Strong Mainshocks Research Articles

Forecasting strong subsequent events in the Italian territory: a national and regional application for NESTOREv1.0

AbstractThe Italian territory is one of the most seismically active areas in Europe, where Strong Subsequent Events (SSEs), in combination with the strong mainshock effects, can lead to the collapse of already weakened buildings and to further loss of lives. In the last few years, the machine learning-based algorithm NESTORE (Next STrOng Related Earthquake) was proposed and used to forecast clusters in which the mainshock is followed by a SSE of similar magnitude. Recently, a first new version of a MATLAB package based on this algorithm (NESTOREv1.0) has been developed and the code has been further improved. In our analysis, we considered a nationwide and a regional catalogue for Italy to study the seismicity recorded over the last 40 years in two areas covering most of the Italian territory and northeastern Italy, respectively. For both applications, we obtained statistical information about the clusters in terms of duration, productivity and release of seismic moment. We trained NESTOREv1.0 on the clusters occurring approximately in the first 30 years of catalogues and we evaluated its performance on the last 10 years. The results showed that 1 day after the mainshock occurrence the rate of correct SSE forecasting is larger than 85% in both areas, supporting the application of NESTOREv1.0 in the Italian territory. Furthermore, by training the software on the entire period available for the two catalogues, we obtained good results in terms of near-real-time class forecasting for clusters recorded from 2021 onward.

Mainshock–aftershock sequence simulation via latent space encoding of generative adversarial networks

AbstractAftershocks (ASs) following strong mainshocks (MSs) can exacerbate structural damage or lead to collapse. However, the scarcity of recorded data necessitates reliance on artificial sequences, which have difficulty in characterizing the time‐frequency correlation between MSs and ASs. This study innovatively converts the AS time history prediction into an image translation task, exploiting the invertible transformation between accelerograms and time‐frequency representations. An encoder–decoder neural network is developed to encode the MS information into the latent space of a pre‐trained generative adversarial network, enabling accurate AS predictions through the decoder. The integration of seismic parameters further improves the AS prediction performance. Comparative analyses demonstrate that the proposed method outperforms the traditional ones on accuracy and robustness and reproduces the non‐stationarity of ASs.

Preliminary report of field reconnaissance on the 6 February 2023 Kahramanmaras Earthquakes in Türkiye

AbstractOn February 6, a successive rupture of major faults in the Eastern Anatolian Fault Zone and Cardak-Surgu fault triggered a strong mainshock (Mw 7.7) and a major aftershock (Mw 7.6) in Kahramanmaras. The successive earthquake sequence hit southern provinces in Türkiye and northern regions in Syria, causing severe fatality and economic loss. After the earthquakes, the International Consortium on Geo-disaster Reduction (ICGdR) organized an investigation team, involving specialists from China, Japan and Türkiye, to conduct a primary field reconnaissance on seismic damage of infrastructure and ground failures. The 10-day reconnaissance, including a mini-symposium at the Istanbul Technical University (ITU), was conducted from 25 March to 3 April and specifically focused on fault ruptures, liquefaction, landslide, rockfall and lateral spreading along the major ruptured faults from Antakya in Hatay to Goksun in Kahramanmaras, passing through provinces of Gaziantep, Adıyaman and Malatya. By this reconnaissance, a large amount of original seismic data was collected and a primary understanding was established for further steps on mitigation and reduction of seismic damages and its secondary geohazards.

Seismic Response Control of a Nonlinear Tall Building Under Mainshock–Aftershock Sequences Using Semi-Active Tuned Mass Damper

Under the excitation of strong mainshock (MS), the structure will enter the nonlinear stage, whose hysteretic characteristic can be well-simulated by the Bouc–Wen (BW) model. The accompanying aftershock (AS) may increase structural damage and cause the safety problem, therefore, the advanced structural control technology is worthy of being applied to nonlinear structure under MS–AS sequences. The tuned mass damper (TMD) is a popular vibrational absorber, however, it is sensitive to the frequency deviation and may lose its effect for nonlinear dynamic response control in this issue. To protect the nonlinear tall building under MS–AS sequences effectively, a semi-active TMD (STMD) is investigated in this study, which can vary its frequency and damping in real time simultaneously. It can adapt to structural nonlinear behavior in each time segment through varying its stiffness according to the wavelet transform (WT) based algorithm, and meanwhile, enhance the energy dissipation through switching its damping coefficient in real time. A 10-story nonlinear tall building is proposed as a case study and for comparison, a passive TMD (PTMD) is used. Nine MS–AS sequences are elected and nonlinear structural control performance of STMD is highlighted. Results show that STMD can control structural displacement responses and base shear effectively and performs better than PTMD in reducing the plastic development of the structure as well. Further, energy analysis is proposed and it is verified that STMD has an excellent effect in decreasing the structural input energy from MS–AS sequences and dissipating more energy than PTMD. Especially, it is found that a strong AS may cause a larger response to the nonlinear structure than MS, while the nonlinear control effect of STMD is still significant in that case.

Study of steel buildings with LCF system under critical mainshock-aftershock sequence: Evaluation of fragility curves and estimation of the response modification factor by artificial intelligence

In the seismic active zones, the stored energy in faults is continuously released which can lead to the seismic sequence phenomenon. So that, strong main shocks may involve numerous foreshocks and aftershocks with a magnitude close to the main shock. Hence, it is expected that the behavior of buildings under successive shocks would differ compared to one earthquake. Thus, this paper examined the effects of the seismic sequence phenomenon on the performance of steel buildings with the Linked Column Frame (LCF) system under critical successive earthquakes. As LCF is relatively new lateral force resisting system and considering the successive scenarios still seems necessary, this paper estimates the Response modification factors (R factors) by artificial intelligence, besides evaluating the fragility curves under critical mainshock-aftershock sequence. Therefore, building frames with 3, 5, 7, 9, and 11 stories equipped with the LCF lateral load resisting system had been analyzed in the OpenSees software consist of incremental dynamic, nonlinear static, and linear and nonlinear dynamic analyses. R factors were calculated under the seismic scenarios with/without sequence after performing more than 18 thousand nonlinear dynamic analyses. For more comprehensive examination of R factors, in addition to the number of stories, the effect of other parameters, including the length and the behavior of link beams (shear/flexural) were studied. An ideal artificial neural network was moreover designed based on the parameters, and experimental equations were proposed to determine the R factor under consecutive earthquakes with acceptable accuracy. The results indicated that the designed buildings for the R factor based on single earthquake could not provide the expected performance when experiencing successive shocks because the R factors caused by consecutive earthquakes was 26% lower than the single case.

Reliability analysis of an inter-story isolated structure under a main-aftershock sequence based on the Laplace asymptotic method

After a strong mainshock, subsequent ground motion is the result of a sequence of multiple aftershocks, and the damage to a structure under these conditions is more severe than from a single earthquake. Most seismic studies are based on a single earthquake event. To explore the influence of a main-aftershock sequence on an isolated inter-story structure, we constructed a three-dimensional finite-element model of such a structure and subjected it to repeated main-aftershock sequences. The Laplace asymptotic method of second-order second-moment was used to calculate the reliability of the structure under the action of a single mainshock and after a main-aftershock sequence at different seismic levels. The effects of the number of aftershocks, the location of the isolation layer, and the stiffness of the isolation bearing in the structure were analyzed. The results showed that aftershocks increased the failure probability of each sub-structural part of the inter-story isolated structure. The failure probability of the lower structure had the greatest influence, which was about 3.89 times that for the mainshock alone. The probability of failure from multiple vs single aftershocks was similar, but the magnitude of the aftershock plays a major role in failure. The number of aftershocks reduced the overall reliability of an inter-story isolation structure. In the case of different isolation layer positions, the placement of the isolation layer at the top of the seventh story under an extremely rare earthquake level resulted in a reduction of 6.01%. With isolation bearings of different stiffness, the largest decrease was 7.88% when the stiffness was 50%.

Seismic fragility analysis of the inter-story isolated structure for the influence of main-aftershock sequences

The inter-story isolated structure is an effective and feasible structure seismic technology and system, but most studies on inter-story isolated structures only consider the mainshock. A strong mainshock is usually accompanied by multiple aftershocks, the structure will be damaged under the action of the mainshock. Because of the short time interval between the main shock and the aftershocks, the structure is often not repaired in time, so it will be further damaged under the action of the aftershock. Therefore, it is meaningful to study the fragility of inter-story isolated structures under the action of main-aftershock sequences. In this study, the incremental dynamic analysis method was used, and the inter-story isolated structure of a frame shear wall was established. The vulnerability curves of each substructure under the action of a single mainshock and main-aftershock sequence were compared. A series structure system was used to calculate the overall vulnerability of the inter-story isolated structure. The vulnerability curves of different isolation layer setting positions and isolation bearing stiffness under the action of a single mainshock and main-aftershock were compared, and the collapse margin ratio (CMR) of the structure given. The results show that aftershocks increase the exceedance probability of each substructure, and with an increase in the limit state, the influence of aftershocks is more obvious. An appropriate isolation layer design reduces the influence of aftershocks and the exceedance probability of the entire structure.

Seismic performance of regular frame under near field and far field earthquakes

In the past few decades, it is observed that the occurrence of earthquakes is in the form of sequences where the mainshock is followed by one or more aftershocks. The earthquakes excite the foundation of a structure in sequence over some hours/days, out of which few seismic sequences may last for years. The aftershocks that accompany the mainshock take a toll on the seismic demands of the buildings which go on increasing in each successive event as the building may not be retrofitted or strengthened after the mainshock due to time constraints or some other reasons. The seismic design codes consider only a single seismic event of earthquake i.e., mainshock and neglect possibility of the aftershocks. Under a strong mainshock event, even a regular structure may be severely damaged and is highly vulnerable under subsequent events may get the severity of its damage worsened and even collapse. In the present study, an 11-storey RC 2-D frame has been analyzed under10 different episodes of sequential earthquakes with each with the same mainshock followed by six aftershocks of increasing PGA from 0.1 g to 0.6 g of near field and far field following nonlinear time history analyses using SAP2000 software. Inelastic response of the structure is studied with respect to the location of plastic hinges, maximum top displacement, residual displacement, and storey drift. Fragility curves for damaged states are plotted for the far-field and near field sequential earthquakes and discussed.

Comparative Study on Seismic Fragility Assessment of Self-Centering Energy-Absorbing Dual Rocking Core versus Buckling Restrained Braced Systems under Mainshock–Aftershock Sequences

AbstractA strong mainshock may cause several aftershocks within a short interval. These aftershocks can make buildings damaged during the mainshock more vulnerable to collapse. Hence, it is critica...

On the aftershock polarity to assess residual displacement demands

A strong mainshock is able to trigger several aftershocks during a short time interval, causing additional damage to structures. The effect of the polarity of aftershock ground motions (positive and negative) is of great significance, particularly on the residual displacement (RD) demand. The positive and negative polarity occurs when the direction of aftershock is the same and the opposite of that of mainshock, respectively, while these are only two of the various situations of the incident angle of aftershock with respect to that of mainshock. This study evaluates the sufficiency of the positive and negative polarity in multiple earthquakes for the determination of critical residual displacements of structures by considering various relative differences between the incident angles of sequential earthquakes. The structures are schematized as bi-linear single-degree-of-freedom systems to develop constant-strength spectra in terms of the Normalized Residual Displacement (NRDθ). The results reveal that considering only the conventional aftershock polarity (positive and negative) may not necessarily lead to critical residual displacement demands, and also accounting for the variability of various incident angles for consecutive ground motions is necessary for the reliable evaluation of structures.

Unified Scaling Law for Earthquakes: Space-Time Dependent Assessment in Friuli-Venezia Giulia Region

The concept of the Unified Scaling Law for Earthquakes (USLE), which generalizes the Gutenberg-Richter relationship making use of the fractal distribution of earthquake sources in a seismic region, is applied to seismicity in the Friuli-Venezia Giulia region, FVG (Northeastern Italy) and its surroundings. In particular, the temporal variations of USLE coefficients are investigated, with the aim to get new insights in the evolving dynamics of seismicity within different tectonic domains of FVG. To this purpose, we consider all magnitude 2.0 or larger earthquakes that occurred in 1995–2019, as reported in the catalog compiled at the National Institute of Oceanography and Applied Geophysics (OGS catalog), within the territory of its homogeneous completeness. The observed variability of seismic dynamics for three sub-regions of the territory under investigation, delimited based on main geological and tectonic features, is characterized in terms of several moving averages, including: the inter-event time,τ; the cumulative Benioff strain release, Ʃ; the USLE coefficients estimated for moving six-years time intervals, and the USLE control parameter,η. We found that: 1) the USLE coefficients in FVG region are time-dependent and show up correlated; 2) the dynamical changes ofτ, Ʃ, andηin the three sub-regions highlight a number of different seismic regimes; 3) seismic dynamics, prior and after the occurrence of the 1998 and 2004 Kobarid (Slovenia) strong main shocks, is characterized by different parameters in the related sub-region. The results obtained for the FVG region confirm similar analysis performed on a global scale, in advance and after the largest earthquakes worldwide. Moreover, our analysis highlights the spatially heterogeneous and non-stationary features of seismicity in the investigated territory, thus suggesting the opportunity of resorting to time-dependent estimates for improving local seismic hazard assessment. The applied methods and obtained parameters provide quantitative basis for developing suitable models and forecasting tools, toward a better characterization of future seismic hazard in the region.

A comprehensive spatiotemporal evaluation of the current earthquake activity in different parts of the Frakull-Durrës fault zone, Albania

This study presents a detailed spatiotemporal analysis for the Frakull-Durrës (F-D) fault zone at the beginning of 2020. For this purpose, the most frequently used statistical seismicity parameters such as magnitude completeness, Mc-value, Gutenberg-Richter b-value, recurrence times, annual probabilities and standard normal deviate, Z-value, were mapped. The F-D fault zone was divided into two seismogenic subregions, and Mc-value was taken as 2.5 for both the south and north parts. The b-value was estimated as 0.83 ± 0.06 for the south part and 0.85 ± 0.06 for the north part. b-values for both zones are smaller than 1.0 and these values may be considered to be a larger stress accumulation to build up over time and to be released by the next possible earthquakes. Clear decreasing trends were observed in time variations of b-values before the occurrences of several strong main shocks. Analyses of annual probabilities and recurrence times suggest that the study region has an intermediate/long-term earthquake hazard in comparison to occurrences of strong/destructive earthquakes in the short term. Some anomaly regions of a small b-value and a large Z-value were found along the F-D fault zone at the beginning of 2020: i) among Lushnje-Tirana-Durrës including the middle part of the F-D fault zone, ii) in and around Lezha including the north end of the F-D fault zone. Thus, the combination of regions with the lowest b-value and largest Z-value may supply preliminary results for earthquake hazard, and these regions may be considered to be the most likely regions for future strong/large earthquakes in the F-D fault zone.

Parametric and non-parametric estimation of extreme earthquake event: the joint tail inference for mainshocks and aftershocks

In an earthquake event, the combination of a strong mainshock and damaging aftershocks is often the cause of severe structural damages and/or high death tolls. The objective of this paper is to provide estimation for the probability of such extreme events where the mainshock and the largest aftershocks exceed certain thresholds. Two approaches are illustrated and compared – a parametric approach based on previously observed stochastic laws in earthquake data, and a non-parametric approach based on bivariate extreme value theory. We analyze the earthquake data from the North Anatolian Fault Zone (NAFZ) in Turkey during 1965–2018 and show that the two approaches provide unifying results.

Effects of incident angles of earthquake sequences on seismic demands of structures

Numerous aftershocks can be triggered by a strong mainshock and may cause more severe and widespread structural damage with respect to a single seismic event. This study investigates the effect of relative differences between the incident angles of consecutive earthquakes on seismic demands of structures as bi-linear Single-Degree-Of-Freedom (SDOF) systems. To this aim, constant-strength spectra are developed based on different Normalized Engineering Demand Parameters (NEDPs) to identify this impact as a new seismic uncertainty. Several seismic sequences with one aftershock and also two aftershocks are generated considering different directions of both mainshocks and subsequent aftershocks as input excitations in nonlinear dynamic analyses of SDOF systems. The results demonstrate that taking into account the relative differences of the incident angles of sequential earthquakes can affect significantly seismic responses.

Insights on nonlinear soil behavior and its variation with time at strong-motion stations during the Mw7.8 Kaikōura, New Zealand earthquake

To the end of investigating the potential nonlinear soil behavior at 44 strong motion stations during the 2016 Mw7.8 Kaikōura, New Zealand earthquake, we analyzed the modifications of the site response under the strong mainshock recording based on the horizontal-to-vertical spectra ratio, including the shift of the predominant frequency, and three nonlinearity indicators, DNL, ADNL, and PNL. Significant modifications to site response were observed at more than half of the stations, where the predominant frequency generally shifted to the lower one, and the DNL, ADNL, and PNL values were generally greater than 4.0, 0.3, and 10%, respectively. The strain proxy represented by PGV/Vs30 was found in high level as the nonlinearity indicators were high. The deformation proxy may be a better indicator predicting the nonlinear soil behavior at sites with available shear-wave velocity. After the nonlinear soil behavior occurred, the soil structure completely recovered and the nonlinearity vanished after a period of time. The recovery process was dependent on the ground motion intensity immediately after the strong ground shaking, and the soil condition.

A probabilistic framework for the economic loss estimation of structures considering multiple aftershocks

The strong aftershocks have the potential to induce the additional economic loss of structures. This manuscript proposes a framework to estimate the additional economic loss induced by multiple aftershocks, and meanwhile to determine the necessary number of aftershocks that should be considered in the economic loss estimation. Four analysis models (hazard model, structural model, damage model, and loss model) are contained in the proposed framework, which is applied to a 4-story reinforced concrete structure. The result indicates that the additional economic loss induced by a given aftershock is lower when more aftershocks occur before this one. From the perspective of probability, at least two aftershocks should be considered in the loss estimation for M6.0 mainshock, while this number increases to 10 and 28 for M7.0 and M8.0 mainshocks respectively. For low-rise reinforced concrete frame structure, the impact of aftershocks on the economic loss increases with the increase of mainshock magnitude, and it can exceed 40% for strong mainshock from the perspective of probability. The proposed framework and results in this study would be helpful for the decision making in the pre- and post-earthquake environment.

Numerical investigation of AP1000 NIB under mainshock-aftershock earthquakes

This manuscript studies the seismic behavior of AP1000 nuclear power plant buildings (NPPB) under different mainshock and aftershock earthquakes. Three mainshock-aftershock seismic sequences, which satisfy RG 1.60 spectrum, are adopted in time history analysis. A 3D finite element model (FEM) is established to analyze the seismic response of AP1000 NPPB. Parametric analysis of maximum acceleration, peak displacement, acceleration response spectrum, and stress distribution are carried out to study the effects of mainshock-aftershock earthquakes. It is indicated that strong aftershock may aggravate the potential cumulative damage and take a significant influence on the seismic performance of the nuclear island building (NIB). Mainshock and aftershock also influence the floor acceleration response spectrum in horizontal directions significantly. Therefore, to assess the safety of AP1000 NIB, aftershock should be considered in the seismic design of strong mainshock with strong aftershock, especially for the isolated NPPB.

Stochastic procedure for the simulation of synthetic main shock‐aftershock ground motion sequences

SummaryAccording to the current seismic codes, structures are designed to resist the first damaging earthquake during their service life. However, after a strong main shock, a structure may still face damaging aftershocks. The main shock‐aftershock sequence may result in major damage and eventually the collapse of a structure. Current studies on seismic hazard mainly focus on the modeling and simulation of main shocks. This paper proposes a 3‐step procedure to generate main shock‐aftershock sequences of pairs of horizontal components of a ground motion at a site of interest. The first step generates ground motions for the main shock using either a source‐based or site‐based model. The second step generates sequences of aftershocks' magnitudes, locations, and times of occurrence using either a fault‐based or seismicity‐based model. The third step simulates pairs of ground motion components using a new empirical model proposed in this paper. We develop prediction equations for the controlling parameters of a ground motion model, where the predictors are the site condition and the aftershock characteristics from the second step. The coefficients in the prediction equations and the correlation between the model parameters (of the 2 horizontal components of 1 record and of several records in 1 sequence) are estimated using a database of aftershock accelerograms. A backward stepwise deletion method is used to simplify the initial candidate prediction equations and avoid overfitting the data. The procedure, based on easily identifiable engineering parameters, is a useful tool to incorporate effects of aftershocks into seismic analysis and design.

Seismic Damage Assessment of SMA-Retrofitted Multiple-Frame Bridge Subjected to Strong Main Shock–Aftershock Excitations

Seismic Damage Assessment of SMA-Retrofitted Multiple-Frame Bridge Subjected to Strong Main Shock–Aftershock Excitations

Application of complex network theory to the recent foreshock sequences of Methoni (2008) and Kefalonia (2014) in Greece

Seismic hazard evaluation before recent strong main shocks in the area of Greece is attempted using prior seismicity on the basis of earthquake network theory. The connections of earthquake networks are constructed from successive earthquakes and the nodes are represented by cells of normal grids that were considered superimposed on the study areas. The dynamic evolution of the network structure is examined at sliding windows for identifying periods of statistically significant change, i.e., the network structure differentiation from that of a random network, where the structure is characterized by selected network measures, including the index of small-worldness property. By studying the structure of complex earthquake network, a distinct dynamic evolution is revealed, 2 months before the main shock occurrence. Particularly, the network measures, such as clustering coefficient and small-worldness index, tend to increase before and exhibit an abrupt jump at the time of the main shock occurrence, and then slowly decrease and become stable with small variations as before.