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The research on the liquefaction and reliquefaction trend of sand under the action of earthquake sequence is of great significance for the evaluation of seismic and geological disasters at engineering sites. Through the shaking table test, the changes in site conditions after multiple liquefaction under the action of a simulated earthquake sequence are analyzed. Based on the data collected and recorded by various sensors during the test, combined with the light dynamic penetration test and the geotechnical test data before and after liquefaction, the variation laws of pore water pressure, earth pressure, acceleration, soil moisture content, and relative compactness of soil in the process of site model soil liquefaction are obtained. The results show that under the action in simulated earthquake sequence, with the increase of vibration times, the sand compactness at the same depth increases significantly, the increment decreases gradually, and the pore water pressure increment decreases gradually. However, the sand compactness and pore water pressure increment at different depths gradually decreased from deep to shallow, resulting in the liquefaction potential of deep sand being significantly lower than that of shallow sand. [ABSTRACT FROM AUTHOR]

On June 10, 2022, the MS6.0 Ma'erkang earthquake sequence occurred in the Bayan Har block. In this paper, the temporal and spatial distribution and attenuation characteristics of earthquake sequence is analyzed based on the regional structure, the temporal and spatial distribution of earthquake sequence, the focal mechanism solution, and the parameters of earthquake sequence, using the data of Sichuan Seismic Network and the temporary stations incorporated into the network. The results show that: (1) The MS6.0 Ma'erkang earthquake sequence is generally distributed in NW-SE direction, and the long axis of this series is basically consistent with the nearby Songgang fault. (2) As the earthquake sequence is formed by three large events with MS ≥5.0, the frequency of small earthquakes in the earthquake sequence area generally attenuates relatively slowly, while the activity level (magnitude) of aftershocks attenuates rapidly. (3) After the MS5.2 earthquake, the sequence parameters obtained show that the h-value is 1.09 and the p-value is stable at 1.02, indicating that the intensity and frequency attenuation of the earthquake sequence are gradually stable and tend to be normal. The b-value is 0.95, indicating that the maximum aftershock magnitude of the series is estimated to be ML5.1, and the b-value gradually tended to be stable, indicating that the stress in the region gradually tended to be balanced after the MS5.2 earthquake. The MS4.4 (ML5.0) earthquake that occurred at 4:37 on June 10 (local time) is the largest aftershock after the MS5.2 earthquake in the sequence. (4) The Songgang fault with NW-SE trend is presumed to be the main seismogenic tectonics, but the migration of three earthquakes with MS≥5.0 may also indicate that the Songgang fault is not a single seismogenic tectonics, which requires further field scientific investigation and analysis.

Mt. Ruapehu is an active andesitic stratovolcano, consisting of several peaks with the summit plateau at 2,797 m, making it the tallest active volcano in New Zealand. The extent of the volcano spreads 40 km across with a series of complex faults encompassing almost the entire base of the volcano. A series of earthquakes occurring 20 km west of the summit of Mt. Ruapehu, near the small town of Erua, which preceded the 1995/1996 major volcanic eruption sequence has been proposed as a medium-term precursor for eruptions at Mt. Ruapehu. We use unsupervised machine learning clustering algorithms HDBSCAN and DBSCAN to define anomalous earthquake swarms in the region and determine whether the Erua swarm was unique by identifying key characteristics in space, time and magnitude distribution. HDBSCAN found six spatial cluster zones to the west of Mt. Ruapehu, which have temporal seismic bursts of activity between 1994 and 2023. DBSCAN identified the seismic swarm that preceded the 1995/1996 major eruption, along with one other similar cluster in the same region, which did not coincide with any documented magmatic unrest, suggesting distal seismic swarms at Mt. Ruapehu may not serve as a reliable eruption precursor when observed in isolation. We instead found that earthquake swarms are relatively common at Mt. Ruapehu and the temporal evolution of the earthquake clusters west of Mt. Ruapehu share similar characteristics to seismic swarms identified in other settings related to fluid migration, typical of fault-valve models. [ABSTRACT FROM AUTHOR]

Significant seismic activity has been witnessed in the area of Ridgecrest (Southern California) over the past 40 years, with the largest being the Mw 5.8 event on 20 September 1995. In July 2019, a strong earthquake of Mw 7.1, preceded by a Mw 6.4 foreshock, impacted Ridgecrest. The mainshock triggered thousands of aftershocks that were thoroughly documented along the activated faults. In this study, we analyzed the spatiotemporal variations of the frequency–magnitude distribution in the area of Ridgecrest using the fragment–asperity model derived within the framework of non-extensive statistical physics (NESP), which is well-suited for investigating complex dynamic systems with scale-invariant properties, multi-fractality, and long-range interactions. Analysis was performed for the entire duration, as well as within various time windows during 1981–2022, in order to estimate the qM parameter and to investigate how these variations are related to the dynamic evolution of seismic activity. In addition, we analyzed the spatiotemporal qM value distributions along the activated fault zone during 1981–2019 and during each month after the occurrence of the Mw 7.1 Ridgecrest earthquake. The results indicate a significant increase in the qM parameter when large-magnitude earthquakes occur, suggesting the system's transition in an out-of-equilibrium phase and its preparation for seismic energy release. [ABSTRACT FROM AUTHOR]

The prompt identification of faults responsible for moderate‐to‐large earthquakes is fundamental for understanding the likelihood of further, potentially damaging events. This is increasingly challenging when the activated fault is an offshore buried thrust, where neither coseismic surface ruptures nor GPS/InSAR deformation data are available after an earthquake. We show that on 9 November 2022, an Mw 5.5 earthquake offshore Pesaro ruptured a portion of the buried Northern Apennines thrust front (the Cornelia thrust system [CTS]). By post‐processing and interpreting the seismic reflection profiles crossing this thrust system, we determined that the activated fault (CTS) is an arcuate 30‐km‐long, NW‐SE striking, SW dipping thrust and that older structures at its footwall possibly influenced its position and geometry. The activation of adjacent segments of the thrust system is a plausible scenario that deserves to be further investigated to understand the full earthquake potential of this offshore seismogenic source. Plain Language Summary: The Northern Apennines chain is characterized by thrust faults running from the Po Plain to the Adriatic Sea on the northeastern side of peninsular Italy. These thrusts are buried below ≈2,000 m cover of Plio‐Pleistocene deposits. Controversies arose about these thrust faults' activity and earthquake potential based on their hidden geological signature and the scanty seismicity that could be associated with them. The earthquake (magnitude 5.5) that occurred on 9 November 2022, offshore Pesaro revived this argument. In this work, we analyze the geological structure of the crustal volume affected by the seismic sequence, exploiting seismic reflection profiles and well‐log data to identify the earthquake causative fault. Our results demonstrate that the earthquake ruptured a well‐known fault of the Northern Apennines' buried thrust front, supporting that it is indeed active and seismogenic. The size and architecture of this thrust front suggest that it could generate even larger earthquakes (Mw > 6.5). This type of geological study is instrumental to understanding the geometry of earthquake faults, particularly in offshore areas, because they constitute reliable inputs for earthquake hazard models and, when done promptly after an earthquake, provide key elements for other studies on the seismic source and the unfolding of the ongoing seismic sequence. Key Points: 9 November 2022, earthquake consistent with activity of the Cornelia thrust, a fault system running off the central Adriatic coastThe seismic reflection profiles in the area allowed for delineating the thrust and its earthquake potential with a much finer resolutionThe properties of the causative fault suggest that the activation of adjacent segments is a plausible scenario that deserves consideration [ABSTRACT FROM AUTHOR]

Despite the fact that earthquake occurrence can be strongly influenced by the architecture of pre‐existing faults, it remains challenging to obtain information about the detailed subsurface geometries of active fault systems. Current geophysical methods for studying such systems often fail to resolve geometrical complexities at sufficiently high spatial resolutions. In this work, we present a novel method for imaging the detailed 3D architectures of seismically active faults based on high‐precision hypocenter catalogs, using nearest neighbor learning and principal component analysis. The proposed approach enables to assess variations in fault instabilities and kinematics. We apply the method to the relatively relocated St. Léonard (max. ML = 3.2) and Anzère (max. ML = 3.3) microearthquake sequences in the Southwestern Swiss Alps, revealing strike‐slip fault systems with interconnecting stepovers at depths of 3–7 km and lengths ranging from 0.5 to 2 km. In combination with additional information about fault instabilities and kinematics, we observe significantly reduced earthquake migration velocities and fault locking processes within the stepovers. Understanding such processes and their role in the propagation of strain across stepovers is of great relevance, as these structures can potentially limit earthquake ruptures but also represent possible locations for the nucleation of larger ruptures. Our proposed method is expected to be broadly useful for further applications such as monitoring hydraulic fracture stimulations or geothermal exploration of natural, fluid‐bearing faults. Conducting similar high‐resolution spatiotemporal analyses of microseismic sequences has the potential to greatly enhance our comprehension of how the 3D fault architecture impacts seismogenic fault reactivation. Plain Language Summary: Earthquakes commonly occur on planar geological structures, called faults. Profound knowledge of the presence and geometries of earthquake‐generating faults is crucial to understand the regional earthquake hazards. Especially for regions with distributed and rather small earthquakes, it is however challenging to detect such faults, because of the fact that these faults often do not reach the surface. Here, we present a new method that uses the locations of small earthquakes to resolve the geometries of earthquake‐generating faults at depth. This is only possible due to the recent advances in earthquake location methods, which can reduce the uncertainties of the locations from the kilometers down to some tens of meters for precise measurements. With our method, we image the complex geometries of two fault systems in the Southwestern Swiss Alps at high resolution. The knowledge obtained by applying our method cannot only help to detect previously unknown, potentially hazardous earthquake‐generating faults but might also improve our understanding of earthquake processes in general. Key Points: We present a novel approach that uses relocated hypocenters to image 3D geometries and instabilities of active faultsApplication to two earthquake sequences in the Southwestern Swiss Alps provides detailed insights into these strike‐slip faults systemsThis study documents the dynamics of earthquake‐migration processes across stepover faults at high resolution [ABSTRACT FROM AUTHOR]

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

Abstract The prompt identification of faults responsible for moderate‐to‐large earthquakes is fundamental for understanding the likelihood of further, potentially damaging events. This is increasingly challenging when the activated fault is an offshore buried thrust, where neither coseismic surface ruptures nor GPS/InSAR deformation data are available after an earthquake. We show that on 9 November 2022, an Mw 5.5 earthquake offshore Pesaro ruptured a portion of the buried Northern Apennines thrust front (the Cornelia thrust system [CTS]). By post‐processing and interpreting the seismic reflection profiles crossing this thrust system, we determined that the activated fault (CTS) is an arcuate 30‐km‐long, NW‐SE striking, SW dipping thrust and that older structures at its footwall possibly influenced its position and geometry. The activation of adjacent segments of the thrust system is a plausible scenario that deserves to be further investigated to understand the full earthquake potential of this offshore seismogenic source.

On 21 May 2021, the MS 6.4 earthquake struck Yangbi County, Dali City, Yunnan Province. Minor seismic activities were common both before and after the earthquake; the foreshock sequence activity characteristics of this earthquake were studied to gain a better understanding and more perspective. First, between May 18 and 5 July 2021, we collected data of the seismic events from the Yangbi MS 6.4 earthquake sequence, determined the minimum magnitude of completeness of the sequence based on the magnitude-frequency relationship, filtered out the more complete earthquake sequences, and performed double-difference earthquake relocation using the HypoDD method. Then, we improved the nearest-neighbor distance algorithm and used it in conjunction with the Gaussian mixed model fitting method to conduct a comprehensive multiple factor analysis of the Yangbi MS 6.4 earthquake sequence. The main findings of our preliminary analysis are as follows: 1) We distinguished the foreshocks, mainshocks, and aftershocks of the Yangbi earthquake sequence as well as their higher-order aftershocks, using the method described in this paper, and we can obtain clear intergenerational relationships between them. We identified eight “foreshocks” with statistically greater physical significance than the others and found that majority of the shocks occurred shortly after the mainshock were direct aftershocks, with secondary or higher-order aftershocks gradually increasing in the later stage of the sequence. 2) Combining the double-difference earthquake relocation and the event distribution on the fault plane, we found that the b-value of the Yangbi foreshock sequence was clearly on the lower end of the spectrum, with an obvious nonlinear amplification process, and can be divided into three foreshock sub-sequences: sub-sequence I with the lowest b-value and a concentrated spatial distribution, which is an F-value foreshock sequence; subsequence II with an overall shallower source depth and an obvious rebound in b-value, which is an explosive aftershock sequence; subsequence III with a lower b-value and a rapid rupture spread, which is a typical U-F-ρ foreshock. 3) We constructed a topological tree of the Yangbi foreshock sequence in Yunnan Province using the nearest-neighbor distance algorithm, combining the correlation between earthquake sequence type and fault rupture intensity. We analyzed the distribution and topology of the three sub-sequences (combined) of the Yangbi MS 6.4 foreshock sequence and found the following: the topology of subsequence I was more linear than the sequence II, each was the parent event of the subsequent event, and its rupture mode was similar to the fluid intrusion rupture in a specific channel; sequence II had a relatively simple topology, exhibiting a spray topology, and independently formed a main-aftershock mini-sequence, probably triggered by its shallow brittle rupture, which caused the fluid intrusion rupture to spread faster and eventually triggered the Yangbi MS6.4 mainshock.

Significant seismic activity has been witnessed in the area of Ridgecrest (Southern California) over the past 40 years, with the largest being the Mw 5.8 event on 20 September 1995. In July 2019, a strong earthquake of Mw 7.1, preceded by a Mw 6.4 foreshock, impacted Ridgecrest. The mainshock triggered thousands of aftershocks that were thoroughly documented along the activated faults. In this study, we analyzed the spatiotemporal variations of the frequency–magnitude distribution in the area of Ridgecrest using the fragment–asperity model derived within the framework of non-extensive statistical physics (NESP), which is well-suited for investigating complex dynamic systems with scale-invariant properties, multi-fractality, and long-range interactions. Analysis was performed for the entire duration, as well as within various time windows during 1981–2022, in order to estimate the qM parameter and to investigate how these variations are related to the dynamic evolution of seismic activity. In addition, we analyzed the spatiotemporal qM value distributions along the activated fault zone during 1981–2019 and during each month after the occurrence of the Mw 7.1 Ridgecrest earthquake. The results indicate a significant increase in the qM parameter when large-magnitude earthquakes occur, suggesting the system’s transition in an out-of-equilibrium phase and its preparation for seismic energy release.

Abstract Afterslip inside a coseismic slip patch is rarely observed, though some previous studies suggest that it is driven by poroelastic rebound (PER). These studies assume constant frictional strength, whereas time-dependent strengthening (healing) of a fault is expected from laboratory experiments, which provide a basis for a rate- and state-dependent friction law (RSF). In this study, quasistatic poroelasticity (PE) was implemented in a dynamic earthquake sequence simulation using a spectral boundary integral equation method, and the effect of PER on the behavior of a fault governed by RSF was examined. Spatio-temporal convolution for PE would significantly affect the resolution of the numerical simulation affordable. This problem has been resolved by numerical approximation of the time dependency of Green’s function of PE in the wavenumber domain, definition of memory variables, and reformulation of the temporal convolution into ordinary differential equations of them. In the novel method, the additional numerical costs due to PE are negligible. A planar fault with a rate-weakening patch embedded in the rate-strengthening region was simulated. Because it is the healing of the fault that competes against PER, both the aging law and slip law were examined, which have different characteristics in the evolution of the fault strength. The simulation results indicate that PER causes postseismic loading to the patch, but the healing efficiently suppresses afterslip not only for the aging law, but also for the slip law. When cases with different friction laws are compared, the healing is more significant for the aging law, which has log- $$t$$ t strengthening at a limit of $$V\to 0$$ V → 0 . However, the effect of PER on the slip rate is minor for the slip law. The slip law yields additional healing if the fault is accelerated by loading owing to PER. The simulation results are consistent with the absence of afterslip within the coseismic slip patches in the observations. Graphical Abstract

The 2008 Mw 7.9 Wenchuan earthquake is the most disastrous seismic event in China since 1976. Both field and seismological investigations suggest a multi‐stage coseismic rupture with most damage associated with the Yingxiu‐Beichuan Fault (YBF) of spatially variable fault strike and dip angles. To investigate the effect of fault geometric complexity on coseismic rupture and paleoseismic pattern on the YBF, we perform earthquake sequence modeling on 3D fault geometry in the framework of rate‐and‐state friction law. Our model produces a long‐term earthquake sequence with quasi‐regular recurrences of whole‐fault ruptures and segmented ruptures. The along‐strike rupture segmentation, earthquake propagation speed, and slip rate are mainly controlled by along‐strike variations of fault dip and strike angles. Particularly, the YBF can be divided into two segments: the southern segment featured by large events (Mw > 8.0) and the northern segment with smaller events (Mw < 7.5), with recurrence intervals primarily determined by the tectonic loading rate. In a whole‐rupture event, the rupture speed mainly correlates with the fault dip angle; a smaller dip angle results in a wider seismogenic zone and higher rupture speeds, whereas the small‐scale variation in rupture speed is regulated by the fault strike angle. The effect of strike variation on the total coseismic slip amount is more pronounced due to the large strike angle gradient along the YBF. After varying the slip vector to reflect the northward transition from the dominant thrust‐slip to strike‐slip faulting during the Wenchuan coseismic rupture, we obtain reasonably good agreement of model simulated coseismic surface displacements with GPS observations. Plain Language Summary: The Yingxiu‐Beichuan Fault (YBF) hosting the 2008 Mw 7.9 Wenchuan earthquake has a complex nonplanar interface. Seismic studies after the Wenchuan earthquake have unveiled a complex rupture process. Field trenching studies find a complex recurrence interval and rupture pattern of paleoseismic events. Here, we conduct a numerical simulation of earthquake sequences on the 3D YBF to study the effect of fault geometry on the coseismic rupture and paleoseismic behaviors. Our modeling results are quantitatively comparable with most field observations. Both whole‐fault ruptures and partial rupture events are produced. The larger events (magnitude >8.0) initiate near the southern end of the YBF. The smaller events (magnitude <7.5) initiates near the northern end of the YBF. Their repeating time is mainly controlled by the background tectonic loading rate. In the partial rupture events, coseismic rupture stops near Nanba area when encountering a geometrical barrier due to a sharp change of fault dip. While the lateral rupture propagation speed is controlled by variation of both fault dip and strike angles, coseismic slip distribution is more controlled by the variation of fault strike (lateral fault orientation) than dip. We find that the orientation of slip vector is important in coseismic rupture modeling of Wenchuan‐type event. Key Points: A long‐term earthquake sequence with quasi‐regular recurrences is simulated, including whole‐fault and segmented rupturesFault dip angle and hence the seismogenic zone width has first‐order control on the along‐strike rupture segmentationFault strike angle controls small‐scale variations in rupture propagation speed and slip rate, which affect the cumulative coseismic slip [ABSTRACT FROM AUTHOR]

Ensuring the safety and functionality of petrochemical plants after a strong earthquake is critical, particularly due to the presence of hazardous chemical materials stored in pressurized vessels and reaction tanks that can be flammable or toxic. Strong earthquakes in chemical units can cause severe structural damage, leading to adverse consequences like human injuries and economic losses. In harsh atmospheric environments, existing steel structures that have not been maintained adequately throughout their lifetime become even more vulnerable to ground motions due to corrosion-induced capacity loss. This study focuses on the vulnerability assessment of an existing aging petrochemical plant located in a high-corrosive atmospheric condition, used as a case study. The risk assessment employs probabilistic approaches to estimate the damage level and associated repair costs at different intensities of ground motion, corresponding to each state of damage (loss). Moreover, the study investigates the effect of non-structural components and their interaction with the primary structure by employing advanced finite element modelling of reaction tanks. The results indicate that aftershock motions can slightly alter the performance of the structure, whereas the effect of corrosion is more significant. Particularly, for the corroded structure, the seismic risk is significantly affected by the demand imposed by aftershocks. These results underscore the importance of comprehensive risk assessments to ensure the safety and integrity of existing petrochemical plants with aging effects and located in seismic zones. • Aftershock motions have a slight impact on altering the probability of structural damage and losses in petrochemical plants. • The assessment of seismic risk is more prominently influenced by the considerable effect of corrosion loss, as compared to the impact of aftershocks. • In the presence of corrosion, the susceptibility of structures to damage induced by aftershocks is worsened. • The dynamic response of the structure can be influenced by non-structural components like steel tanks. [ABSTRACT FROM AUTHOR]

This investigation examines the contemporary documentation of a sequence of low-magnitude earthquakes at the fringes of the Kingdom of Sweden, today Southeastern Finland, in 1751–1752. A total of 11 pages of original correspondence sent from the target village of Svenskby to the Swedish capital Stockholm are reviewed. Newspaper accounts from Sweden and Russia are included in the analysis, and a timeline of the reporting is constructed. A newly created catalog shows over 30 distinct events between the end of October and December 1751 (Julian calendar). The assignment of macroseismic intensity to the earthquakes is hampered by loud acoustic effects that accompany and/or constitute the observations. Maximum intensities are assessed at IV–V (European Macroseismic Scale 1998), and maximum macroseismic magnitudes in the range of MM1.9–2.4, and were probably observed at short epicentral distances close to the ground surface. Comparisons to macroseismic data related to instrumentally recorded earthquakes in the region support the notion of low magnitudes. The data from 1751 provide an analog to modern macroseismic observations from geothermal stimulation experiments. Such experiments have acted as a spur for considering seismic risk from low-magnitude earthquakes whose consequences have seldom previously been a matter for concern. [ABSTRACT FROM AUTHOR]

High‐resolution computer simulations of earthquake sequences in three or even two dimensions pose great demands on time and energy, making lower‐cost simplifications a competitive alternative. We systematically study the advantages and limitations of simplifications that eliminate spatial dimensions in quasi‐dynamic earthquake sequence models, from 3D models with a 2D fault plane down to 0D or 1D models with a 0D fault point. We demonstrate that, when 2D or 3D models produce quasi‐periodic characteristic earthquakes, their behavior is qualitatively similar to lower‐dimension models. Certain coseismic characteristics like stress drop and fracture energy are largely controlled by frictional parameters and are thus largely comparable. However, other observations are quantitatively clearly affected by dimension reduction. We find corresponding increases in recurrence interval, coseismic slip, peak slip velocity, and rupture speed. These changes are to a large extent explained by the elimination of velocity‐strengthening patches that transmit tectonic loading onto the velocity‐weakening fault patch, thereby reducing the interseismic stress rate and enhancing the slip deficit. This explanation is supported by a concise theoretical framework, which explains some of these findings quantitatively and effectively estimates recurrence interval and slip. Through accounting for an equivalent stressing rate at the nucleation size h* into 2D and 3D models, 0D or 1D models can also effectively simulate these earthquake cycle parameters. Given the computational efficiency of lower‐dimensional models that run more than a million times faster, this paper aims to provide qualitative and quantitative guidance on economical model design and interpretation of modeling studies. Plain Language Summary: Computer simulations are a powerful tool to understand earthquakes and they are often simplified to save time and energy. Dimension reduction—using 1D or 2D models instead of 3D models—is a commonly used simplification, but its consequences are not systematically studied. Here we find that both the overall earthquake recurrence pattern and the magnitude of stress changes on the fault caused by earthquakes remain relatively unchanged by model simplification by dimension reduction. However, some key observations such as the total slip and rupture speed achieved during an earthquake, as well as the precise recurrence interval are larger in lower‐dimensional models. These changes are related to the elimination of lateral creeping regions that transmit stress onto the fault, which is an unavoidable consequence of the elimination of a physical dimension. We use simple theoretical calculations to reproduce these observations and justify this causal relationship. As simplified models are still popular due to their computational efficiency, this contribution helps their users and developers to understand and anticipate the potential discrepancies of their results with respect to the three‐dimensional situation that exists in nature. Therefore users can design their models and interpret their results with this work as a guideline. Key Points: Models with dimension reduction simulate qualitatively similar quasi‐periodic earthquake sequences with quantitative differencesReduced influence of velocity‐strengthening patches due to dimension reduction increases recurrence interval, slip and rupture speedWe provide guidelines on how to interpret lower‐dimensional modeling results of interseismic loading and earthquake ruptures [ABSTRACT FROM AUTHOR]

An Ms 6.4 earthquake occurred in Yangbi, Yunnan, China on May 21, 2021, which has obvious foreshock activity and abundant aftershocks. Based on the seismic observation data recorded by the Yunnan Seismic Network three days before and seven days after the mainshock, a double-difference location method was used to relocate 2 133 earthquakes of the Yangbi sequence. Aftershocks are mostly distributed to the southeast of the mainshock in a unilateral rupture pattern. This sequence exhibits a SE-trending linear alignment with a length of about 25 km, and most of the focal depth is above 12 km. Integrated with the seismic distribution and focal mechanism results, we infer that the strike of the seismogenic fault is about 140°, and dipping to the SW. The fault structure revealed by the seismic sequence is complex, with the NW segment exhibiting a steep dip and relatively simple structure of strike-slip rupture and the SE segment consisting of several branching ruptures. The Yangbi Earthquake is a typical foreshock-mainshock-aftershock sequence, and the mainshock is likely triggered by the largest foreshock. This earthquake occurred in the boundary between high- and low-velocity anomalous zone, where is susceptible to generate large earthquakes. [ABSTRACT FROM AUTHOR]

Abstract Dynamic earthquake sequence simulation is an important tool for investigating the behavior of a fault that hosts a series of earthquakes because it solves all interrelated stages in the earthquake cycle consistently, including nucleation, propagation and arrest of dynamic rupture, afterslip, locking, and interseismic stress accumulation. Numerically simulating and resolving these phenomena, which have different time and length scales, in a single framework is challenging. A spectral boundary integral equation method (SBIEM) that makes use of a fast Fourier transform is widely used because it reduces required computational costs, even though it can only be used for a planar fault. The conventional SBIEM has a periodic boundary condition as a result of the discretization of the wavenumber domain with a regular mesh; thus, to obtain an approximate solution for a fault in an infinite medium, it has been necessary to simulate a region much longer than the source distribution. Here, I propose a new SBIEM that is free from this artificial periodic boundary condition. In the proposed method, the periodic boundaries are removed by using a previously proposed method for the simulation of dynamic rupture. The integration kernel for the elastostatic effect, which reaches infinitely far from the source, is expressed analytically and replaces the one in the conventional SBIEM. The new method requires simulation of a region only twice as long as the source distribution, so the computational costs are significantly less than those required by the conventional SBIEM to simulate a fault in an infinite medium. The effect of the distance λ between the artificial periodic boundaries was investigated by comparing solutions for a typical problem setting between the conventional and proposed SBIEM. The result showed that the artificial periodic boundaries cause overestimation of the recurrence interval that is proportional to λ −2. If λ is four times the fault length, the interval is overestimated by less than 1%. Thus, the artificial periodic boundaries have only a modest effect on the conclusions of previous studies.

Afterslip inside a coseismic slip patch is rarely observed, though some previous studies suggest that it is driven by poroelastic rebound (PER). These studies assume constant frictional strength, whereas time-dependent strengthening (healing) of a fault is expected from laboratory experiments, which provide a basis for a rate- and state-dependent friction law (RSF). In this study, quasistatic poroelasticity (PE) was implemented in a dynamic earthquake sequence simulation using a spectral boundary integral equation method, and the effect of PER on the behavior of a fault governed by RSF was examined. Spatio-temporal convolution for PE would significantly affect the resolution of the numerical simulation affordable. This problem has been resolved by numerical approximation of the time dependency of Green's function of PE in the wavenumber domain, definition of memory variables, and reformulation of the temporal convolution into ordinary differential equations of them. In the novel method, the additional numerical costs due to PE are negligible. A planar fault with a rate-weakening patch embedded in the rate-strengthening region was simulated. Because it is the healing of the fault that competes against PER, both the aging law and slip law were examined, which have different characteristics in the evolution of the fault strength. The simulation results indicate that PER causes postseismic loading to the patch, but the healing efficiently suppresses afterslip not only for the aging law, but also for the slip law. When cases with different friction laws are compared, the healing is more significant for the aging law, which has log- t strengthening at a limit of V → 0 . However, the effect of PER on the slip rate is minor for the slip law. The slip law yields additional healing if the fault is accelerated by loading owing to PER. The simulation results are consistent with the absence of afterslip within the coseismic slip patches in the observations. [ABSTRACT FROM AUTHOR]

Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower crust. The mainshock triggered a significant increase in the number of small-magnitude events spread over an unusually large area in the ECAB. The paper’s goal is to compute the source parameters of the earthquakes that occurred during the aforementioned sequence, by empirical application of Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple methods and seismicity evolution at regional scale is investigated. Our results highlight a still active deformation regime at the edge of the EE Craton, while the source parameters reveal a complex fracture of the mainshock and a very high-stress drop.

The Amatrice–Visso–Norcia seismic sequence struck Central Italy across the Apenninic normal fault system in 2016. Fluids likely triggered the sequence and reduced the stability of the fault network following the first earthquake (Amatrice, Mw 6.0), with their migration nucleating the Visso (Mw 5.9) and Norcia (Mw 6.5) mainshocks. However, both spatial extent and mechanisms of fluid migration and diffusion through the network remain unclear. High fluid content, enhanced permeability, and pervasive microcracking increase seismic attenuation, but different processes contribute to different attenuation mechanisms. Here, we measured and mapped peak delay time and coda attenuation, using them as proxies of seismic scattering and absorption before and during the sequence. We observed that the structural discontinuities and lithology control the scattering losses at all frequencies, with the highest scattering delineating carbonate formations within the Gran Sasso massif. The Monti Sibillini thrust marks the strongest contrasts in scattering, indicating a barrier for northward fracture propagation. Absorption does not show any sensitivity to the presence of these main geological structures. Before the sequence, low-frequency high-absorption anomalies distribute around the NW-SE-oriented Apennine Mountain chain. During the sequence, a high-absorption anomaly develops from SSE to NNW across the seismogenic zone but remains bounded north by the Monti Sibillini thrust. We attribute this spatial expansion to the deep migration of CO2-bearing fluids across the strike of the fault network from a deep source of trapped CO2 close to the Amatrice earthquake. Fluids expand SSE-NNW primarily during the Visso sequence and then diffuse across the fault zones during the Norcia sequence.

The relationships between kinematics and fault geometry for the coseismic ruptures from the 24th and February 25, 1981 earthquake sequence in the eastern Gulf of Corinth (Ms 6.7 and 6.4) are analysed. The two earthquakes ruptured faults located across strike rather than along strike as typifies other earthquake sequences. In detail, surface ruptures formed on the sub-parallel Pisia and Skinos Faults, with an 8 km along-strike overlap zone, separated across strike by < 2 km. The largest coseismic offsets occurred in the overlap zone. The 41-year-old ruptures are still well preserved as bedrock fault plane lichen-free stripes and colluvial ruptures, allowing detailed structural mapping at 213 rupture localities. A comparison between our measurements and Jackson et al. (1982) showed no overall consistent signal of post-seismic slip as some of our measurements were greater and some smaller than those recorded in 1981. The ruptures produced a single maximum asymmetric profile (Pisia: maximum throw of 223 cm) and a double maxima profile (Skinos: maximum throw of 109 cm and 130 cm). The shapes of the profiles differed in previous earthquakes on these faults, as evidenced by an older lichen-free stripe, implying non-characteristic earthquakes. Summing the two overlapping throw profiles across-strike reveals a single maximum symmetric bell-like profile. Using the above observations on coseismic offsets, kinematic information, and the geometry of faults, a rupture scenario has been proposed in terms of fault bends and corrugation orientations which suggests that parts of each fault may have ruptured in each earthquake. • Fault geometry and slip kinematic observations help understand the rupture style formed during Greece's 1981 earthquakes. • The across-strike Pisia and Skinos Faults appear to interact based on their combined surface slip distributions. • The Pisia and Skinos Faults exhibit no resolvable post-seismic slip in 41 years since the earthquakes. • The Pisia and Skinos Faults produce non-characteristic earthquakes. • Slip vector convergence and divergence toward the hangingwall may reveal the presence of major fault bends. [ABSTRACT FROM AUTHOR]

In 2018, four deadly (Mw 6.2–6.9) earthquakes struck the north coast of Lombok Island on 28 July, 5 August, and 19 August. The slip distributions of the three mainshocks are modeled in this study by inverting the co-seismic deformation imaged using an interferometric analysis of Sentinel-1 synthetic aperture radar measurements (InSAR), based on rectangular dislocations embedded in a multi-layered elastic half-space. Our best-fit co-seismic slip model suggests the estimated maximum fault slips of 1.3 m, 2.2 m, and 2.5 m for the mainshocks from July to August, located at depths of 9.6 km, 13.6 km, and 22.2 km, respectively. We applied an unsupervised learning method (ST-DBSCAN) to cluster the relocated aftershocks so that we could identify the source of each aftershock. The clustered aftershocks are primarily distributed in the areas with increased Coulomb stress and are less abundant in the maximum slip patch on the three rupture faults, indicating consistency with our estimated co-seismic slip model. In addition, we use an InSAR time series, consisting of 337 descending and 177 ascending Sentinel-1 acquisitions to investigate the time-dependent, post-seismic deformation in the two years following the Lombok 2018 earthquake sequence, based on a pure afterslip model and a combined model that simulates viscoelastic relaxation and afterslip simultaneously. The best-fit combined model suggests a Maxwell viscosity of 1 × 1018 Pa s for both the lower crust and asthenosphere, and it reveals that the maximum of the cumulative afterslip within two years is ∼0.7 m, along the northwestward up-dip continuation of the co-seismic rupture area. • InSAR-derived rupture model for each earthquake estimated using layered half-space. • Coulomb stress triggering possible for the 2nd, less likely for the 3rd mainshock. • Aftershock clusters distinguished for each event and found to decay over 1–2 weeks. • InSAR time-series used to determine afterslip considering viscoelastic lower crust. • Afterslip persisted for many months, even after aftershock sequences decayed. [ABSTRACT FROM AUTHOR]

Based on the existing seismic activity data，past moderate-strong earthquake cases and the foregoing researches in Wuzhong-Lingwu area，this paper summarizes the moderate-strong earthquake sequence，the seismic activity before the mainshock and temporal and spatial features of major earthquakes in mainland China. Characteristics of small and moderate earthquake activity are analyzed based on minimum magnitude of completeness during 1970—2019. The generalized foreshock of moderate-strong earthquake and the small-moderate earthquake activity have many marked features. It reveals that 1 to 2 years before moderate-strong earthquakes，the regional earthquake concentration with ML≥2.0 was formed in the Wuzhong-Lingwu area and its vicinity. The signal earthquake with ML≥3.0 mostly appeared in the AzuoqiYaoba area which is in the northwest of Wuzhong-Lingwu area. The generalized foreshock was actually abundant before mainshock occurred，existing some degree of earthquakes intensify. Statistical analysis of the earthquake sequence shows that the multiple mainshock type，double-shock type and swarm type take a greater proportion and the multiple conjugate tectonics is probably the main reason. Besides，from large seismic situation perspective，the leading factor of most previous moderate-strong earthquake with M≥5 is probably caused by strong earthquakes active period of the North China block，and the high earthquake risk of the area has close connection not only with local special tectonic and regional foreshock activity，but also with the effect of major earthquakes activity from North China block to Northeast Asia block. In comparison，the major earthquakes activity of Tibetan plateau，Longmenshan fault zone，Qilianshan structure belt and other areas have no obvious effect on triggering moderate-strong earthquake in Wuzhong-Lingwu area，and there may be tectonic response between moderate-strong earthquake in Wuzhong-Lingwu area and major earthquakes activity from North China block to Northeast Asia block.

The increase of available seismic data prompts the need for automatic processing procedures to fully exploit them. A good example is aftershock sequences recorded by temporary seismic networks, whose thorough analysis is challenging because of the high seismicity rate and station density. Here, we test the performance of two recent Deep Learning algorithms, the Generalized Phase Detection and Earthquake Transformer, for automatic seismic phases identification. We use data from the December 2019 Mugello basin (Northern Apennines, Italy) swarm, recorded on 13 permanent and nine temporary stations, applying these automatic procedures under different network configurations. As a benchmark, we use a catalog of 279 manually repicked earthquakes reported by the Italian National Seismic Network. Due to the ability of deep learning techniques to identify earthquakes under poor signal‐to‐noise‐ratio (SNR) conditions, we obtain: (a) a factor 3 increase in the number of locations with respect to INGV bulletin and (b) a factor 4 increase when stations from the temporary network are added. Comparison between deep learning and manually picked arrival times shows a mean difference of 0.02–0.04 s and a variance in the range 0.02–0.07 s. The improvement in magnitude completeness is ∼0.5 units. The deep learning algorithms were originally trained using data sets from different regions of the world: our results indicate that these can be successfully applied in our case, without any significant modification. Deep learning algorithms are efficient and accurate tools for data reprocessing in order to better understand the space‐time evolution of earthquake sequences. Plain Language Summary: We focus on the testing and comparison of recently published frameworks for seismic events and seismic waves detection based on deep learning. As an example we use data from an earthquake swarm that occurred in December 2019 in the Mugello basin (N of Florence, Italy). Earthquake swarms analysis is particularly challenging because of the high seismic rates, with events closely occurring in time and space. The results of the automatic analysis are compared with results reviewed by human analysts. The deep learning algorithms used in this application were originally trained on data collected from different regions of the world: Our results indicate their capability to apply the knowledge gained in different contexts to totally unseen data. However, some differences, due to the different design of the algorithms and to training data sets, arise and suggest that careful tuning and benchmarking are required to achieve stable and reliable results in seismic data analysis workflows. Key Points: We analyze the Mw 4.5 Mugello (Italy) earthquakes sequence, December 2019, using deep learning algorithms, and compare with bulletin dataDeep learning performances on detections and phase picking works very well even without ad‐hoc retrainingDeep learning based earthquake catalogs show differences due to different algorithm designs [ABSTRACT FROM AUTHOR]

Dynamic earthquake sequence simulation is an important tool for investigating the behavior of a fault that hosts a series of earthquakes because it solves all interrelated stages in the earthquake cycle consistently, including nucleation, propagation and arrest of dynamic rupture, afterslip, locking, and interseismic stress accumulation. Numerically simulating and resolving these phenomena, which have different time and length scales, in a single framework is challenging. A spectral boundary integral equation method (SBIEM) that makes use of a fast Fourier transform is widely used because it reduces required computational costs, even though it can only be used for a planar fault. The conventional SBIEM has a periodic boundary condition as a result of the discretization of the wavenumber domain with a regular mesh; thus, to obtain an approximate solution for a fault in an infinite medium, it has been necessary to simulate a region much longer than the source distribution. Here, I propose a new SBIEM that is free from this artificial periodic boundary condition. In the proposed method, the periodic boundaries are removed by using a previously proposed method for the simulation of dynamic rupture. The integration kernel for the elastostatic effect, which reaches infinitely far from the source, is expressed analytically and replaces the one in the conventional SBIEM. The new method requires simulation of a region only twice as long as the source distribution, so the computational costs are significantly less than those required by the conventional SBIEM to simulate a fault in an infinite medium. The effect of the distance λ between the artificial periodic boundaries was investigated by comparing solutions for a typical problem setting between the conventional and proposed SBIEM. The result showed that the artificial periodic boundaries cause overestimation of the recurrence interval that is proportional to λ−2. If λ is four times the fault length, the interval is overestimated by less than 1%. Thus, the artificial periodic boundaries have only a modest effect on the conclusions of previous studies. [ABSTRACT FROM AUTHOR]

This investigation examines the contemporary documentation of a sequence of low-magnitude earthquakes at the fringes of the Kingdom of Sweden, today Southeastern Finland, in 1751–1752. A total of 11 pages of original correspondence sent from the target village of Svenskby to the Swedish capital Stockholm are reviewed. Newspaper accounts from Sweden and Russia are included in the analysis, and a timeline of the reporting is constructed. A newly created catalog shows over 30 distinct events between the end of October and December 1751 (Julian calendar). The assignment of macroseismic intensity to the earthquakes is hampered by loud acoustic effects that accompany and/or constitute the observations. Maximum intensities are assessed at IV–V (European Macroseismic Scale 1998), and maximum macroseismic magnitudes in the range of MM1.9–2.4, and were probably observed at short epicentral distances close to the ground surface. Comparisons to macroseismic data related to instrumentally recorded earthquakes in the region support the notion of low magnitudes. The data from 1751 provide an analog to modern macroseismic observations from geothermal stimulation experiments. Such experiments have acted as a spur for considering seismic risk from low-magnitude earthquakes whose consequences have seldom previously been a matter for concern.

Recently an earthquake sequence occurred in Haenam, southwestern part of the Korean Peninsula. Earthquake swarms are not unusual in the western region of Korea, which already experienced comparable series of earthquakes before the Haenam earthquake: in the Boryeong offshore (2013) and Baekryeong Island (2013 and 2019) regions. The ML 3.1 earthquake occurred 21km WNW of Haenam on May 3, 2020 and the earthquake sequence occurred from April 26 to May 9 intensively. For two weeks, a dozen small foreshocks preceded the mainshock and more aftershocks occurred. In this paper, relocation of events with focal mechanisms based on the seismicity of the 2020 Haenam earthquake sequence is proposed. The distribution of earthquake sequence in Haenam area is WNW-ESE direction, which is approximately parallel to the fault plane solutions of events with ML ≥ 2.0 indicating strike-slip faults. The sequential events concentrate within the elliptical area of about 0.18 km2 on the surface and the focal depth distributions show some variations between 19.6 km and 21.6 km for different velocity models but their ranges are within about 500–600 m. The depth variations for different models can be due to the different boundary depths between upper and lower crusts. Based on a comparison of the earthquake swarms in the western region of the Korean Peninsula, this study provides seismological information for understanding the seismotectonic structures in the Korean Peninsula. [ABSTRACT FROM AUTHOR]

We explore the three‐dimensional structure of the 2016–2017 Central Italy sequence using ~34,000 ML ≥ 1.5 earthquakes that occurred between August 2016 and January 2018. We applied cross‐correlation and double‐difference location methods to waveform and parametric data routinely produced at the Italian National Institute of Geophysics and Volcanology. The sequence activated an 80 km long system of normal faults and near‐horizontal detachment faults through the MW 6.0 Amatrice, the MW 5.9 Visso, and the MW 6.5 Norcia mainshocks and aftershocks. The system has an average strike of N155°E and dips 38°–55° southwestward and is segmented into 15–30 km long faults individually activated by the cascade of MW ≥ 5.0 shocks. The two main normal fault segments, Mt. Vettore‐Mt. Bove to the North and Mt. della Laga to the South, are separated by an NNE‐SSW‐trending lateral ramp of the Sibillini thrust, a regional structure inherited from the previous compressional tectonic phase putting into contact diverse lithologies with different seismicity patterns. Space‐time reconstruction of the fault system supports a composite rupture scenario previously proposed for the MW 6.5 Norcia earthquake, where the rupture possibly propagated also along an oblique portion of the Sibillini thrust. This dissected set of normal fault segments is bounded at 8–10 km depth by a continuous 2 km thick seismicity layer of extensional nature slightly dipping eastward and interpreted as a shear zone. All three mainshocks in the sequence nucleated along the high‐angle planes at significant distance from the shear zone, thus complicating the interpretation of the mechanisms driving strain partitioning between these structures. Key Points: 3D fine‐scale geometry and kinematics of the 2016–2017 Central Italy Sequence by HypoDD locations of data produced in monitoring environmentSpace‐time evolution of the seismic activity and its relation with a shallow shear zone bounding the system at depthFocus on faults segmentation and role of the preexisting compressional structures in modulating the extensional deformation pattern [ABSTRACT FROM AUTHOR]

This paper presents a novel precast energy dissipation shear wall (PEDSW) structure system that using mild steel dampers as dry connectors at the vertical joints to connect adjacent wall panels. Analytical studies are systematically conducted to investigate the seismic performance of the proposed PEDSW under sequence-type ground motions. During earthquake events, earthquake sequences have the potential to cause severe damage to structures and threaten life safety. To date, the damage probability of engineering structures under earthquake sequence has not been included in structural design codes. In this study, numerical simulations on single-story PEDSW are carried out to validate the feasibility and reliability of using mild steel dampers to connect the precast shear walls. The seismic responses of the PEDSW and cast-in-place shear wall (CIPSW) are comparatively studied based on nonlinear time-history analyses, and the effectiveness of the proposed high-rise PEDSW is demonstrated. Next, the foreshock-mainshock-aftershock type earthquake sequences are constructed, and the seismic response and fragility curves of the PEDSW under single mainshock and earthquake sequences are analyzed and compared. Finally, the fragility analysis of PEDSW structure under earthquake sequences is performed. The influences of scaling factor of the aftershocks (foreshocks) to the mainshocks on the fragility of the PEDSW structure under different damage states are investigated. The numerical results reveal that neglecting the effect of earthquake sequence can lead to underestimated seismic responses and fragilities, which may result in unsafe design schemes of PEDSW structures. [ABSTRACT FROM AUTHOR]

A strong offshore earthquake (M w 6.0) struck Béjaia city (eastern Algeria) on March 18th, 2021. This earthquake was followed by several aftershocks among the M w 5.2 that occurred 13 min after the main shock. Moreover, another earthquake (M w 5.0) occurred in the same zone one year later on March 19th, 2022. Near-field digital accelerograph records were used to study the earthquake and its related aftershocks. First, the March 2021 (M w 6.0) main shock, six of its main aftershocks, and the March 19th, 2022 (M w 5.0) earthquake were located. These epicentres are distributed in a 10 km-long and 2 to 3 km-wide NE–SW-trending area, with depths ranging between 8 km and 14 km. Second, using waveform inversion, the seismic moment and the focal mechanism of the three events (the March 18th, 2021, main shock and its strongest aftershock (M w 5.2) that occurred 13 min after the main shock and the March 19th, 2022 (M w 5.0) earthquake) were determined. These focal mechanisms exhibit reverse faulting with a short lateral component. Third, the source rupture process of the March 18th, 2021 (M w 6.0), earthquake was calculated from waveform inversion to obtain the moment–release distribution on a finite fault. The nodal plane oriented N74E seems to be associated with the activated fault plane. Considering the seismotectonic framework of the region, the fault that activated during the 2021 earthquake sequence is offshore. This fault, called the Western Segment, which is situated in the western part of the reverse fault system, is also at the origin of the Djidjelli historical earthquakes of August 21st, and 22nd, 1856 (Io = VIII-IX, M ≥ 6.6). [Display omitted] • Locations of the 18 March 2021 (Mw6.0) and 19 March 2022 (Mw5.2) earthquakes (Bejaia, Algeria). • Source modelling from strong-motion waveforms of the first main shock (18 March 2021, Mw6.0, Béjaia, Algeria). • Source parameters estimated for the 3 main seismic events (18/03/2021, Mw6.0, Mw5.2; 19/03/2022, Mw5.0). • The Djijeli–Bejaia active fault system and the western fault originated by the 22 August 1856 Djidjelli earthquake. [ABSTRACT FROM AUTHOR]

The seismic behaviors of retaining walls were evaluated considering only a single earthquake. However, aftershock or foreshock ground motions can occasionally be as strong as mainshock ground motions and can aggravate damage to structures. This study investigated the seismic responses of a cantilever retaining wall under earthquake sequences by performing a series of finite-difference-based numerical simulations. We collected a suite of mainshock-aftershock and foreshock-mainshock sequential ground motions, which were used as the input ground motions in the numerical simulations. This study focused on revealing the differences in wall responses under a single earthquake and multiple earthquake sequences, considering a wide range of ground motion intensities. The relative wall movements caused by mainshock-aftershock and foreshock-mainshock sequential ground motions can be much larger than those caused by a single mainshock motion. In addition, the sequence itself had the effect of increasing the relative wall movements under strong sequential ground motions. However, the effects of the aftershock and foreshock motions on the wall responses were insignificant under weak sequential ground motions. • Responses of a cantilever retaining wall are studied by numerical simulations. • nput motions are collected from actual MS-AS and FS-MS sequences. • RWD by earthquake sequences can be much larger than that by single earthquakes. • Strong earthquake sequences themselves have the effect of increasing RWD. • The effects of AS and FS on RWD are insignificant under weak earthquake sequences. [ABSTRACT FROM AUTHOR]

In this research the Coulomb stress changes due to earthquake sequence that began in the 1936 to 1997 earthquakes in Eastern Iran and their triggering effect on the rupturing of adjacent faults were investigated. Obtained results revealed a well triggering relation between main shocks and later events on the Dasht-e-Bayaz and Abiz faults, respectively. The calculated stress maps indicate that positive and negative zones of Coulomb stress changes with increasing lobes of 0.1‒1 bar are found in the off-faults tips, located at rupture terminates while the decreasing Coulomb stress lobes lie in the high angle relative to the rupture plane or relieve stress along the ruptures. Cross sections drawn respect to the source and received faults confirm the results. Also, the stress perturbations resulted from successive earthquakes and the spatial patterns of the following earthquake distribution have a good consistency. This correlation shows that the population of secondary faults and subsequent earthquakes can be encouraged in the future by coseismic Coulomb stress changes due to mainshock and background loading. [ABSTRACT FROM AUTHOR]

The M ≥ 6 earthquakes occurred in the South-North Seismic Belt, Mainland China (longitudes from 98 - 107°E and latitudes from 21 - 41°N) during 1900 - 2016 are taken to measure the multifractal dimensionsspatial distribution and time sequence of events and the dominant periods. The multifractal dimensions, Dq, are measured from the log-log plots of Cq(r) versus r and Cq(t) versus t, where Cq(r) and Cq(t) are the generalized correlation integrals for the epicentral distribution and time sequence of events, respectively. r and t are the epicentral distance and inter-event time, respectively, at positive q. The log-log plot of Cq(r) versus r shows a linear portion when log(r1) ≤ log(r) ≤ log(ru). The r1 and ru values are, respectively, 120 and 560 km for M ≥ 6 events, 100 and 560 km for M ≥ 6.5 events, and 63 and 560 km for M ≥ 7 events. The r1 value decreases with the lower-bound magnitude. Dq monotonically decreases with increasing q. The Dq values are between 1.618 and 1.426 for M ≥ 6 events, between 1.562 and 1.108 for M ≥ 6.5 events, and between 1.365 and 0.841 for M ≥ 7 events. The log-log plot of Cq(t) versus t show a linear distribution when log(tl) ≤ log(t) ≤ log(tu), where tl and tu are, respectively, 5 and 50.1 years for M ≥ 6 events, 5 and 50.1 years for M ≥ 6.5 events, and 16 and 63.1 years for M ≥ 7 event, thus suggesting that the time sequences of earthquake in the study region are multifractal. The Dq values are between 0.830 and 0.703 for M ≥ 6 events, between 0.835 and 0.820 for M ≥ 6.5 events, and between 0.786 and 0.685 for M ≥ 7 events. The Morlet wavelet technique is applied to analyze the dominant periods of temporal variations in numbers of yearly earthquakes for the three magnitude ranges, i.e., M ≥ 6, M ≥ 6.5, and M ≥ 7. The resultant dominant period is 2.94 years for M ≥ 6 events and cannot be evaluated for M ≥ 6.5 and M ≥ 7 events. [ABSTRACT FROM AUTHOR]

In this study, joint inversions of Synthetic Aperture Radar (SAR) and Global Position System (GPS) measurements are used to investigate the source parameters of four Mw > 5 events of the 2016-2017 Central Italy earthquake sequence. The results show that the four events are all associated with a normal fault striking northwest-southeast and dipping southwest. The observations, in all cases, are consistent with slip on a rupture plane, with strike in the range of 157˚ to 164˚ and dip in the range of 39˚ to 44˚ that penetrates the uppermost crust to a depth of 0 to 8 km. The primary characteristics of these four events are that the 24 August 2016 Mw 6.2 Amatrice earthquake had pronounced heterogeneity of the slip distribution marked by two main slip patches, the 26 October 2016 Mw 6.1 Visso earthquake had a concentrated slip at 3-6 km, and the predominant slip of the 30 October 2016 Mw 6.6 Norcia earthquake occurred on the fault with a peak magnitude of 2.5 m at a depth of 0-6 km, suggesting that the rupture may have reached the surface, and the 18 January 2017 Mw 5.7 Campotosto earthquake had a large area of sliding at depth 3-9 km. The positive static stress changes on the fault planes of the latter three events demonstrate that the 24 August 2016 Amatrice earthquake may have triggered a cascading failure of earthquakes along the complex normal fault system in Central Italy. [ABSTRACT FROM AUTHOR]

Schools are the gathering place of the young generation. They are expected to act as a refuge after an earthquake. Even though Nepal has no building supervision system, the quality of school buildings receives necessary attention. During a field study conducted in mid-June, we summarised the seismic damage to 12 schools with various types of structures, including local characteristic reinforced concrete frames, cement-bonded or mud-bonded masonry and timber frames. The pros and cons of local construction technologies are discussed. Some interesting earthquake phenomena, such as the effects of forward directivity, site configuration, earthquake sequence and safe distance from a slope, are discussed. The geo-security evaluation of a proposed school site in mountainous Baramchi is also conducted. [ABSTRACT FROM AUTHOR]

The approach in China since the last 1.5 decade for using apparent stress in time-dependent seismic hazard assessment or earthquake forecast is summarized. Retrospective case studies observe that apparent stress exhibits short-term increase, with time scale of several months, before moderate to strong earthquakes in a large area surrounding the 'target earthquake'. Apparent stress is also used to estimate the tendency of aftershock activity. The concept relating apparent stress indirectly to stress level is used to understand the properties of some 'precursory' anomalies. Meanwhile, different opinions were reported. Problems in the calculation also existed for some cases. Moreover, retrospective studies have the limitation in their significance as compared to forward forecast test. Nevertheless, this approach, seemingly uniquely carried out in a large scale in mainland China, provides the earthquake catalogs for the predictive analysis of seismicity with an additional degree of freedom, deserving a systematic review and reflection. [ABSTRACT FROM AUTHOR]

An M 6.8 ( Mw 6.5) deep-focus earthquake occurred beneath the Bonin Islands at 21:18 (JST) on June 23, 2015. Observed high-frequency (>1 Hz) seismograms across Japan, which contain several sets of P- and S-wave arrivals for the 10 min after the origin time, indicate that moderate-to-large earthquakes occurred sequentially around Japan. Snapshots of the seismic energy propagation illustrate that after one deep-focus earthquake occurred beneath the Sea of Japan, two deep-focus earthquakes occurred sequentially after the first ( Mw 6.5) event beneath the Bonin Islands in the next 4 min. The United States Geological Survey catalog includes three Bonin deep-focus earthquakes with similar hypocenter locations, but their estimated magnitudes are inconsistent with seismograms from across Japan. The maximum-amplitude patterns of the latter two earthquakes were similar to that of the first Bonin earthquake, which indicates similar locations and mechanisms. Furthermore, based on the ratios of the S-wave amplitudes to that of the first event, the magnitudes of the latter events are estimated as M 6.5 ± 0.02 and M 5.8 ± 0.02, respectively. Three magnitude-6-class earthquakes occurred sequentially within 4 min in the Pacific slab at 480 km depth, where complex heterogeneities exist within the slab. [ABSTRACT FROM AUTHOR]