to generate the artificial mainshock-aftershock (MSAS) ground motions showing pulse characteristics. The efficiency of period normalization is discussed from the point of view of dispersion, and the effects of the aftershocks are quantitatively studied. It is found that strong aftershock can contaminate the efficiency of period normalization, and the effects of strong aftershocks in the near-fault region on cumulative damage can exceed 20% and reach 40%. The equations are proposed for the prediction of damage induced by pulse-like MSAS sequences.

Vulnerability assessment of AP1000 NPP under mainshock-aftershock sequences

Researches explicitly indicate that the seismic events, consisting of the several shock waves (the first followed by the aftershock or train) may result in cumulative damage even if the intensity is not strong. This manuscript presents the vulnerability assessment formulation to estimate the influence of aftershocks on the damage of the nuclear power plant (NPP). In the formulation, the engineering demand (ED) parameter is derived. Four categories of performance levels (PL), three limit states (LS) and the relative intensity measure (∇IM) are defined to calculate the fragility of the AP1000 NPP under 30 groups of seismic sequences. The effect of aftershock and mainshock-aftershock ratio ∇Sa on the vulnerability of the structure are quantitatively analyzed. The observed results indicated that the aftershock can cause additional cumulative damage to the NPP. The fragility increases by 10–40% for different damage levels with an increase of ∇Sa from 0.5 to 1.0. The fragility framework can thus be used to assess the vulnerability of NPP under mainshock-aftershock sequences.

Probabilistic fragility and resilience assessment and sensitivity analysis of bridges incorporating aftershock effects

ABSTRACT This paper discusses a computationally efficient methodology for probabilistic fragility and resilience analysis of bridges explicitly incorporating the effects of aftershock seismic events, based on nonlinear time history analysis, stochastic simulation and kriging surrogate modeling. Aftershock fragilities dependent on mainshock-induced damage states are developed and used in conjunction with restoration models for estimating bridge functionality and resilience. The input parameter space for the surrogate models consists of uncertain hazard and structural parameters and deterministic structural/geometrical bridge parameters, leading to probabilistic estimates of bridge performance metrics (i.e., fragility, functionality, resilience). Kriging surrogate modeling is used for estimating these metrics, facilitating efficient adoption of these complex structural and probability models. Relative importance of the predictive parameters is quantified by extending and integrating a probabilistic sensitivity analysis into the proposed framework. An illustrative example of the proposed framework is demonstrated through fragility and resilience assessment of a typical bridge in California subjected to mainshock and aftershock.

Development of the Performance-Based Plastic Design Method by Considering the Effects of Aftershocks

ABSTRACT One of the efficient methods for seismic design of structures is Performance-based Plastic Design (PBPD) in which by choosing the desired yield mechanism and the target drift at the beginning of the design, unlike that in conventional design methods, there is no need for a time-consuming process of trial and error to achieve the desired performance. On the other hand, recent earthquakes have shown that aftershocks can increase the damage to a building after the main earthquake and thereby threaten the safety of the inhabitants. However, the seismic hazard of aftershocks is still not explicitly included in the current design codes. In this study, attempts have been made to develop the PBPD method for considering the effects of aftershocks. For this purpose, by performing an incremental dynamic analysis (IDA), the ductility factor μ and the ductility reduction factor Rμ of six PBPD frames (2-, 4-, 6-, 8-, 10-, and 12-story) were calculated under mainshock and mainshock-aftershock sequences. Then, the energy modification factor γ, which is related to the two parameters μ and Rμ, was calculated for both the mainshock and the mainshock-aftershock sequences. An equation is presented in order to calculate the ratio of the energy modification factor of the mainshock-aftershock sequence to the mainshock energy modification factor () according to the design period, and the factor obtained is multiplied by the energy modification factor γ so that the effect of aftershock can be considered for the frame design, and by using this modification, the performance of the frame under the mainshock-aftershock sequence can be improved. The results of the time-history analysis performed on the modified PBPD frames show better performance by the frames designed on the basis of modified PBPD than the frames designed on the basis of the original PBPD method under MS-AS sequences in terms of inter-story drift and rotation of plastic hinges.

Damage Evolution and Failure Behavior of Post-Mainshock Damaged Rocks under Aftershock Effects

Rock stability has long been a hot topic during underground energy exploitation, but the failure process of rock materials under earthquake effects is extremely complicated, and the failure mechanism still remains unclear. In order to investigate the fatigue damage and failure behavior of rocks under aftershock effects considering the post-mainshock damage states, a series of laboratory tests were conducted on marble specimens subjected to stepwise cyclic loading. Four levels of peak stress (i.e., 10, 30, 50, and 70 MPa) were applied in the first cycle, to simulate mainshock damage. The results indicate that, with the increase of initial cycle amplitude, mainshock damage has a significant effect on deformation behavior, dissipated energy, P-wave velocity, and AE characteristics of tested specimens during aftershock process. The increasing amplitude of initial cycle enhances irreversible deformation and weakens the resistance to deformation, which accelerates the expansion of specimen volume and results in the reduction of bearing capacity. Furthermore, the increasing amplitude of initial cycle obviously changes the failure morphologies and intensifies the final macro-fracture scale of tested specimens, which are verified by acoustic emission AF-RA value and b-value, respectively.

Ground motion rotation for mainshock-aftershock sequences: Necessary or not?

This manuscript studies the effects of rotating mainshock-aftershock (MSAS) sequences to different orientations on the seismic performance of structures. A total of 222 recorded MSAS ground motions are additionally rotated the given angle away from the fault-parallel direction. Four different engineering demand parameters (EDPs) are used to quantify the seismic performance. The results indicate that the aftershock would change the critical angle of ground motions with respect to the mainshock, and the change in the critical angle can exceed 30°. The effect of ground motion rotation is significant for MSAS sequences, and can exceed 25% for all EDPs considered. The effects of aftershocks on the maximum and median EDPs over all rotation angles are generally comparable. Rotating MSAS sequences into the critical angle is necessary for seismic design and performance evaluation.

Seismic performance evaluation of typical dampers designed by Chinese Code subjected to the main shock-aftershocks

A practical design method in the current Chinese code is introduced and used to design various dampers, which assumes that dampers provide building structures with the effective stiffness and effective damping ratio. Aftershocks followed by the main shock are likely to cause further structural damage. However, the influence of aftershocks is not considered in this design method. It is not clear for the seismic performance of code-designed dampers under the main shock-aftershock sequences. This paper concentrates on the seismic performance of dampers designed by the Chinese code by considering the effect of aftershocks. Five types of dampers, namely buckling-restrained braces (BRBs), viscoelastic(VE) dampers, viscous dampers(VDs), friction dampers(FDs), and self-centring(S–C) dampers that are installed on a high-rise building structure are designed by the Chinese code. Using the incremental dynamic analysis and fragility analysis, the seismic performance of five types of dampers under the main shock-aftershock sequences are compared from the respective of structural collapse, damage and the equipment's acceleration failure. The results show that the design method of dampers in the Chinese code are applicable even without considering the influence of aftershocks. The seismic responses of structure-damper systems are mainly caused by the main shock. Different dampers with different effective stiffness and effective damping ratio have their own distinctive advantages of enhancing the building structural safety. FDs and S–C dampers are superior to other dampers in controlling the structural collapse. VDs perform best in reducing the structural damage and equipment's acceleration failure probabilities.

Generalized Earthquake Counting Processes for Sequence‐Based Hazard

AbstractSequence‐based probabilistic seismic hazard analysis (SPSHA) allows us to account for the effect of aftershocks in the assessment of seismic structural‐design actions (Iervolino et al., 2014, 2018). In fact, it generalizes classical probabilistic seismic hazard analysis (PSHA; Cornell, 1968), combining it with aftershock‐PSHA (Yeo and Cornell, 2009). SPSHA associates in time aftershocks to mainshocks and, therefore, retains a desirable property of classical PSHA; that is, events (earthquakes in PSHA and mainshock–aftershock sequences in SPSHA) occur according to homogeneous Poisson processes (HPPs). Nevertheless, the number of earthquakes in SPSHA is not Poisson‐distributed. This is addressed herein, in which the probability distribution is formulated and discussed for the following random variables: (1) the count of all earthquakes pertaining to sequences originating in any time interval; (2) the count of all earthquakes occurring in any time interval; (3) the count of all earthquakes that cause exceedance of an arbitrary ground‐motion intensity threshold at the site of interest, generated by sequences originating in any time interval. An application referring to central Italy is also developed to help the discussion. The three main findings are that: (1) the formulated SPSHA counting processes further generalize PSHA; that is, they degenerate in the corresponding mainshock HPPs, if aftershocks are neglected; (2) to associate the aftershocks to the corresponding mainshocks in time is fit for hazard assessment purposes; and (3) the variance‐to‐mean ratio of the counting distributions is significantly larger than one; consequently, the occurrence processes cannot be approximated by Poisson processes. These results, which complete the SPSHA framework, can be a reference for model calibration exercises when SPSHA is computed via simulation and in those cases in which the probability of an exact number of exceedances is of interest, rather than that of observing at least one exceedance (e.g., for seismic damage accumulation studies).

Probabilistic seismic demand assessment of self-centering concrete frames under mainshock-aftershock excitations

This paper investigates the effect of aftershocks on the seismic performance of self-centering (SC) prestressed concrete frames using the probabilistic seismic demand analysis methodology. For this purpose, a 4-story SC concrete frame and a conventional reinforced concrete (RC) frame are designed and numerically analyzed through nonlinear dynamic analyses based on a set of as-recorded mainshock-aftershock seismic sequences. The peak and residual story drifts are selected as the demand parameters. The probabilistic seismic demand models of the SC and RC frames are compared, and the SC frame is found to have less dispersion of peak and residual story drifts. The results of drift demand hazard analyses reveal that the SC frame experiences lower peak story drift hazards and significantly reduced residual story drift hazards than the RC frame when subjected to the mainshocks only or the mainshock-aftershock sequences, which demonstrates the advantages of the SC frame over the RC frame. For both the SC and RC frames, the influence of as-recorded aftershocks on the drift demand hazards is small. It is shown that artificial aftershocks can produce notably increased drift demand hazards of the RC frame, while the incremental effect of artificial aftershocks on the drift demand hazards of the SC frame is much smaller. It is also found that aftershock polarity does not influence the drift demand hazards of both the SC and RC frames.

Seismic response of RC frames under far-field mainshock and near-fault aftershock sequences

Engineered structures built in seismic-prone areas are affected by aftershocks in addition to mainshocks. Although aftershocks generally are lower in magnitude than that of the mainshocks, some aftershocks may have higher intensities; thus, structures should be able to withstand the effect of strong aftershocks as well. This seismic scenario arises for far-field mainshock along with near-field aftershocks. In this study, four 2D reinforced concrete (RC) frames with different numbers of stories were designed in accordance with the current Iranian seismic design code. As a way to evaluate the seismic response of the case-study RC frames, the inter-story drift ratio (IDR) demand, the residual inter-story drift ratio (RIDR) demand, the Park-Ang damage index, and the period elongation ratio can be useful engineering demand parameters for evaluating their seismic performance under mainshock-aftershock sequences. The frame models were analyzed under a set of far-field mainshock, near-fault aftershocks seismic sequences using nonlinear dynamic time-history analysis to investigate the relationship among IDR, RIDR, Park-Ang damage index and period ratio experienced by the frames. The results indicate that the growth of IDR, RIDR, Park-Ang damage index, and period ratio in high-rise and short structures under near-fault aftershocks were significant. It is evident that engineers should consider the effects of near-fault aftershocks on damaged frames that experience far-field mainshocks as well.

Influence of advanced structural modeling technique, mainshock-aftershock sequences, and ground-motion types on seismic fragility of low-rise RC structures

Large earthquakes are rare natural hazards, having catastrophic impact on society due to loss of lives, damage to constructed facilities, and business interruption. Because of damage accumulation due to the main strong shaking, aftershocks potentially endanger the safety of residents and subsequently increase financial loss due to downtime and repair costs. Therefore, accurate prediction of the seismic performance of structures in the post-earthquake stage is critical for disaster risk mitigation. This paper employs an advanced structural modeling technique, which can simulate various features of cyclic degradation in material and structural components using nonlinear fiber beam-column elements. The model accounts for inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement and can simulate multiple failure modes of reinforced concrete structures under dynamic loading. Furthermore, a comprehensive ground motion selection accounting for multiple types of ground motions, such as shallow crustal, deep inslab, and subduction earthquakes, is implemented. Finally, a new set of fragility curves has been developed for each ground motion type, which accounts for the aftershock effects and influence of ground motion types on cyclic degradation and failure modes of low-rise reinforced concrete structures. It was found that slight and moderate damage is not significantly affected by major aftershocks for different ground motions types. However, considering aftershocks increases the probability of exceedance of damage for extensive and complete damage up to 5% and 10% for inslab and crustal event, respectively. The proposed methodology significantly improves the accuracy of seismic risk and vulnerability assessment by reducing the uncertainties associated with structural modeling and variability of earthquake ground motions.

REASSESS V2.0: software for single- and multi-site probabilistic seismic hazard analysis

Probabilistic seismic hazard analysis (PSHA) is generally recognized as the rational method to quantify the seismic threat. Classical formulation of PSHA goes back to the second half of the twentieth century, but its implementation can still be demanding for engineers dealing with practical applications. Moreover, in the last years, a number of developments of PSHA have been introduced; e.g., vector-valued and advanced ground motion intensity measure (IM) hazard, the inclusion of the effect of aftershocks in single-site hazard assessment, and multi-site analysis requiring the characterization of random fields of cross-correlated IMs. Although software to carry out PSHA has been available since quite some time, generally, it does not feature a user-friendly interface and does not embed most of the recent methodologies relevant from the earthquake engineering perspective. These are the main motivations behind the development of the practice-oriented software presented herein, namely REgionAl, Single-SitE and Scenario-based Seismic hazard analysis (REASSESS V2.0). In the paper, the seismic hazard assessments REASSESS enables are discussed, along with the implemented algorithms and the models/databases embedded in this version of the software. Illustrative applications exploit the potential of the tool, which is available at http://wpage.unina.it/iuniervo/doc_en/REASSESS.htm .

Damage spectra of the mainshock-aftershock ground motions at soft soil sites

The soft soil tends to amplify the damage accumulation induced by mainshock-aftershock (MSAS) sequences due to the significant amplification effect of soft soil on the ground motions. This manuscript proposes the damage spectra of MSAS ground motions at soft soil sites to quantify the accumulated damage experienced by the structures under the MSAS sequences. Artificial MSAS ground motions are firstly generated by the revised randomization method (i.e. combing two random records which belong to two different ground motion groups respectively). The effects of period normalization (i.e. the vibration period is normalized by the predominant period of mainshock ground motion) are discussed from the point of view of dispersion and mean. The effects of the aftershock ground motions at soft soil sites on the damage spectra are quantitatively studied. The results indicate that the period normalization can reduce the dispersion of damage spectra of MSAS sequences, but larger intensity of aftershock ground motion can contaminate the efficiency of period normalization. The effects of strong aftershocks at soft soil sites on the damage spectra can exceed 40%. Strong aftershock ground motions at soft soil sites tend to cause larger additional damage than those at firm soil sites. The prediction equations are finally proposed to predict the mean of damage spectra for MSAS sequences at soft soil sites.

Resilience loss factor for evaluation and design considering the effects of aftershocks

The seismic resilience of a system includes not only the consequences directly related to the intensity of earthquake but also the recovery process and time closely related to the human and government responses in the post-earthquake environment, in which many aftershocks would affect the mainshock-damaged systems. This manuscript proposes a resilience loss factor to incorporate the effects of aftershocks on the resilience loss of structures. This factor is applied on the evaluation of resilience loss for a case-study reinforced concrete (RC) frame structure under mainshock-aftershock (MSAS) sequences. The recovery time and pre-event functionality are introduced in the definition of the resilience loss factor, making this factor concentrate more on the intrinsic characteristics of structures during the recovery process. The replacement threshold based on the repair cost is incorporated into the definition of resilience loss factor to reflect the engineering application. The results indicate that the effects of considering replacement threshold and aftershocks on the resilience loss of structure can exceed 200% and 400% respectively, when the replacement threshold is reached.