Seismic Sequence Research Articles

Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).

Sangay volcano (Ecuador): the opening of two new vents, a drumbeat seismic sequence and a new lava flow in late 2021

On 2 December, 2021 we recorded a sequence of drumbeat seismic events at Sangay volcano. This sequence lasted several hours and resulted in two explosive emissions whose eruptive columns reached 9 km above crater. Unexpectedly, these explosions did not produce any ash fallout in the inhabited areas around the volcano. This drumbeat sequence was produced after a series of morphological changes, including the opening of two new vents (W and N) and a landslide. These occurred during an enhanced period of ground deformation and degassing. Further analysis of satellite images allowed us to determine that this sequence was associated with the widening of the recently open vent north of the main crater, and the extrusion of a new lava flow. Timely communication of this event to the authorities and the population was ensured by the IG-EPN by following internal protocols. The corresponding short reports reached more than 300,000 people.

Permanent displacement reliability analysis of soil slopes subjected to mainshock-aftershock sequences

It is of great significance for disaster prevention to study the influence of mainshock-aftershock sequences on reliability of the soil slope. Considering the uncertainty of the mainshock-aftershock sequences, a new analysis method is proposed for the random dynamic response analysis and permanent displacements reliability evaluation of soil slopes under the action of mainshock-aftershock sequences. Firstly, a simulation method of random seismic sequence based on Copula function considering the correlation between mainshock and aftershock is introduced in this paper. Then, the permanent displacement of soil layered slope under 86 randomly generated mainshock-aftershock sequences and single mainshocks is calculated based on Newmark method. And the influence of aftershocks on the permanent displacement of slope is analyzed through the mean value. Besides, the influence of peak ground acceleration (PGA) on the permanent displacement and sliding surface of slope is also discussed. Finally, based on the probability density evolution method (PDEM), the influence of aftershocks on the permanent displacement reliability of slope is explained from the perspective of probability for the first time. The results show that the randomly generated mainshock-aftershock sequence can meet the needs of random dynamic analysis and probability analysis of soil slope by comparing with the target value. Based on the random dynamic calculation results, compared with single mainshocks, the soil slope has larger permanent displacement under the mainshock-aftershock sequences. However, there are obvious uncertainties in the permanent displacement and the position of the most dangerous sliding surface according to the different soil layer distributions and ground motions. It is very necessary to study the influence of mainshock-aftershock sequence on slope reliability from the perspective of probability. When the threshold is 1 m, the reliability of the slope will be reduced by 2 %∼15 % by aftershocks.

Applying the damage assessment for rapid response approach to the august 24 M6 event of the seismic sequence in central Italy (2016)

Seismic monitoring networks are increasingly being used in urban areas to record and locate earthquakes. Recordings in the proximity of buildings also allow assessing, as a first approximation, the expected building damage. The DARR (Damage Assessment for Rapid Response) method provides local-scale information on expected damage patterns. The potential of this approach is discussed here for the August 24 M6 event of the Central Italy seismic sequence (2016–2017). We focus only on the first event of the sequence because cumulative damage is outside the scope of this study. The earthquake recordings are available from two Italian monitoring networks: the Italian Accelerometric Archive (ITACA) and the OSS (Osservatorio Sismico delle Strutture), which collects data from monitored buildings and bridges in Italy. We selected four target areas (Amatrice, Norcia, Visso and Sulmona) characterized by different epicentral distances and building typologies, that suffered different levels of damage during the M6 event on 24 August 2016. Using recordings either in the free field or in the basement of buildings, the expected relative displacement of building typologies common in the studied areas is calculated with the DARR method. Using predefined damage thresholds from literature, the obtained results allow quantifying the expected damage for dominant building typologies in the surroundings of the recording sites. We investigate and discuss the potential use and applicability of the DARR method in different areas depending on the epicentral distance and building characteristics. The results indicate that the DARR approach is useful for supporting and improving rapid response activities after a seismic event.

Anomalies and transient variations of b-value in Italy during the major earthquake sequences: what truth is there to this?

SUMMARY The Gutenberg–Richter b-value is thought to be a proxy of stress conditions in the crust and therefore able to locate asperities as zones of stress concentration responsible for the nucleation of strong events. The scientific literature contains a broad range of case studies showing precursory drops of b, just before the occurrence of strong events, and subsequent rises, during the early part of aftershocks sequences. Translating these results into hazard assessment, the b-value has assumed the status of a candidate precursor to the occurrence of an imminent large event. This issue is analysed here for three major seismic sequences that occurred recently in Italy. In comparison to previous studies, this investigation indicates that the variability of b may not be a reliable indicator of stress or a significant precursor in these examples, and instead may be assigned to a combination of chance, inhomogeneities in the data and inefficiencies in estimation methodologies. Consequently, extreme caution is required when we interpret b-values both as a proxy of physical processes involved in a seismic sequence and as a precursor to the occurrence of imminent strong events.

Calibration of a Bayesian spatio-temporal ETAS model to the June 2000 South Iceland seismic sequence

SUMMARY The reliable forecasting of seismic sequences following a main shock has practical implications because effective post-event response is crucial in earthquake-stricken regions, aftershocks can progressively cause increased damage and compound economic losses. In the South Iceland Seismic Zone (SISZ), one of two large transform zones in Iceland where earthquake hazard is the highest, an intense seismic sequence took place during 17–24 June 2000, starting with a ${M}_{\rm{w}}$ 6.4 main shock on 17 June 2000, followed by another ${M}_{\rm{w}}$ 6.5 main shock four days later and on a different fault. Both earthquakes caused considerable damage and incurred heavy economic losses. They were immediately followed by intense aftershock activity on the causative faults and triggered earthquakes as far as 80 km away along the transform zone. To investigate the feasibility of forecasting the progression of such complex sequences, we calibrated a spatio-temporal epidemic-type aftershock sequence (ETAS) clustering model to the June 2000 seismic sequence in the framework of Bayesian statistics. Short-term seismicity forecasts were carried out for various forecasting intervals and compared with the observations, the first generated a few hours after the first main shock and followed by daily forecasts. The reliability of the early forecasts was seen to depend on the initial model parameters. By using an adaptive parameter inference approach where the posteriors from each preceding forecasting interval served as informative priors for the next, the fast convergence of the parametric values was ensured. As a result, the 16–84 percentile range of the forecasted number of events captured the actual number of observed events in all daily forecasts, and the model exhibited a strong spatial forecasting ability, even only a few hours after the main shock, and over all subsequent daily forecasts. We present the spatio-temporal ETAS parameters for the June 2000 sequence as ideal candidates of prior estimates for future operational earthquake forecasting of other Icelandic aftershock sequences. Past seismic sequences need to be analysed retrospectively to confirm the stability of the parameters of this study, effectively enable the application of the Bayesian ETAS model as an operational earthquake forecasting system for aftershocks in Iceland.

Exploring the performance of experimentally benchmarked RC bridge pier models when subjected to sequential seismic shocks

In this paper we explore the performance of RC bridge piers to seismic ground motion sequences, using both experimental and numerical models. Four RC columns were tested on the University of Bristol’s shake table. These columns contained both well-confined and poor-confined cases. Spectrally matched by near-field without pulse (NFWP), near-field pulse-like (NFPL) and far-field (FF) ground motion records where employed in a sequential/progressive fashion ranging from (I) slight damage (II) extensive damage (III) complete damage and (IV) aftershock cases. These experimental test results are then used to develop a benchmarked OpenSees model of this bridge pier. The importance of the concrete tension constitutive model is highlighted. The differences between sequential (progressive damage) and neglecting sequential seismic events are discussed. The benchmarked model is then used for a heuristic case using incremental dynamic analyses. A comparison is made between drift and energy dissipation performance measures, that suggests drift cannot identify the increased system damage induced by sequential events.

Quantitative Biostratigraphic Analysis and Age Estimates of Middle Cretaceous Sequences in The Baltimore Canyon Trough, Offshore Mid-Atlantic U.S. Margin

ABSTRACT We applied quantitative methods to previously published biostratigraphic data from the Baltimore Canyon Trough (offshore of the Mid-Atlantic U.S.A.) to provide an improved chronostratigraphic framework for Cretaceous sequences. Here, we successfully used graphic correlation of 228 planktonic foraminifera, nannofossil, and palynological events spanning 22 wells to define assemblage and interval zones as well as major paleoenvironmental changes in the Dawson Canyon, Logan Canyon (three sequences), and Missisauga Formations (two sequences, undifferentiated here). Ranking and scaling techniques were not successful because of the of the limited number of usable biostratigraphic markers. The ages of the sequences previously identified using well logs and seismic profiles were temporally constrained based on chronostratigraphically significant biostratigraphic markers that we identified: the late Cenomanian to Turonian DCx sequence (Rotalipora cushmani and Thalmanninella greenhornensis); the early Cenomanian LC1 sequence; the middle and late Albian LC2 sequence (Braarudosphaera africana, Planomalina buxtorfi, and Spinidinium vestitum); the late Aptian LC3 sequence (Cyclonephelium tabulatum); and the early Aptian to Barremian Missisauga sequences (Aptea anaphrissa, Pseudoceratium pelliferum, and Muderongia simplex). These five biostratigraphic associations are correlated with six prominent seismic reflectors and sequence boundaries that can be traced across the basin. Duration of hiatuses associated with these sequence boundaries are uncertain, though our Monte Carlo analysis allows extraction of age estimates from broad and sometimes contradictory ranges and suggests correlation of hiatuses with global sea-level falls. Together, these seismic and biostratigraphic interpretations can be applied (1) to evaluate reservoir continuity and the viability of offshore carbon storage reservoirs in the Baltimore Canyon Trough, (2) to better define the tectonostratigraphic evolution of the basin, and (3) to contribute to the understanding of regional and global variations in Cretaceous sea level.

Dynamic Source Parameters of Significant Earthquakes in the Gulf of Aqaba, Egypt

Several seismological and paloseismolgical studies and historical reports have indicated that Gulf of Aqaba is characterized by higher seismicity than any other seismogenic sources around Egypt. Accordingly, surrounding areas have been impacted by several destructive earthquakes (AD 1068, AD 1212, AD 1229, AD 1458, and AD 1995). Cities situated around the Gulf of Aqaba have experienced different damage levels due to historical and instrumental earthquakes resulting in considerable fatalities. The Aqaba earthquake which occurred on November 22, 1995 (Moment magnitude (Mw) = 7.3) was the largest recorded earthquake along with the Dead Sea Fault System, strongly felt at Eastern Mediterranean region. Many seismic sequences have ocuuerd in the Gulf of Aqaba in the last decades and some of them continued for almost two years. This paper applies a spectral decomposition method based on a reference site to correct the source spectra from the path and site effects by employing Iterative Least Square analysis. The obtained displacement source spectra are modulated with Brune’s omega square type spectrum. The dynamic earthquakes parameters are computed using the S-wave window for the significant Gulf of Aqaba earthquakes. 50 earthquakes have been used in this study with Ml 3.0 to 6. The calculated seismic moments range spans from 1.48E+19 to 2.193E+22 dyne-cm, and the corner frequency range spans from 4.1 to 8.5 Hz. The source radii span which range from 84.3 m to 173.4 meters. The observed stress drops vary from 0.3 to 371.7 megapascal (Mpa).

Rock and fault rheology explain differences between on fault and distributed seismicity.

Analysis of seismicity can illuminate active fault zone structures but also deformation within large volumes of the seismogenic zone. For the Mw 6.5 2016–2017 Central Italy seismic sequence, seismicity not only localizes along the major structures hosting the mainshocks (on-fault seismicity), but also occurs within volumes of Triassic Evaporites, TE, composed of alternated anhydrites and dolostones. These volumes of distributed microseismicity show a different frequency-magnitude distribution than on-fault seismicity. We interpret that, during the sequence, shear strain-rate increase, and fluid overpressure promoted widespread ductile deformation within TE that light-up with distributed microseismicity. This interpretation is supported by field and laboratory observations showing that TE background ductile deformation is complex and dominated by distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones. Our results indicate that ductile crustal deformation can cause distributed microseismicity, which obeys to different scaling laws than on-fault seismicity occurring on structures characterized by elasto-frictional stick-slip behaviour.

Reliability evaluation for cantilevered retaining walls considering uncertainty of mainshock-aftershock sequences

A method for the probabilistic analysis and reliability evaluation of retaining walls is presented in this paper. This paper examines the stochastic dynamic response of the rigid cantilever retaining wall subjected to mainshock-aftershock sequences. A variable generation strategy for minimizing the lowest generalized F-discrepancy is used to facilitate the simulation of stochastic seismic sequences. The correlation between the main shock and the aftershock is established through a copula function. The direct probability integral method (DPIM) is used to estimate the reliability of the retaining wall when the wall is subjected to stochastic seismic sequences. The results show that the dynamic reliability of different physical quantities at different safety thresholds can be obtained by the above methods without assuming the probability distribution of the response. This study also demonstrates that aftershocks can degrade the structural reliability to varying degrees, with the degradation intensifying in proportion to the increase in design requirements.

Seismic and sequence stratigraphy of the Palaeocene carbonates at the western margin of Cyrenaica platform, NE-Libya

The work of this research focuses on seismic sequence stratigraphy of the Palaeocene succession in the western margin of Cyrenaica Platform. The availability of a good seismic and well data provided the opportunity for testing seismic and sequence stratigraphy techniques in the area. This area, geologically, lies on a hinge-line between the subsiding Sirte Basin to the west and the stable Cyrenaica Platform to the east. The immediate objective of this research is to better understand the stratigraphic and facies variation within the Palaeocene sequence in this area.
 By the end of the Palaeocene the major tectonic rifting event which formed the Sirte Basin had almost ceased. The subsequent history of the basin was represented the post-rift depositional phase of basin development. The carbonate platform on the western margin of the Cyrenaica Platform contains a record of composite relations between the factors that controlled carbonate platform deposition during the Tertiary. Evolution of this carbonate margin is an excellent example since it clearly shows how the history of this type of margin is dominated by interaction between regional effects, and governed by thermal subsidence consequent upon extension. The shelf edge is the most sediment-starved part of the margin, and given suitable climatic conditions, carbonate deposition can occur at rates equal to subsidence thus maintaining and building the platform. The most important facies and stratigraphic relations of Palaeocene sequence are analysed and discussed in terms of platform development and the principal factors that influenced patterns of sedimentation and the production of carbonate. Furthermore, the sequence stratigraphic and seismic facies study of this interval explain the Paleocene geological history of the Sirte Basin, an essential step in any Hydrocarbon exploration program

Long-term triggered seismicity on the Mosha fault by Damavand volcano, Iran: Implications on the seismic hazard of Tehran metropolis

The seismic history of the Mosha fault, the largest and most active fault of Eastern Tehran metropolis, and its relation to the Damavand active volcano, the highest mountain in the Middle East, is investigated. We deduce that the central Mosha, near the Damavand, has a higher seismicity than either its western or eastern segments. On 7 May 2020, an Mw 5.1 earthquake occurred on the central Mosha, about 40 km east of Tehran and 10 km southwest of the Damavand crest, and it was felt intensely in Tehran. Its rupture is imaged and located in a region that presented a relative seismic quiescence compared to its eastern and western parts, during the last 14 years, suggesting its partial locking and heterogeneous distribution of fault frictional strength on this segment of Mosha. Its significant directivity to the west is confirmed by the mainshock rupture model, its PGA distribution, and distribution of early aftershocks. The rupture model suggests a relatively small stress drop of 2.6 bar, which is consistent with the comparatively high rupture dimension of 9 km for a Mw 5.1 earthquake, and indicates the easy rupture expansion on the central Mosha near the Damavand Volcano. The central Mosha experienced earthquakes in 1930, 1955, and 1983, as well as high microseismic activity and the 2020 seismic sequence, all of which strongly point to a possible influence of the Damavand Volcano on the seismicity of the central Mosha. This is corroborated by the observation of hydrothermal zones on the Mosha fault and the extension of a sill-like Damavand young magma chamber until central Mosha in tomography studies. We propose that the existing heat may increase the pore pressure on the fault, which lowers the effective normal stress, facilitates the nucleation-expansion of the rupture, and unclamps the fault. Damavand could act as a fuse and nucleate earthquakes, and if the rupture extends toward the west, it could have a significant directivity effect on low-frequency seismic waves that reach Tehran without attenuation and affect tall structures. In addition, high site amplification for frequencies up to 16 Hz due to the deep sedimentary basin, mainly in the mid-city of Tehran, will be remarkable for short buildings.

The Slender Rigid Block: Archetype for the Seismic Analysis of Masonry Structures

ABSTRACT The elasto-plastic oscillator, representing the basic model for reinforced concrete and steel structures, shows its inadequacy when dealing with masonry structures. The column, resting on a rigid plane with friction, is shown to be particularly appropriate to model the dynamic behaviour of masonry structures. At this aim, this paper investigates the motion of a slender rigid block (SRB), initially resting in its vertical configuration and hit by a seismic input, represented through a single impulse constituted by a rectangular or sinusoidal half-wave or a simplified sequence of horizontal acceleration pulses, having rectangular or sinusoidal shapes. An accumulation of the rotation of the SRB exists under the action of sequences of opposite acceleration pulses. The dynamic collapse is shown to take place when the column is led to its critical configuration – where the restoring moment of the weight is lost – so that the last pulse can further increase the rotation of the column. Expressing the behaviour of the SRB by means of the force reduction factor, the ratio between the magnitude of the collapse horizontal acceleration and the static strength of the column, the difference between the responses of the column decreases with increasing the number of the pulses of the sequence. A simple definition of the asymptotic failure domain can be obtained with a good approximation with a limited number of pulses. It is possible to evaluate the minimum collapse acceleration intensity of seismic sequences of pulses of any assigned duration able to produce the collapse of the rigid block, representing the seismic strength of the SRB. Finally, the results obtained in the theoretical analysis are compared with the outcomes of more accurate and time-consuming non-linear dynamic or incremental analyses (IDA).

Crustal permeability changes inferred from seismic attenuation: Impacts on multi-mainshock sequences

We use amplitude ratios from narrowband-filtered earthquake seismograms to measure variations of seismic attenuation over time, providing unique insights into the dynamic state of stress in the Earth’s crust at depth. Our dataset from earthquakes of the 2016–2017 Central Apennines sequence allows us to obtain high-resolution time histories of seismic attenuation (frequency band: 0.5–30 Hz) characterized by strong earthquake dilatation-induced fluctuations at seismogenic depths, caused by the cumulative elastic stress drop after the sequence, as well as damage-induced ones at shallow depths caused by energetic surface waves. Cumulative stress drop causes negative dilatation, reduced permeability, and seismic attenuation, whereas strong-motion surface waves produce an increase in crack density, and so in permeability and seismic attenuation. In the aftermath of the main shocks of the sequence, we show that theM≥ 3.5 earthquake occurrence vs. time and distance is consistent with fluid diffusion: diffusion signatures are associated with changes in seismic attenuation during the first days of the Amatrice, Visso-Norcia, and Capitignano sub-sequences. We hypothesize that coseismic permeability changes create fluid diffusion pathways that are at least partly responsible for triggering multi-mainshock seismic sequences. Here we show that anelastic seismic attenuation fluctuates coherently with our hypothesis.

Effects of reconsolidation degree on reliquefaction resistance under mainshock–aftershock sequences: A DEM investigation

AbstractAftershock events may immediately follow mainshock events. In liquefiable deposits, the excess pore pressure caused by mainshock events may not dissipate completely in a short time interval between the mainshock and the aftershock. Sandy soil with different reconsolidation degrees (Ur) may present different reliquefaction resistances during the aftershock, which was not fully considered into the previous studies of reliquefaction. In this study, discrete element method (DEM) was employed to simulate a series of cyclic triaxial tests to evaluate the effects of Ur on reliquefaction resistance under mainshock–aftershock seismic sequence. The initially liquefied specimen was reconsolidated from two states with small and large residual shear strain respectively. Reconsolidated specimens reliquefied under different cyclic stress ratios (CSRs). Simulation results show that reliquefaction resistance increased gradually with increasing Ur, which was closely related to the previous residual shear strain during the first liquefaction event. Specimens that having large residual shear strain reliquefied easily, and the reliquefaction resistance of those specimens increased slightly with increasing Ur. On the contrary, reliquefaction resistance of the specimens that having small residual shear stain increased obviously with increasing Ur. The ratio of initial mechanical average coordination number to initial mesoscopic fabric anisotropy of reconsolidated specimens had a good correlation to the contraction potential and increased gradually with increasing Ur.

The Role of Faults as Barriers in Confined Seismic Sequences: 2021 Seismicity in the Granada Basin (Betic Cordillera)

AbstractFault barriers are key structures for studying seismic hazard in regions of intense brittle deformation. The interaction between fault sets affects their seismogenic behavior, if some of them act as barriers. The Granada Basin, in the Betic Cordillera, is a region affected by shallow brittle deformation, as it was the scenario for the recent Granada 2021 seismic sequence. This seismicity presented a swarm behavior at the beginning of the sequence, followed by mainshock‐aftershock features. Geological and gravity data presented here reveal that the basement is affected by two sets of NW‐SE and NE‐SW normal faults and intensely deformed by vertical NW‐SE joints. Improved relocation of the Granada 2021 seismicity reveals a confined chimney‐shape seismicity caused by the activity of a 2 km long NW‐SE normal fault segment. The confinement of the sequence is associated with the NE‐SW fault set acting as a barrier that restricts the rupture area, limiting the maximum magnitude, and favoring the recurrence of events with smaller magnitude. The chimney‐shape of the seismic sequence suggests that the deformation is propagated vertically to the surface, facilitated by preexisting fractures. The shallow extensional deformation during the uplift of the central Betic Cordillera drove the activity of the local structures obliquely to the regional extensional trends, as evidenced by the seismic sequence. This multidisciplinary study improves the knowledge on the origin of the Granada Basin and underlies the important role of preexisting fractures on fault segmentation and seismic propagation, decreasing the seismic potential of this area.

An Atypical Shallow Mw 5.3, 2021 Earthquake in the Western Corinth Rift (Greece)

AbstractModerate‐to‐large earthquakes in rifts may occur on leading boundary faults or inner antithetic faults. Here we show a rare case of the 2020–2021 seismic sequence in the Corinth rift, that culminated in the shallow rupture of the antithetic fault, neither preceded nor followed by the leading fault rupture. The hypocenter of the largest shock (Mw 5.3 of 17 February 2021) was located at ∼8 km depth. However, seismic waveform data, supported by satellite‐geodetic and tide gauge measurements, pointed to rupture at shallow depth (∼3 km), where no earthquakes were previously observed. We show that the earthquake most probably ruptured two orthogonal, conjugate fault segments: a weak nucleation phase occurred in the microseismically highly active sub‐horizontal detachment layer, followed – a few seconds later – by a larger, shallow moment release on a high‐angle, south‐dipping normal fault. The latter is the Mornos offshore fault, antithetic to the leading, north‐dipping Psathopyrgos fault. Our study presents the first instrumental/observational evidence of a very shallow Mw 5+ event in this rift – and one of the few reported worldwide. The depth limit of the main shallow slip patch coincides with the expected crossing of the Mornos fault with the Psathopyrgos fault, stressing the importance of fault segmentation and rooting inherited from the rift history. This unusual shallow slip in a depth range with little background seismicity and few aftershocks needs to be further investigated by dynamic modeling as a possible prototype of hazardous events in rift environments.

Investigation of the Factors Controlling the Duration and Productivity of Aftershocks Following Strong Earthquakes in Greece

Strong crustal earthquakes in Greece are typically followed by aftershocks, the properties of which are important factors in seismic hazard assessment. In order to examine the properties of earthquake sequences, we prepared an earthquake catalog comprising aftershock sequences with mainshocks of Mw ≥ 5.5 from 1995 to 2021. Regional aftershock parameters were estimated to highlight variations in aftershock decay and productivity among regions with similar seismotectonic characteristics. A statistically based method of estimating aftershock duration and a metric of relative aftershock productivity to examine the variations among the different cases were employed. From the detailed analysis of the selected seismic sequences, we attempt to unravel the physical mechanisms behind deviations in aftershock duration and productivity and resolve the relative contribution of background seismicity, the Omori–Utsu law parameters and the mainshock faulting properties. From our analysis, the duration of aftershock sequences depends upon the rupture process of the mainshock, independently of its magnitude. The same applies to aftershock productivity, however, other tectonic setting (e.g., seismic coupling) or source-related (e.g., focal depth, stress drop) parameters also contribute. The estimated regional parameters of the aftershock rate models could be utilized as initial ones to forecast the aftershock occurrence rates at the early stage following a mainshock.

Fragility and vulnerability analysis of deteriorating ordinary bridges using simulated ground‐motion sequences

AbstractHighway bridges are critical components of the lifeline transport infrastructure in urban areas. They are designed with the expectation of not sustaining significant structural/non‐structural damage after major earthquake‐induced ground shaking. However, in the current structural performance‐based assessment practice, the effects of a pre‐damaged state during ground‐motion sequences are often neglected. Additionally, environmentally induced deterioration mechanisms (e.g., steel rebar corrosion) may exacerbate the consequences of such ground‐motion sequences on the seismic structural performance during the bridge design lifetime; yet such effects are commonly overlooked. This study proposes a computational methodology to derive state‐dependent fragility and vulnerability relationships (i.e., explicitly depending on the damage state achieved by the bridge structure during a first shock) for bridge structures subjected to chloride‐induced corrosion deterioration and ground‐motion sequences. The methodology is demonstrated for a case‐study ordinary bridge structure (representing a typical bridge vulnerability class in southern California) under seismic sequences assembled from the CyberShake 15.12 (hybrid) simulated ground‐motion database. In the proposed approach, parameterised (i.e., dependent on the corrosion deterioration level) vector‐valued probabilistic seismic demand models are developed for the bridge components (i.e., columns and shear keys). These models, calibrated through sequential cloud‐based nonlinear time‐history analyses, relate the dissipated hysteretic energy in the ground‐motion sequence to a deformation‐based engineering demand parameter induced by the first shock and a ground‐motion intensity measure of the second shock for a given corrosion deterioration level. Fragility relationships are first derived for a single ground motion at the component‐level; state‐dependent fragility relationships are then derived by considering the additional damage induced by a second ground motion within the simulated sequence (structure‐specific damage states are considered). Furthermore, a component‐based simulation approach accounting for the correlations across the components’ response is utilised to derive fragility relationships at the system level (i.e., for the bridge structure). Finally, appropriate damage‐to‐loss models for ordinary bridges are used to derive state‐dependent vulnerability relationships. The results demonstrate the significant impact of earthquake‐induced ground‐motion sequences and environmentally‐induced corrosion deterioration, emphasising the necessity of accounting for this multi‐hazard threat in the structural performance‐based assessment of bridges. Relative variations up to 120% can be found in the system‐level state‐dependent fragility median values comparing the results for the bridge in pristine and deteriorated conditions.