Multiple Earthquake Events Research Articles

Duration and damping for ground motion characterization in frequency‐domain damage estimation of SDOF structures

AbstractIn estimating the damage to a single‐degree‐of‐freedom (SDOF) structure due to multiple earthquake events, it may be convenient to use a frequency‐based approach, in which the anticipated ground motions are characterized through power spectral density functions (PSDFs) compatible with the specified design spectra for those motions. Since a spectrum‐compatible PSDF corresponds to a fictitious stationary ground motion process of a specified duration, the duration of this process should preferably be same as the length of the stationary segment in the actual ground acceleration process for a meaningful computation of the stochastic response. On assuming the strong‐motion segment of a given ground motion to represent its stationary segment, the nonstationarity of the strong‐motion segment is evaluated for the existing strong‐motion duration definitions. It is found that the Trifunac‐Brady definition is the most reasonable strong‐motion duration definition in respect of consistently giving the longest strong‐motion segment with least nonstationarity. Further, a spectrum‐compatible PSDF can be used for estimating the stochastic response of a linear system, provided the system has the same damping ratio as that of the design spectrum from which the PSDF has been computed. This causes errors in estimating the peak response of an equivalent linear system due to its damping ratio being higher than the initial damping ratio of the nonlinear system. To address this, an empirical expression is developed for the modified damping ratio of a linear system, which would lead to the correct peak response of the system on using the PSDF obtained from the 5%‐damping design spectrum.

Modeling damage accumulation during ground-motion sequences for portfolio seismic loss assessments

Conventional earthquake risk assessments use fragility and vulnerability models that are based on seismic demands from individual (mainshock) ground motions, and implicitly assume that a structure is intact before an earthquake hits. This study develops a suite of more realistic state-dependent seismic fragility and vulnerability models for a wide range of building taxonomies, leveraging state-of-the-art methods to account for dynamic damage accumulation in structures due to multiple earthquake events (i.e., ground-motion sequences). Models are developed for 561 building classes (i.e., structural types) from the Global Earthquake Model’s global database of fragility and vulnerability models. Four 2010–2012 Canterbury sequence earthquakes are then used to demonstrate an application of the developed models within a portfolio loss assessment, capturing the time-dependent nature of damage and loss in the vulnerability calculations. The results of this application indicate that accounting for damage accumulation across a series of events can significantly increase expected loss ratios compared to a conventional mainshock-only portfolio risk analysis. This work can help analysts to develop and apply state-dependent fragility and vulnerability models for quantifying the potential impact of damage accumulation in portfolio-scale seismic loss assessments.

Digital twin-based life-cycle seismic performance assessment of a long-span cable-stayed bridge

Long-span cable-stayed bridges often have a design service life of more than a hundred years, during which they may experience multiple earthquake events and accumulate seismic damage if they are located in seismic-prone regions. Earthquake occurrence is discretely and randomly distributed over the life cycle of a long-span cable-stayed bridge and often causes sudden drops in the structural performance instead of yearly fixed seismic performance degradation. This study thus proposes a digital twin-based life-cycle seismic performance assessment method for long-span cable-stayed bridges. The major components of this method include: (1) a seismic hazard analysis-based generation method of earthquake occurrence sequence; (2) a digital twin-based structural response prediction method considering lifetime earthquake occurrence and sequence; and (3) a service life quantification method. The proposed method is applied to a scaled long-span cable-stayed bridge with a series of shake table tests. The results show that the digital twin can closely reproduce the life-cycle seismic response of the bridge under sequential earthquakes. The proposed assessment method provides a more intuitive presentation of the life-cycle seismic damage accumulation process and a more accurate estimation of the service life of a long-span cable-stayed bridge.

Comparing responses of special and intermediate moment frames under repeated earthquakes

Steel moment-resisting frames (MRFs) are designed to resist a design-level single seismic event. However, every earthquake may be accompanied by several foreshocks or aftershocks. The cyclic deterioration of structural elements during early earthquakes may cause the lateral-load-resisting system to collapse under the mainshock or aftershocks. The differences in the seismic performance of special and intermediate MRFs (here called SMFs and IMFs, respectively) subjected to single and multiple earthquake events were investigated. For this purpose, four perimeter steel MRFs with four and eight storeys were designed and subjected to 22 synthesised multi-record earthquake events. The non-linear modelling technique adopted accounts for the cyclic deterioration of structural elements, which is a key parameter in the evaluation of seismic performance under sequential earthquakes. The results showed that structural collapse capacity may be reduced considerably if the structure is subjected to foreshock and aftershock events in addition to the mainshock. In addition, it was found that, for the IMF models, the switch from the life safety (LS) limit state to the collapse prevention limit state was immediate. Therefore, the design of IMFs for the LS performance level may result in a substantial risk of collapse when the structure is exposed to repeated earthquakes.

Constraining coseismic earthquake slip using Structure from Motion from fault scarp mapping (East Helanshan Fault, China)

This study uses fault scarps to investigate past earthquakes and the potential seismic hazard of the East Helanshan Fault (EHF). The EHF is a Holocene active normal fault located at the complex transition from compression in the northeastern corner of the Tibetan Plateau to apparently east-west extension in the west margin of Ordos Block, as implied by the Yinchuan Faulted Basin and its boundary normal faults. Repeated normal faulting events, including the 1739 CE event on the EHF, offset alluvial fans of Late Pleistocene and Holocene age and formed remarkable fault scarps, especially along its Suyukou and Hongguozi segments. We selected three fault sections in the Suyukou and Hongguozi segments, where fault scarps of different heights are well preserved on different terrace surfaces. By using Structure-from-Motion-Multiview Stereo (SfM-MVS) techniques on Unmanned Aerial System (UAS) imagery, we acquired high-quality topography of the selected fault sections. Detailed mapping and surveying of the offset geomorphic features and their associated fault scarps, with distinct free faces and multiple slope breaks, suggest that surface-rupturing earthquakes occurred repeatedly along the EHF. Densely spaced cross scarp topographic profiles were extracted to evaluate the vertical slip distributions using newly acquired SfM-derived topography. Through statistical analysis of the displacement dataset, we constrain multiple earthquake events and their cumulative offsets on the Suyukou and Hongguozi segments. Large earthquakes on the EHF produced different characteristic co-seismic slips on the Suyukou segment and the Hongguozi segment of ~2.6 m and ~1.7 m, respectively. The scaling relationships between the moment magnitude, surface rupture length, and the average co-seismic slip indicate possible magnitudes within a range of Mw 6.8–7.2, and a potential magnitude of approximately Mw 7.4 as an upper bound in the case of a multi-fault rupture along the EHF. This study demonstrates the UAS-based SfM photogrammetry as an effective means for mapping fault zone topography in areas of sparse vegetation, as well as its potential for identifying and measuring subtle geomorphic features associated with past earthquakes.

Probabilistic seismic loss estimation of aging highway bridges subjected to multiple earthquake events

This study investigates the seismic life-cycle cost of aging highway bridges under chloride exposure when subjected to multiple main shock earthquake events along the design lifetime of the structure. While chloride induced corrosion manifests as a continuous deterioration process, earthquake occurrences are typically intermittent. Although relatively weak seismic events that induce none-to-minor bridge damage may not instigate immediate intervention and repair, resulting structural cracks may further accelerate the corrosion process. This study presents a novel Monte Carlo based methodology that explicitly considers potentially altered deterioration pattern between seismic shocks as well as uncertainty in earthquake occurrences, record-to-record variability, repair decisions and repair processes for seismic life-cycle cost computations. The proposed framework also eliminates the need for the often improbable assumption in past literature on seismic life-cycle cost assessment that necessitates structural rehabilitation to pre-hazard states regardless of the seismic damage level. Case-study application of the proposed framework is demonstrated on a single column integral bridge located in the marine splash zone in California, United States. Results from multiple main shock analysis of both non-deteriorating and aging case-study bridge reveals a significant contribution of cumulative damage from corrosion deterioration and shock sequences towards seismic loss and life-cycle cost assessment.

Expected seismic fragility of code-conforming RC moment resisting frames under twin seismic events

Seismic design codes require buildings to be designed only for one seismic event, neglecting the possibility of multiple earthquake events. However considerable damage and loss is observed due to aftershocks where buildings have been affected by the first event. The purpose of this research is to introduce fragility curves of code-conforming reinforced concrete moment resisting frames (RCMRFs) under aftershock. Three RC moment resisting frames of 4, 8 and 15 stories, are loaded and designed based on the latest three editions of the Iranian code of seismic design for buildings referred to as Standard No. 2800 (STD-2800). The nine designed frames are then numerically simulated in OpenSees software to perform nonlinear dynamic analysis. Twenty natural ground motion sequences, each including two seismic events, were selected from previous studies for the purpose of nonlinear incremental dynamic analysis. Maximum inter-story drift was employed as a damage index, to capture performance of the RC frames under the sequences. The accepted performance for the collapse level was taken from Iranian instructions for seismic rehabilitation of existing buildings, PBO-Publication No. 360, 2007, which is similar to ASCE 41-13. Seismic fragility values are calculated for the buildings in the second event after being damaged in different first event intensity scenarios. Assuming a log-normal distribution for the failure probability function, corresponding values of mean, μ, and standard deviation, β, for each case are calculated and discussed. Results indicate how the probability of failure in the seismic fragility curves may differ as a result of the effects of a first event scenario. It is also shown how updating the design criteria as well as the height of frames can affect fragility in a second event.

Structural performance of deteriorating reinforced concrete columns under multiple earthquake events

This study investigates deteriorating reinforced concrete (RC) columns subjected to multiple earthquake events, taking into consideration how a combination of environmental stressors and extreme loads can adversely affect the performance of RC columns during their expected service life. For this purpose, nonlinear finite-element models are developed with all the necessary details, validated with a set of experimental test results, and then employed to predict the extent of structural degradation as a function of age and prior earthquake-induced damage. This is through a systematic approach to capture the consequences of corrosion-induced deterioration, in terms of rebar cross section loss, drop of steel yield and ultimate strength, and degradation of bond. The consequences of earthquake events are evaluated with not only structural damage caused by lateral loads, but also the corrosion process expedited as a result of residual damage. Such a holistic assessment strategy paves the way to obtain realistic predictions for the performance of RC columns under future earthquake events. To study this critical aspect, a set of representative scenarios that consist of two earthquake events of different intensities are defined. Considering that RC columns in service commonly experience more than one earthquake during their lifetime, the outcome of this study contributes to transforming the accuracy of predictions made on the residual capacity and seismic response of structures subjected to multiple stressors and hazards.

Cumulative vulnerability assessment of highway bridges considering corrosion deterioration and repeated earthquake events

Civil infrastructure systems when located in high seismic zones may experience multiple earthquake events along their service life. Highway bridges that comprise critical elements of the transportation network are often exposed to harsh environments that exacerbate the structural performance to withstand such repeated shocks. While earthquake occurrences are intermittent in nature, environmental degradation mechanisms such as corrosion deterioration continually weakens the structural capacity. Presently there exists a gap in literature that simultaneously considers such temporally varying multi-hazard threats within a probabilistic framework. This study presents a novel methodology that considers repeated main shock sequences and corrosion deterioration for an index-based estimation of cumulative damage along the service life while considering the associated uncertainties with the earthquake occurrence and the deterioration process. A newly introduced analytical strategy in this study enables the updating of bridge pier section properties reflecting deterioration of an already damaged bridge between subsequent earthquake shock events. The proposed framework is demonstrated on a representative case-study multi-span continuous concrete box-girder bridge in California, United States. The results reveal a significant impact of corrosion deterioration on the seismic damage accumulation under multiple earthquakes and underlines the necessity to incorporate aging effects within bridge asset management in high seismic zones.

Comparative Cost Analysis of Possible Seismic Retrofitting Schemes for Multi-Story Unreinforced Masonry Building

The core objective of this study is to identify the cost effective seismic retrofitting option for multi-story unreinforced masonry (URM) buildings. A seismically deficient 3 story URM structure assumed to be located in the moderate seismic zone of Bangladesh is taken as a reference for this work. Depending on the expected seismic performance level, 5 possible retrofitting schemes are designed for the building and then compared in terms of cost of construction. It is evident from the study that, the ‘Splint and Bandage’ and the ‘Internal Concrete Box’ are the cheapest solutions considering ‘Life Safety’ and ‘Immediate Occupancy’ performance level respectively. Expensive retrofitting options like ‘Seismic Isolation’ and ‘Internal Concrete Box’ have added advantage to withstand multiple earthquake events over the cheaper methods such as ‘Splint and Bandage’ and ‘jacketing’.

A novel framework integrating downhole array data and site response analysis to extract dynamic soil behavior

Seismic site response analysis is commonly used to predict ground response due to local soil effects. An increasing number of downhole arrays are deployed to measure motions at the ground surface and within the soil profile and to provide a check on the accuracy of site response analysis models. Site response analysis models, however, cannot be readily calibrated to match field measurements. A novel inverse analysis framework, self-learning simulations (SelfSim), to integrate site response analysis and field measurements is introduced. This framework uses downhole array measurements to extract the underlying soil behavior and develops a neural network-based constitutive model of the soil. The resulting soil model, used in a site response analysis, provides correct ground response. The extracted cyclic soil behavior can be further enhanced using multiple earthquake events. The performance of the algorithm is successfully demonstrated using synthetically generated downhole array recordings.

The Influence of Ground-Motion Variability in Earthquake Loss Modelling

Earthquake loss models are subject to many large uncertainties associated with the input parameters that define the seismicity, the ground motion, the exposure and the vulnerability characteristics of the building stock. In order to obtain useful results from a loss model, it is necessary to correctly identify and characterise these uncertainties, incorporate them into the calculations, and then interpret the results taking account of the influence of the uncertainties. An important element of the uncertainty will always be the aleatory variability in the ground-motion prediction. Options for handling this variability include following the traditional approach used in site-specific probabilistic seismic hazard assessment or embedding the variability within the vulnerability calculations at each location. The physical interpretation of both of these approaches, when applied to many sites throughout an urban area to assess the overall effects of single or multiple earthquake events, casts doubts on their validity. The only approach that is consistent with the real nature of ground-motion variability is to model the shaking component of the loss model by triggering large numbers of earthquake scenarios that sample the magnitude and spatial distributions of the seismicity, and also the distribution of ground motions for each event as defined by the aleatory variability.

Generalised precursory swarm hypothesis.

The precursory swarm hypothesis as developed in New Zealand has been generalised on the basis of a systematic study of multiple earthquake events at moderate to large magnitudes in the Japan region. In the generalised hypothesis the precursory seismic sequence consists of one or more swarms followed by one or more mainshock events in the same sub-region. The predictor is the characteristic swarm magnitude, defined as the average magnitude of the three largest earthquakes in the swarm or swarms. The observed value of the predictor yields estimates of the magnitude and time of occurrence of the largest mainshock in the sequence; these parameters are estimated as soon as the first swarm occurs, and are revised on the occurrence of successive swarms in the sequence. Further statistical analysis of secondary mainshock events is required, after which a score-sheet of past successes, failures and false-alarms can be compiled, and the hypothesis can then be rigorously tested against future earthquake activity in the region.

Multiple earthquake events at moderate-to-large magnitudes in Japan.

A systematic study of multiple earthquake events to be found in the Japan Meteorological Agency catalogue, 1926-1979, has been made by restricting the data to magnitudes M≥5.0, and identifying multiple events by the three-in-one-day rule. Of the 74 events identified, 33 are classed as mainshock events, 29 as swarms, and 12 as multiplets. The locations of many mainshock events have been close to those of swarms (although no swarms have been located in western Honshu), and this may indicate the occurrence of local changes in the mechanical condition of the rock, and in the complexity of the stress field, with time. The results encourage the search for a possible precursory relationship between swarms and mainshock events in Japan such as has been proposed for New Zealand.