Strong Aftershocks Research Articles

Seismic clusters and fluids diffusion: a lesson from the 2018 Molise (Southern Italy) earthquake sequence

The identification of seismic clusters is essential for many applications of statistical analysis and seismicity forecasting: uncertainties in cluster identification leads to uncertainties in results. However, there are several methods to identify clusters, and their results are not always compatible. We tested different approaches to analyze the clustering: a traditional window-based approach, a complex network-based technique (nearest neighbor—NN), and a novel approach based on fractal analysis. The case study is the increase in seismicity observed in Molise, Southern Italy, from April to November 2018. To analyze the seismicity in detail with the above-mentioned methods, an improved template-matching catalog was created. A stochastic declustering method based on the Epidemic Type Aftershock Sequence (ETAS) model was also applied to add probabilistic information. We explored how the significant discrepancies in these methods’ results affect the result of NExt STrOng Related Earthquake (NESTORE) algorithm—a method to forecast strong aftershocks during an ongoing cluster—previously successfully applied to the whole Italian territory. We performed a further analysis of the spatio-temporal pattern of seismicity in Molise, using the Principal Component Analysis (PCA), the ETAS algorithm, as well as other analyses, aimed at detecting possible migration and diffusion signals. We found a relative quiescence of several months between the main events of April and August, the tendency of the events to propagate upwards, and a migration of the seismicity consistent with a fluid-driven mechanism. We hypothesize that these features indicate the presence of fluids, which are also responsible for the long duration of the sequence and the discrepancies in cluster identification methods’ results. Such results add to the other pieces of evidence of the importance of the fluid presence in controlling the seismicity in the Apennines. Moreover, this study highlights the importance of refined methods to identify clusters and encourages further detailed analyses when different methods supply very different results.Graphical

Study on secondary collapse of the bottom frame masonry structure in semi-ruined state based on FEM–FDEM

The bottom frame masonry structure (BFMS) in a semi-ruined state is vulnerable to secondary collapse under strong aftershocks, posing a significant risk to rescuers during post-earthquake operations. Therefore, investigating the mechanisms and characteristics of secondary collapse of BFMS in a semi-ruined state (BFMS-SR) under aftershocks is critical. This paper proposes a numerical modeling framework for BFMS under mainshock-aftershock conditions, utilizing a combination of the finite element method (FEM) and the finite discrete element method (FDEM). The validation demonstrates that FEM–FDEM can effectively reproduce the transition of BFMS from an intact state to a semi-ruined state, ultimately leading to a secondary collapse state. Subsequently, the mechanisms and structural response characteristics of BFMS-SR under aftershocks are analyzed. The secondary collapse of the BFMS-SR under aftershocks is primarily governed by column hinge development. Furthermore, according to the time-history curve of absolute vertical velocity, the secondary collapse of BFMS-SR exhibits four distinct stages: stabilization, structural response development, secondary collapse, and post-collapse. The end of the stabilization phase is proposed as the early-warning threshold for secondary collapse of BFMS-SR under aftershocks, aiding in post-earthquake rescue operations.

Automatically Detected CSES Ionospheric Precursors Before Part of the Strong Aftershocks of the 23 January 2024 Wushi MS 7.1 Earthquake in Northwest China

Earthquake prediction is still a large challenge worldwide so far. In this paper, an automatic detection method was put into service immediately after the Wushi MS 7.1 earthquake on 23 January 2024 to weekly detect possible CSES (China Seismo-Electromagnetic Satellite) precursory information before impending aftershocks. An electron perturbation with an enhanced magnitude of 38.3% was first detected on 24 January 2024 at night orbit 33175 and the corresponding variations in different plasma parameters measured at this orbit presented a typical feature of electron depletion or plasma bubble with an abrupt decrease and then an increase after one minute. The Kp index was also checked during this period and the values reached 3.7 once on 23 and 24 January, which indicates that these ionospheric variations probably originated from solar activities instead of three strong aftershocks with a magnitude more than five in the following three days. However, uncertainties still exist. Then, an electron perturbation with amplitude of 24.6%, as well as an O+ one of 27.3%, was successfully searched automatically at the same revisiting orbit 33251 on 3 February 2024 in a magnetically quiet period. These two plasma variations, as well as ones of other ionospheric parameters, were characterized by highly synchronous properties, which increase the availability as seismic precursors. However, no obvious variations were observed at other revisiting orbits or other orbits near the aftershock areas during this period. An aftershock with magnitude of MS 5.3 and the strongest one of MS 5.8 took place on 24 and 25 February, respectively, 20 days after and 1000 km away.

Case Study on Behavior of a Masonry Buildings Under Seismic Loading October 2023 Earthquake in Nepal

Abstract: This paper assesses the behavior of the corners of unreinforced masonry walls during a magnitude 5.3 earthquake that struck western Nepal on October 3 at 14:40 local time. A strong aftershock with a 6.3 Richter scale occurred immediately after the main shock at 15:06, followed by a second aftershock of 5.1 Richter scale. The epicenter of the quake was located in Bajhang district, with tremors felt in neighboring districts (Doti, Achham, Bajura, Dadeldhura, Baitadi, and Darchula), as well as in Kathmandu. According to the National Society of the Red Cross, 334 houses have been fully damaged, and 1,185 have been partially damaged. The assessment is still ongoing in the affected areas, with displaced people staying in schools or with neighbors. The response is slow due to access constraints, requiring one to two days of walking to reach the affected communities. The majority of the buildings in the affected region are constructed with masonry, most of which were formed with random or coursed stone and mud brick walls without any reinforcement. Many of these buildings were damaged or had collapsed. The cracking and failure patterns of the buildings are examined and interpreted according to current provisions for earthquake resistance of masonry structures. The damages are attributed to several factors, such as poor construction quality and workmanship. This study aims to contribute to the body of knowledge necessary for improving earthquake resilience in Nepal and similar regions around the world

Seismic performance of near-fault girder bridges based on laminated rubber bearings under the impact of sequential earthquakes

Strong aftershocks frequently follow near-fault earthquakes, exacerbating the damage to bridges already impacted by the main shock. However, current seismic design codes worldwide overlook the adverse effects of aftershocks on near-fault bridges. Therefore, it is imperative to conduct research on the seismic performance of near-fault girder bridges under sequential earthquakes. This study proposes a seismic isolation design for near-fault girder bridges subjected to sequential earthquakes, utilizing laminated rubber bearings, sacrificial shear keys, and cable-type unseating prevention devices. Seismic ground motion records from typical stations during the Turkey-Syria earthquakes were collected to assess the seismic safety of bridges. Nonlinear time-history analysis systematically investigated the seismic displacements of the laminated rubber bearings and girder, as well as the seismic damage of shear keys and pier. The results show that the lateral displacement of the girder induced by the sequential earthquake was approximately 1.5 times greater than that caused by a single main shock, while the longitudinal displacement of the girder remained similar. The seismic isolation design proposed in this study not only controlled the relative displacement between the piers and the girder but also effectively protected the pier from severe seismic damage, ensuring the bridge’s functionality after the sequential earthquake.

Time-Dependent Aftershock Probabilistic Forecasting and Ensemble Model Implementation: Insights from the 2021 Ms 7.4 Maduo Earthquake

Abstract On 22 May 2021, an Ms 7.4 earthquake with a focal depth of 17 km struck the Maduo region of Qinghai province, breaking a 3.8-year quiescence of strong earthquakes (magnitude >7.0) in mainland China. This event has increased stress on the Maqin–Maqu segment of the Kunlunshan fault, heightening the potential for a large earthquake in the region. In this study, we employed the epidemic-type aftershock sequence (ETAS) model and Reasenberg–Jones (R–J) model to fit the aftershock sequence following the mainshock, analyzing the temporal response and its ability to influence future generations. Concurrently, ensemble models were explored to leverage the strengths of both the ETAS and R–J models. Short-term forecasts of the probability and occurrence rate of aftershock events with varying magnitudes were conducted for the next three days. Then statistical methods, that is, receiver-operating characteristic diagrams and information gain, were used to evaluate the unconditional and relative performance. Our findings include that the ETAS model indicates a high decay rate with many aftershocks triggered by previous events, whereas the R–J model shows a normal decay rate with a higher proportion of strong aftershocks. The ETAS and R–J models perform better than random guesses for different aftershock magnitudes, but the ETAS model is somewhat affected by the problem of missing small earthquakes for a short period of time after the mainshock. The ensemble model that chooses the minimum strategy shows promise, especially in the early period, suggesting a reasonable and cautious decision approach should be chosen during the unstable stage. This study highlights the importance of short-term aftershock probability estimation for seismic research and decision-making. In the absence of more accurate models, current analytical approaches within the operational earthquake forecasting framework remain valuable. Continuous testing, feedback, and refinement of forecasting models, along with the development of ensemble models, are essential for enhancing seismic risk assessment and mitigation strategies.

The rupture process of the Hualien M7.3 sequence on April 3, 2024

The Hualien M7.3 earthquake on April 3, 2024, was a significant and strong earthquake in Taiwan, China in the past two decades. The rupture process of the main shock and strong aftershocks is of great significance to the subsequent seismic activity and seismogenic tectonic research. Based on local strong-motion data, we used the IDS (Iterative Deconvolution and Stacking) method to obtain the rupture process of the mainshock and two strong aftershocks on the 23rd. The rupture of the mainshock was mainly unilateral, lasting 31 s, with a maximum slip of 2 m, and the depth of the large slip zone is about 41-49 km. There is a clear difference between the rupture depth of the main shock and the two strong aftershocks. The depths of the large slip zones of the latter two are 3-9 km and 8-10 km, respectively. There is also a significant difference in the seismogenic fault between the mainshock and the aftershocks, and we believe that there are two seismogenic fault zones in the study area, the deep and the shallow fault zone. The slip of the deep faults activates the shallow faults.

Spatio-temporal characterization of earthquake sequence parameters and forecasting of strong aftershocks in Xinjiang based on the ETAS model.

In this paper, the Integrated Nested Laplace Algorithm (INLA) is applied to the Epidemic Type Aftershock Sequence (ETAS) model, and the parameters of the ETAS model are obtained for the earthquake sequences active in different regions of Xinjiang. By analyzing the characteristics of the model parameters over time, the changes in each earthquake sequence are studied in more detail. The estimated values of the ETAS model parameters are used as inputs to forecast strong aftershocks in the next period. We find that there are significant differences in the aftershock triggering capacity and aftershock attenuation capacity of earthquake sequences in different seismic regions of Xinjiang. With different cutoff dates set, we observe the characteristics of the earthquake sequence parameters changing with time after the mainshock occurs, and the model parameters of the Ms7.3 earthquake sequence in Hotan region change significantly with time within 15 days after the earthquake. Compared with the MCMC algorithm, the ETAS model fitted with the INLA algorithm can forecast the number of earthquakes in the early period after the occurrence of strong aftershocks more effectively and can forecast the sudden occurrence time of earthquakes more accurately.

Possible Interrelations of Space Weather and Seismic Activity: An Implication for Earthquake Forecast

The statistical analysis of the impact of the top 50 X-class solar flares (1997–2024) on global seismic activity as well as on the earthquake preparation zones located in the illuminated part of the globe and in an area of 5000 km around the subsolar point was carried out. It is shown by a method of epoch superposition that for all cases, an increase in seismicity is observed, especially in the region around the subsolar point (up to 33%) during the 10 days after the solar flare in comparison with the preceding 10 days. The case study of the aftershock sequence of a strong Mw = 9.1 earthquake (Sumatra–Andaman Islands, 26 December 2004) after the solar flare of X10.16 class (20 January 2005) demonstrated that the number of aftershocks with a magnitude of Mw ≥ 2.5 increases more than 17 times after the solar flare with a delay of 7–8 days. For the case of the Darfield earthquake (Mw = 7.1, 3 September 2010, New Zealand), it was shown that X-class solar flares and M probably triggered two strong aftershocks (Mw = 6.1 and Mw = 5.9) with the same delay of 6 days on the Port Hills fault, which is the most sensitive to external electromagnetic impact from the point of view of the fault electrical conductivity and orientation. Based on the obtained results, the possible application of natural electromagnetic triggering of earthquakes is discussed for the earthquake forecast using confidently recorded strong external electromagnetic triggering impacts on the specific earthquake preparation zones, as well as ionospheric perturbations due to aerosol emission from the earthquake sources recorded by satellites.

The influence of aftershocks on seismic hazard analysis: a case study from Xichang and the surrounding areas

Abstract. In some instances, a strong aftershock can cause more damage than the mainshock. Ignoring the influence of aftershocks may lead to the underestimation of the seismic hazard of some areas. Taking Xichang and its surrounding areas as an example and based on the Seismic ground motion parameters zonation map of China (GB 18306-2015), this study used the Monte Carlo method to simulate synthetic mainshock sequences. Additionally, the Omi–Reasenberg–Jones (Omi–R–J) aftershock activity model is used to simulate the aftershock sequences that follow mainshocks above a certain magnitude threshold. Then, the mainshock and the aftershocks are combined to calculate the regional seismic hazard using ground motion prediction equations (GMPEs). Finally, the influence of aftershocks on seismic hazard analysis is examined and considered. The results show that in areas with moderate to strong seismic backgrounds, the influence of aftershocks on probabilistic seismic hazard analysis can exceed 50 %. These results suggest that the impact of aftershocks should be properly considered for future probabilistic seismic hazard analyses, especially in areas with moderate to strong seismic activity backgrounds and in areas prone to secondary disasters such as landslides and mudslides.

Effect of aftershock characteristics on the fragility curve of post-mainshock RC frames

Earthquakes are not isolated events but are often followed by aftershocks. While structures are typically designed to withstand the mainshock, the presence of aftershocks poses additional challenges. In some cases, a damaged structure could collapse under a strong aftershock because of the damage evolution and cumulative damage as well as strength and stiffness degradation. Thus, it is essential to understand the structural behavior for post-mainshock decision-making and seismic assessment. The current study explores the impact of aftershock characteristics on the collapse capacity of buildings in seismic zones in accordance with a probabilistic point of view. In this regard, two non-ductile reinforced concrete (RC) frames with 3 and 6 stories were investigated and analyzed using 62 real aftershock records with varying characteristics. Frames were modeled in OpenSees software using Beam With Hinges Element with appropriate plastic hinge length and fiber section to provide suitable accuracy for dynamic analysis. The incremental dynamic analysis (IDA) was used to generate collapse diagrams and fragility curves. Some important seismic parameters including the mean period (Tm), predominant period (Tp), bandwidth frequency (Ω), significant duration of aftershock (Ds) and site characteristics were considered. The results revealed that the probability of collapse would increase in aftershocks with longer periods. For example, for Sa (T1, 5 %) of 1 g and 3-story frame, the collapse probability is increased ≈25 % in Tm > 0.5s compared to Tm < 0.5s which is remarkable. Also, site characteristics could increase the probability of collapse by ~ 6 % in the 6-story frame. The same results can also be seen for aftershocks with a larger predominant period (Tp > T1), greater significant duration (Ds > 11 s) and lower bandwidth frequency (omega <0.5). The findings emphasize the significance of aftershock characteristics on structural collapse capacity, necessitating careful selection of seismic records for accurate evaluation of seismic behavior.

THE BAVRIAN EARTHQUAKE ON FEBRUARY 13, 2022 at 22:25 LOCAL TIME WITH MAGNITUDE MS= 5.4 WITH INTENSITY AT THE EPICENTRE I0=7 POINTS

After the Spitak earthquake on December 7, 1988, the Bavrian earthquake on February 13, 22 hours 25 minutes local time, with a magnitude of MS= 5.4 and an intensity at the epicenter of I0=7 points, was the second with such intensity, lasted quite a long time due to strong aftershocks.

Source rupture process of the March 18th, 2021, Mw6.0 Béjaia (Algeria) earthquake associated with the Western segment – A link with the August 1856 Djidjelli earthquakes (Io = VIII-IX, M ≥ 6)

A strong offshore earthquake (Mw6.0) struck Béjaia city (eastern Algeria) on March 18th, 2021. This earthquake was followed by several aftershocks among the Mw5.2 that occurred 13 min after the main shock. Moreover, another earthquake (Mw5.0) 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 (Mw6.0) main shock, six of its main aftershocks, and the March 19th, 2022 (Mw5.0) 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 (Mw5.2) that occurred 13 min after the main shock and the March 19th, 2022 (Mw5.0) earthquake) were determined. These focal mechanisms exhibit reverse faulting with a short lateral component. Third, the source rupture process of the March 18th, 2021 (Mw6.0), 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).

A New Statistical Perspective on Båth’s Law

Abstract The empirical Båth’s law states that the average magnitude difference (ΔM) between a mainshock and its strongest aftershock is ∼1.2, independent of the size of the mainshock. Although this observation can generally be explained by a scaling of aftershock productivity with mainshock magnitude in combination with a Gutenberg–Richter frequency–magnitude distribution, estimates of ΔM may be preferable because they are directly related to the most interesting information, namely the magnitudes of the main events, without relying on assumptions. However, a major challenge in calculating this value is the bias introduced by missing data points when the strongest aftershock is below the observed cut-off magnitude. Ignoring missing values leads to a systematic error because the data points removed are those with particularly large magnitude differences ΔM. The error can be minimized by restricting the statistics to mainshocks that are at least 2 magnitude units above the cutoff, but then the sample size is strongly reduced. This work provides an innovative approach for modeling ΔM by adapting methods for time-to-event data, which often suffer from incomplete observations (censoring). In doing so, we adequately account for unobserved values and estimate a fully parametric distribution of the magnitude differences ΔM for mainshocks in a global earthquake catalog. Our results suggest that magnitude differences are best modeled by the Gompertz distribution and that larger ΔM are expected at increasing depths and higher heat flows.

Effects of Aftershocks on the Seismic Performances of Reinforced Concrete Eccentric Frame Structures

Strong aftershocks have the potential to cause accumulated damage in structures, a feature which has been reported in post-earthquake reconnaissance studies, particularly for eccentric or irregular structures. This study aims to investigate the seismic behaviors of eccentric RC structural models under mainshock–aftershock (MSAS) sequences. In this study, three-dimensional structural models with eccentricities of 5%, 10%, 15%, 20%, 25%, and 30%, and an eccentricity of 0 (symmetric structural model) are developed by changing the positions of the centers of the structural mass. A static pushover analysis and a nonlinear time history analysis are conducted on the structural models with different eccentricities considering unidirectional and bidirectional earthquake loading (including mainshock ground motion and MSAS sequences). The amplitude of the aftershock ground motion is scaled according to the structural damage levels calibrated with the inter-story drift ratio (IDR). Furthermore, the differences in seismic responses between the unidirectional and bidirectional eccentric structures are discussed. The results show that the peak displacements of the unidirectional eccentric structures under MSAS sequences are nearly 1.4 times higher than those under mainshock ground motions. The structural seismic responses under unidirectional earthquake loading are more sensitive to the intensity of aftershock ground motions than those under bidirectional earthquake loading. Compared with unidirectional eccentric structures, bidirectional eccentric structures are more sensitive to the intensity of aftershock ground motions and have larger torsional angles and more complex displacement trends. The maximum displacement and the maximum IDR of bidirectional eccentric structures under MSAS sequences can reach 1.5 times and 1.4 times of those under mainshock ground motions, respectively.

Collapse strength ratios for structures under mainshock-aftershock subduction seismic sequences

The effects of aftershocks on civil structures have called the attention of the earthquake engineering community mainly in the last two decades. However, neither seismic design codes nor seismic assessment guidelines for existing structures worldwide consider the possible detrimental effects of strong aftershocks nowadays. From this aim, this paper presents the results of a statistical study focused on computing the relative lateral strength to avoid collapse, named lateral strength ratio Rc, of degrading (i.e., having stiffness and in-cycle strength degradation) single-degree-of-freedom, SDOF, systems under mainshock-aftershock seismic sequences. For this purpose, the degrading SDOF systems have a trilinear force–displacement backbone mainly defined by their displacement ductility capacity, μc, and the negative slope after peak strength, αc. Therefore, median Rc ratios were computed for a site-specific mainshock-aftershock scenario at rock sites considering as-recorded and artificial seismic sequences. From the results of this investigation, it is shown that median Rc ratios that take into account strong aftershocks are smaller than those computed for only the mainshocks, which means that the yield strength coefficient, Cy, for structures subjected to mainshock-strong aftershock pairs should be greater than that for structures under only the mainshocks. The ordinates of median Rc ratios depend on the site class, period of vibration, T, as well as μc and αc. A predictive equation for obtaining estimates of period-dependent Rc ordinates under mainshock and mainshock-aftershock sequences at rock sites is introduced whose fitted coefficients depend on μc and αc. The predictive equation of Rc allows obtaining estimates of the yield strength coefficient required to avoid collapse, Cyc, under mainshock-aftershock sequences for the design of new structures.

Coseismic Faulting Model and Post-Seismic Surface Motion of the 2023 Turkey–Syria Earthquake Doublet Revealed by InSAR and GPS Measurements

On 6 February 2023 (UTC), an earthquake doublet, consisting of the Mw 7.8 Pazarcik earthquake and the Mw 7.5 Elbistan earthquake, struck south-central Turkey and northwestern Syria, which was the largest earthquake that occurred in Turkey since the 1939 Erzincan earthquake. The faulting model of this earthquake was estimated based on the coseismic InSAR and GPS displacements. In addition, the best-fitting coseismic faulting model indicates that both the Pazarcik earthquake and the Elbistan earthquake were controlled by predominated left-lateral strike-slip motion, with slip peaks of 9.7 m and 10.8 m, respectively. The Coulomb failure stress (CFS) change suggests that the Pazarcik earthquake has a positive effect in triggering the rupture of the seismogenic fault of the Elbistan earthquake. Furthermore, these two main shocks promoted the occurrence of the Mw 6.3 strong aftershock. Additionally, it is found that the 2023 Turkey-Syria earthquake doublet increased the rupture risk of the Puturge segment of the EAF fault and the northern segment of the Dead Sea Fault (DSF). It is crucial to note that the northern segment of the DSF has not experienced a large earthquake in several centuries, highlighting the need for heightened attention to the potential seismic hazard of this segment. Finally, a deformation zone adjacent to the DSF was identified, potentially attributed to the motion of a blind submarine fault.

A new seismic damage assessment method for single-layer spherical reticulated shells based on structural residual displacement under sequential earthquakes

Single-layer Spherical Reticulated Shell (SSRS) is usually utilized as an emergency shelter or a command post for disaster relief. However, the structural damage indicators in current studies are difficult to observe or record so as to hardly assess the damage state after the main shock. In order to explore the damage state assessment timely and accurately for SSRS, a new seismic damage assessment method based on structural residual displacement under main shock-aftershock ground motions is proposed in this paper. The finite element model of Kiewit 8-SSRS (K8-SSRS) is established in the ANSYS with 40 groups of actual sequential earthquake from the Pacific Earthquake Engineering Research Center (PEER) seismic database. Then, the dynamic responses of the model are analyzed to calculate the damage index and eight structural dynamic response indicators. Moreover, the correlation study between the damage index and eight indicators is carried out to make the comparison analyses. Finally, it is verified that the structural plasticity appeared under main shock can develop further under strong aftershock to cause greatly additional damage or even result in structural strength failure. It is concluded that the maximum residual displacement has the strongest correlation with the damage index of the structure under sequential earthquake, which means the most feasible indicator to characterize the structural damage under sequential earthquake.

Forecasting Strong Subsequent Earthquakes in Greece with the Machine Learning Algorithm NESTORE.

Aftershocks of earthquakes can destroy many urban infrastructures and exacerbate the damage already inflicted upon weak structures. Therefore, it is important to have a method to forecast the probability of occurrence of stronger earthquakes in order to mitigate their effects. In this work, we applied the NESTORE machine learning approach to Greek seismicity from 1995 to 2022 to forecast the probability of a strong aftershock. Depending on the magnitude difference between the mainshock and the strongest aftershock, NESTORE classifies clusters into two types, Type A and Type B. Type A clusters are the most dangerous clusters, characterized by a smaller difference. The algorithm requires region-dependent training as input and evaluates performance on an independent test set. In our tests, we obtained the best results 6 h after the mainshock, as we correctly forecasted 92% of clusters corresponding to 100% of Type A clusters and more than 90% of Type B clusters. These results were also obtained thanks to an accurate analysis of cluster detection in a large part of Greece. The successful overall results show that the algorithm can be applied in this area. The approach is particularly attractive for seismic risk mitigation due to the short time required for forecasting.

Three-dimensional electrical structure and seismogenic environment of the crust–mantle in the Lushan earthquake region, China

The Longmenshan (LMS) southern section is the front of the eastwards material migration from the Qinghai–Tibet Plateau. To understand whether the 2022 Ms6.1 Lushan earthquake was a strong aftershock of the 2013 Ms7.0 Lushan earthquake and why the aftershocks of the two earthquakes were concentrated relative to the 2008 Wenchuan earthquake, a three-dimensional electrical structural model of the Lushan earthquake region is established based on magnetotelluric (MT) array data. The model shows high-conductor layers (HCL) in the Songpan–Ganzi terrane (SGT) middle crust, Sichuan Basin (SCB) shallow surface, western Sichuan foreland depression (WSCFD) and LMS Moho. The analysis suggests that the HCL in the study area at a depth of 10 km is caused by saline fluids in the fracture zone or porous sedimentary rock voids, while the high-conductor body in the middle and lower crust and upper mantle is caused by partial melting of rocks. The Lushan earthquake area is subject to compressional deformation horizontally as the eastwards migration of Qinghai–Tibet Plateau material is blocked by the SCB and vertically by the upwards extension of high-conductor plastic material at depth. The aftershocks of the Lushan earthquake area are mainly confined within the Baoxing–Zhonglin–Daxing–Lushan high-conductor region, which has noticeable electrical structural differences compared to the overall high-resistivity structure of the LMS southern section; these differences explain why the aftershocks of the two earthquakes did not expand in the northeast and southwest directions of LMS. Because the 2022 and 2013 Lushan earthquakes both occurred in the same high-conductor region in the deep part of the Dachuan–ShuangShi fault to the Danyi blind fault, the 2022 Lushan earthquake was a strong aftershock of the 2013 Lushan earthquake in terms of electrical structural characteristics.