Mainshock-aftershock Sequences Research Articles

Fault structure and earthquake clustering in Aswan region (Egypt) revealed by high-precision earthquake location from 35 years of recorded natural and induced seismicity

Starting from an initial catalogue of 7833 natural and reservoir-induced seismic events collected in Aswan region (south Egypt) from 1982 to 2016, we investigate the fault structure and triggering mechanisms by determining high-resolution hypocenters for 2562 earthquakes. Despite the complex network of intersecting fractures, high-precision earthquake locations reveal numerous discrete fault strands (some not mapped yet), whose kinematics have been determined from the focal mechanisms of Ml ≥ 2.5 earthquakes. Furthermore, the analysis of the space-time evolution of seismicity indicates an eastward migration of earthquakes and a progressive activation of different faults. This process could reflect fluid migration in the Wadi Kalabsha embayment likely controlled by permeability barriers at depth acting as seals, and the subsequent build-up of the pore pressure as the dominant driving mechanism of the observed seismicity in the region. A strong spatio-temporal earthquake clustering is observed in the region characterized by seismic sequences, repeated earthquakes, and long-lasting swarms. High variability of b-value in correspondence of the occurrence of mainshock-aftershock sequences also suggests the activation of faults and fractures within their respective fault zones under lower differential stress conditions due to fluids. The projection of earthquake hypocenters on an E-W vertical depth section of the study area shows a seismic gap with an approximate length of about 11 km along the Kalabsha fault, which suggests the presence of a locked fault patch that may generate a Mw 5.9 earthquake if this segment were to break in a single rupture episode.

Slope displacement reliability analysis considering rock parameters spatial variability subjected to stochastic mainshock-aftershock earthquake

A new and efficient framework for analyzing the stochastic dynamic response and reliability of the rock slope was constructed based on the generalized probability density evolution method (GPDEM). The left bank slope of Jinping dam was taken as an illustrative example and the effects of mainshock-aftershock sequence randomness, spatial variability of geotechnical materials and coupled uncertainties on the slope dynamic response and reliability were compared and analyzed for the first time. Firstly, a series of mainshock-aftershock sequences and parameter random fields were generated as stochastic factors inputs. Then, the effectiveness of GPDEM was validated by comparing with Monte Carlo Simulation (MCS). The Newmark slip displacement method was combined with GPDEM to analyze the slope dynamic response and reliability subjected to single and coupled uncertainties from a probabilistic viewpoint. Finally, the fragility curves of three risk levels were established based on the reliabilities with different seismic intensities. The result suggests that parameter spatial variability affects the dynamic response and reliability of the slope to some extent, but this effect will be overshadowed by the uncertainty of mainshock-aftershock sequences when it is considered simultaneously. Besides, aftershocks will cause the decrease of slope reliability and the impact of aftershock cannot be overlooked.

Pseudo-dynamic tests of steel frame with disc spring self-centering energy dissipation braces under doublet earthquake and mainshock-aftershock sequence

To provide valuable insights into the seismic performance and self-centering behavior of self-centering braced systems, a 1/2-scale 3-story 1-bay steel frame equipped with disc spring self-centering energy dissipation braces (DS-SCBs) was designed, fabricated, and subjected to pseudo-dynamic testing. This testing encompassed various earthquake hazard levels, a doublet earthquake, and a mainshock-aftershock sequence. The experimental findings revealed the stable hysteretic response of the DS-SCB, with no observed degradation in stiffness or activation forces. Additionally, the test frame exhibited outstanding seismic performance, sustaining only confined damage under very rare earthquakes. Due to the presence of the DS-SCBs, the test frame performed commendable self-centering behavior, characterized by minimal residual deformations. Peak story drifts and floor accelerations under two earthquakes of the same hazard level were regarded as equivalent. Furthermore, the subsequent aftershocks exhibited no discernible impact on the seismic performance of the test frame. Floor acceleration spikes were observed at locations where the brace stiffness underwent changes or where adjacent stories moved in opposite directions. The steel frame equipped with DS-SCBs proved to be an attractive, low-damage alternative to conventional steel systems for the development of resilient urban areas.

Vine-copula-based multi-dimensional fragility analysis of nuclear power plant under sequential earthquakes

Seismic resilience of critical infrastructure, such as nuclear power plants, is paramount in ensuring nuclear safety. This study presents a comprehensive analysis of the seismic fragility of nuclear power plants under sequential earthquakes, employing the innovative vine-copula theory. The methodology integrates advanced modeling techniques, including layered shell elements and plastic damage softening constitutive modeling, to capture the intricate behavior of nuclear power plants under seismic loading. The seismic sequence is derived from the Wenchuan earthquake data, considering both mainshocks and aftershocks. A set of random seismic peak ground accelerations (PGAs) is generated based on the distribution of giant earthquake PGAs. Utilizing seismic attenuation theory, corresponding random aftershock PGAs are generated. The resulting mainshock-aftershock sequence, modulated within the real seismic sequence, serves as the input for numerical simulations. The vine-copula theory is employed for multi-dimensional fragility analysis, providing a flexible framework to model the complex nonlinear dependencies among structural response parameters. The vine-copula model is applied to fit seismic response data, allowing the construction of fragility surfaces under sequential earthquakes. This approach, rooted in performance-based earthquake engineering (PBEE), enables a more realistic representation of the seismic risk profile. The findings demonstrate that seismic fragility trends for nuclear power plants increase with higher mainshock and aftershock intensity measures (IMs). The impact of aftershocks on the structural performance, often overlooked in traditional studies, is elucidated through the proposed methodology. The study contributes valuable insights into nuclear safety assessments by quantifying the influence of sequential earthquakes on the fragility of nuclear power plants.

Fragility Analysis of a Concrete Gravity Dam under Mainshock–Aftershock Sequences considering Sliding and Cracking

Fragility Analysis of a Concrete Gravity Dam under Mainshock–Aftershock Sequences considering Sliding and Cracking

Optimal design of chevron braced friction damper for mainshock–aftershock vulnerability control

This study proposes the optimum design of chevron braced friction dampers (CBFDs) for the vulnerability control of steel moment–resisting frames (SMRFs) under mainshock-aftershock (MS-AS) sequences. First, the optimum parameters of the CBFD systems installed in the stories of an inelastic SMRF are found through minimizing the maximum damage index of stories averaged over seven scaled earthquake excitations. The uniform distribution of the story damage along the height of the controlled SMRF is adopted as constraint in the optimization procedure. The damage index is calculated based on the Park-Ang damage model which is expressed based on a linear combination of deformation, moment, and absorbed hysteretic energy of structural elements imposed by an earthquake excitation. Results reveal that the optimized CBFDs-equipped SMRF exhibits better distribution of the story damage than that of the uncontrolled SMRF. Finally, the vulnerability assessment of the optimized CBFDs-equipped SMRF under MS and MS-AS ground motions is investigated by the fragility curves. The fragility assessment emphasize that the optimized CBFDs effectively mitigate the seismic vulnerability of the controlled SMRF under MS-AS sequences at different damage states.

Seismic damage analysis of shallow buried subway station in clayey sand soil under mainshock-aftershock earthquakes

During earthquake events, a mainshock may trigger a number of following aftershock earthquakes in a short time. Usually, structures will be damaged to a certain extent by the mainshock, and aftershocks can further exacerbate the damage to structures, making post-disaster rescue and rehabilitation extremely challenging. Yet, most of research on seismic performance of underground structures only considered the single mainshock, ignoring the effect of aftershocks. In this study, a nonlinear numerical model of the shallow buried shallow subway station in clayey sand soil under mainshock-aftershock sequences was constructed to replicate the damage evolution process of underground structures. A limited numerical sensitivity analysis was performed considering the influences of the magnitude of mainshock and aftershock intensity. The results indicate that the damage of subway station is closely related to the magnitude of mainshock intensity. The aftershocks might cause more serious incremental damage to underground structures and secondary displacement of the site, potentially lead to collapse of the subway station, when the mainshock magnitude is greater. With the superposition of the mainshock and aftershock, the relative horizontal displacement of the subway station could reach the inter-story drift limit value. Therefore, the aftershock should be taken into account for seismic safety design of shallow underground structures.

Multi-metric evaluation of the optimal intensity measure for mainshock-aftershock fragility analysis of transmission towers

Identifying the optimal intensity measure (IM) contributes to reducing uncertainties in probabilistic seismic demand models (PSDM) and enhancing confidence of their results. Although many studies have discussed appropriate IMs for mainshocks, relatively less attention is paid to the potential aftershock effect. In particularly, the optimal IMs for developing PSDM of transmission towers subjected to sequential earthquakes has not been well studied previously. Therefore, this paper conducts a multi-metric evaluation to identify the optimal IM in the context of mainshock-aftershock (MSAS) fragility analysis of transmission towers. This evaluation process includes collecting a set of ground motions recorded with varying magnitudes and fault distances to construct MSAS sequences, followed by comprehensive discussions on their seismic characteristics by comparing thirty-six potential IMs candidates. Additionally, a finite element model of a transmission tower is established to numerically investigate different engineering demand parameters (EDP) of the tower under the selected MSAS sequences. The optimal PSDM of the transmission tower is determined by evaluating each couple of IM−EDP on a logarithmic space using multiple evaluation metrics. Through this analysis, the first-mode spectral acceleration Sa(T1,ξ) is identified as the optimal IM as it exhibits excellent correlation, efficiency, practicality and proficiency with the EDPs of the transmission tower, especially with the inter-segment displacement ratio. Consequently, seismic fragility curves of the transmission tower are developed using the optimal IM. The research outcome can contribute to improving the understanding of the probabilistic assessment of transmission towers in the MSAS scenario.

Seismic evaluation of post-tensioned precast concrete frames with non-grouted fuse-type dissipaters under successive excitations

In recent years, demands for mitigating structural damage and post-earthquake repair costs have spurred considerable research on self-centering seismic force-resisting systems. This study evaluates the Post-tensioned Precast concrete Frame with non-grouted fuse-type Dissipaters (PPFD) through experimental and numerical methods. Quasi-static tests were conducted on a one-bay, single-story PPFD to investigate its seismic response. The tested PPFD was shown to effectively control damage to concrete components and limit residual displacements; imperfections at the beam-column interface joint had a negative effect on the initial stiffness and self-centering performance of the PPFD. Parametric analyses were then performed to discuss the force and deformation responses of the beam-column joint in the PPFD with dissipaters solely beneath the beam. A design method for the PPFD was developed based on the formulas derived and verified from the parametric analyses. Nonlinear dynamic analyses were then employed to evaluate prototype designed multi-story PPFDs subjected to individual earthquakes in addition to mainshock-aftershock sequences. The findings indicated that both configurations of PPFD, with dissipaters either symmetrically arranged on the beam or located exclusively below the beam, exhibited comparable maximum inter-story drifts and significantly smaller residual inter-story drifts than a comparable reinforced concrete frame. Moreover, the PPFD demonstrated only a minor increase in both the maximum and residual inter-story drifts when subjected to aftershocks, compared to a much larger increase for the reinforced concrete frame.

A direct analytical derivation of the multi-dimensional fragility spaces of structures under nonstationary mainshock-multi-aftershock sequences

Performance-based earthquake engineering (PBEE) is a popular direction in the earthquake community, and at this stage, risk-based PBEE has become mainstream. In the risk-based probabilistic framework, seismic fragility analysis constitutes the most important link, and corresponding research on the mainshock–aftershock sequence has received widespread attention in recent years. Since a mainshock is often accompanied by multiple aftershocks and there is great uncertainty in the vibration characteristics of aftershocks, a seismic fragility analysis of structures under a stochastic mainshock-multi-aftershock sequence is meaningful and necessary. The corresponding questions, such as how to derive the multi-dimensional analytical fragility form under a stochastic mainshock-multi-aftershock sequence and how to correlate multiple intensity measures with multiple demand parameters, still require further investigation. In this paper, a direct analytical derivation of the multi-dimensional seismic fragility spaces of structures under nonstationary stochastic mainshock-multi-aftershock sequences is introduced. The methodology framework, implementation steps, and application examples are also provided in detail. Moreover, two scenarios, the one-mainshock-one-aftershock and one-mainshock-two-aftershocks, are considered, and the obtained multi-dimensional analytical fragility spaces for both scenarios are validated. In general, the matching accuracy of the fragility results for both scenarios is proven to be high, and the direct analytical derivation of the multi-dimensional fragility spaces is validated to be ideally consistent, which further provides a reference for multi-dimensional risk analysis under nonstationary stochastic mainshock-multi-aftershock sequences in future work.

Endurance time analysis for seismic performance of underground structures subjected to mainshock–aftershock sequences

The seismic response analysis of subway underground structures only considers the impact of a single mainshock, ignoring the aftershocks, which may cause more serious secondary damage within a short time after strong earthquakes. To investigate the seismic performance of underground structures under sequential earthquakes and improve the understanding of aftershocks, we analyzed the dynamic response of subway station structures under mainshock–aftershock sequences. A typical two-story and three-span subway station was selected as the prototype, and a finite element model of a soil–underground structure interaction was established. Based on the basic concept of endurance time and design response spectrum of relevant seismic standards in China, an endurance time acceleration function was generated, and nine mainshock–aftershock sequences were constructed. Thereafter, the seismic responses of subway stations under the action of mainshock–aftershock sequences, such as lateral deformation characteristics and damage failure law, were discussed and analyzed. The results show that the endurance time method can be used as a new and efficient method to study the seismic performance of underground structures subjected to mainshock–aftershock sequences. The effects of aftershocks on the damage and lateral displacement of underground structures were preliminarily revealed. The structural deformation response of each layer varies greatly due to the different damage states of the mainshock. The maximum story drift θ of the structure increases with the increase of loading seismic intensity. From the perspective of seismic damage and relative deformation, the structural dynamic response, considering the effect of sequential earthquakes, is greater than that of the mainshock alone, which reflects the disadvantage of aftershocks in seismic design. The results provide a reference for seismic analysis and damage assessment of structures under sequential earthquakes.

A CFD-FEA coupled model for simulating dynamic response of offshore jacket platform under earthquake considering wind, wave, current and aftershock loads

Offshore platforms as the key equipment in marine oil and gas exploitation are facing challenges from various environmental conditions, such as winds, waves, currents and even earthquakes. Due to the combined action of those concurrent loads, there is potential risk of accidents. The aim of this paper is to systematically study the dynamic response and safety assessment of the large scaled offshore structures against the concurrent loads considering the fluid-structure interaction (FSI) effects. Three-dimensional numerical wave flume including a jacket platform is established by CFD software Fluent, based on which the simulation of wave generation and propagation is carried out. Then the FSI effects between waves and platform are analyzed using the CFD- FEA coupling method. Subsequently, the seismic ground accelerations are inputted into the platform, correspondingly the structural response is analyzed. Moreover, multiple influencing factors including earthquake direction and mainshock-aftershock sequence are discussed. The results show that the structure response under concurrent loads is not a simple linear superposition of different load. The seismic response would be underestimated if wave action is ignored when the earthquake acts on certain angles. The methodology proposed could be regarded as an effective way to analyze the FSI problem under realistic offshore environment.

Seismic Performance of Buildings Equipped with Four-Joint Rotational Friction Dampers in Mainshock–Aftershock Sequences

Seismic Performance of Buildings Equipped with Four-Joint Rotational Friction Dampers in Mainshock–Aftershock Sequences

Fragility analysis of a concrete gravity dam under mainshock-aftershock sequences

This study presents the fragility analysis of a concrete gravity dam under the mainshock-aftershock sequences using the incremental dynamic analysis. Nonlinear dynamic analysis is carried out on a concrete gravity dam considering reservoir and foundation for various intensities of mainshock-aftershock sequences using a two-dimensional finite element model. Several as-recorded mainshock-aftershock sequence data is collected from the available database and used in the dynamic analysis as ground motion input. The cumulative impact resulting from the aftershocks is quantified in terms of crack length and an energy-based damage index. Based on the analysis, different limit states are defined namely slight, moderate, extensive, and severe. Fragility curves are subsequently developed based on these defined limit states. Results indicate that aftershocks increase the fragility of gravity dams significantly. For a typical spectral acceleration of 0.5 g, the probabilities of attaining slight, moderate, extensive, and severe limit states are 94%, 89%, 42%, and 3%, respectively. However, when aftershocks are considered, these probabilities increase to 99.8%, 94.8%, 67.8%, and 7%, respectively.

Robustness of remediation measures against liquefaction induced manhole uplift under mainshock-aftershock sequence

Earthquakes generally consist of one mainshock and subsequent aftershocks. Although effects of aftershocks following the mainshock on surface structures has been studied extensively, similar studies on underground structures are rarely reported in the literature. With the fast advances in underground space development, robustness of remediation measures against underground structures uplift induced by soil liquefaction shall be examined to ensure their functioning subject to not only the mainshock but also the subsequent aftershocks. This paper studies the uplift behaviour of a conventional manhole subjected to the mainshock-aftershock sequence. It was found that, in the ground that becomes liquefied during the mainshock, manholes become more vulnerable when faced with aftershocks. Due to this reason, some of the previously proposed remediation measures, such as increasing the manholes’ self-weight, roughening the sidewalls, were examined using centrifuge modeling in this study. It was found that such measures had little effect in the aftershocks despite their effectiveness in the precedent mainshock. In contrast, the methods that mitigates manhole uplift by enhancing the manhole’s base permeability, demonstrated better performance in the aftershock than in the mainshock, indicating its promising application potential in future mitigation design.

Intersection between tectonic faults and magmatic systems promotes swarms with large-magnitude earthquakes around the Tengchong volcanic field, southeastern Tibetan Plateau

Abstract Volcanic regions commonly host swarms comprising small to moderate-sized earthquakes while tectonic faults host mostly mainshock-aftershock sequences that can include very large earthquakes. In the southeastern Tibetan Plateau, large tectonic faults formed by the collision between the Indian and Eurasian plates intersect with the intraplate Tengchong volcanic field, and the seismic behavior of such an environment remains unclear. Here, we built a deep-learning-based high-precision earthquake catalog for the Tengchong volcanic field and found that (1) ~59% of the seismicity occurred as swarms but on faults aligned with the regional tectonic stress field; (2) all swarms contained fluid-diffusion-like migration fronts, with some occurring where high CO2 emissions have been detected; and (3) a year-long swarm, including two ML 5.2 earthquakes within two months, revealed complex fluid-fault interaction. Combined with the historical occurrences of M >6 earthquake swarms around the Tengchong volcanic field, our observations suggest potential increased likelihood of swarms with large-magnitude earthquakes where large tectonic faults and magmatic systems intersect.

Seismic behaviour of self-centring hybrid steel frames equipped with SMA plates under mainshock–aftershock sequences

This paper focuses on the mechanical behaviour, seismic application in steel frame connections and steel structure of shape memory alloy (SMA) plates. This study commenced by examining the hysteretic response of SMA plates subjected to cyclic tension tests, considering two different loading paths. Subsequently, a self-centring steel frame connection equipped with SMA plates and washers (SC-PW) was examined via quasi-static cyclic loading tests, and comparatively satisfactory self-centring behaviour was observed. However, the degradation of the initial stiffness of the SC-PW due to an initial imperfection was observed. An improvement strategy was proposed to overcome the degradation of the hysteretic behaviour (e.g., initial stiffness) of the SC-PW. Disc springs with a larger stiffness and preloads were introduced to overcome the degradation of the initial stiffness of the SC-PW, and the validation was confirmed via detailed finite element (FE) analyses. To improve the computation efficiency, a simplified FE model was developed to reproduce the hysteretic responses of the connection. Based on the simplified FE model, a six-storey prototype structure, i.e., a self-centring hybrid steel frame (SCHSF), equipped with the improved connection was developed, and the structural dynamic responses were investigated via nonlinear response history analyses, where mainshock–aftershock sequences were considered. The influence of the SMA material properties on the seismic behaviour of the prototype structure was investigated.

Performance-based plastic design and seismic fragility assessment for chevron braced steel frames considering aftershock effects

Observations from past earthquakes demonstrated that buildings in high seismic regions might be exposed to several aftershocks following the mainshock which can significantly increase the damage level. Since aftershocks have the potential to increase the vulnerability of buildings damaged under mainshocks, it is crucial to consider aftershock effects in the structural design phase. Performance-based Plastic Design (PBPD) method is an innovative seismic design method in which the building designed by this method shows a desirable performance under severe ground motions. PBPD allows the designer to avoid time-consuming trial and error process of analyses that are usually done in Performance-based Design method to reach a desirable performance. However, this efficient method is only valid for mainshocks only and does not consider aftershock effects in the structural design process. In this study, initially, the seismic performance (interstory drift and plastic hinges distribution) of the three 3-, 6-, and 9- story PBPD designed Chevron-braced Steel frames (CBSF) are compared under mainshocks (MS) and mainshock-aftershock (MS-AS) sequences. Subsequently, a PBPD method is developed for CBSFs considering the aftershock effects in the structural design process. Incremental dynamic analysis (IDA) has been performed using a suite of mainshocks and aftershock ground motions to develop a design base shear modification factor. Using the developed method, the performance of CBSFs under MS and MS-AS sequences is evaluated and compared with the existing PBPD method. Fragility curves are developed to compare the seismic vulnerability of the frames designed based on PBPD and proposed PBPD methods under MS-AS sequences. The outcomes of this study provide a practical design approach for CBSFs under MS-AS sequences and shows that the structures designed using developed PBPD method show more desirable behavior under MS-AS sequences compared with the structures designed using the conventional PBPD method.

Time-dependent and spatio-temporal statistical analysis of seismicity: application on the complete data set of the 2010 Beni–Ilmane earthquake sequence

SUMMARY The Beni–Ilmane (BI) seismic sequence, situated in the north-central region of Algeria, began on 2010 May 14 with a main shock of ML 5.4 followed by two other shocks on May 16 and 23 of ML 5.3 for both. Using the complete data set from May 14 to August 31 and the Epidemic Type Aftershock Sequences (ETAS) model to separate background from aftershocks contribution gave a first insight around the uncertainty that surrounds the causes and mechanisms of the seismicity. According to our findings, two phases have been determined, the first one, beginning from May 14, shows low b- and p-values with most of the seismicity being self-triggered. In contrast, the second phase, beginning from May 23, demonstrates an increase of both the b- and p-values with a high number of background events. In the second phase, the background contribution involves 53 per cent of all seismic activity after May 23’s peak which is high compared to typical main shock–aftershocks sequences. A possible explanation is that the main shocks began or assisted aseismic processes in addition to causing aftershocks. A sealed source at depth may have been broken by the third main shock, opening the way for the following incursion of highly pressurized fluids (hydrocarbures) confirmed by a high Vp/Vs ratios.

Seismotectonics of the Querétaro Region (Central Mexico) and the 1934 MI 4.8 Earthquake North of Celaya

Abstract The Querétaro region (central Mexico) is located in the trans-Mexican volcanic belt, an active volcanic arc related to the subduction of oceanic plates along the Pacific margin of Mexico. It is characterized by north–south-striking normal faults of the southern Basin and Range Province, up to 40 km long and with morphologically pronounced scarps, such as the San Miguel de Allende fault and the faults forming the Querétaro graben. These faults are located directly north of a major regional-scale system of east–west striking, seismically active intra-arc normal faults that are oriented parallel to the axis of the volcanic arc. Where the two orthogonal normal fault systems interfere, the outcrop-scale observations show that the east–west intra-arc fault system overprints the Basin and Range Province structures. Here we document a 1934 earthquake in a region previously not known for seismic activity. Our study is mostly based on an unpublished contemporary dossier preserved at Archivo Histórico del Instituto de Geología de la Universidad Nacional Autónoma de México, a recently inventoried archive that also preserves several unpublished macroseismic and instrumental studies of major Mexican subduction zone earthquakes between 1911 and 1954. A mainshock–aftershock sequence that initiated 14 July 1934 is documented by instrumental recordings at the Tacubaya observatory and by macroseismic observations at ten population centers, ranging in intensity between five and seven on the modified Mercalli scale. Based on the size of the damage area, the intensity magnitude of the mainshock is estimated at 4.8 ± 0.5. Based on the intensity distribution, the epicenter was located in the Laja River valley north-northeast of the town of Celaya, in the south-southwestern extrapolated continuation of the San Miguel de Allende normal fault scarp, which suggests that this fault extends to the epicentral region of the 1934 earthquake and is characterized by recurrent Quaternary tectonic activity.