Occurrence Of Aftershocks Research Articles

Analysis of borehole strain anomalies before the 2017 Jiuzhaigou Ms 7.0 earthquake based on a graph neural network

Abstract. On 8 August 2017, a strong earthquake of magnitude 7.0 occurred in Jiuzhaigou, Sichuan Province, China. To assess pre-earthquake anomalies, we utilized variational mode decomposition to preprocess borehole strain observation data and combined them with a graph WaveNet neural network model to process data from multiple stations. We obtained 1-year data from four stations near the epicenter as the training dataset and data from 1 January to 10 August 2017 as the test dataset. For the prediction results of the variational mode decomposition–graph WaveNet model, the anomalous days were extracted using statistical methods, and the results of anomalous-day accumulation at multiple stations showed that an increase in the number of anomalous days occurred 15–32 d before the earthquake. The acceleration effect of anomalous accumulation was most obvious 20 d before the earthquake, and an increase in the number of anomalous days also occurred in the 1 to 3 d post-earthquake. We tentatively deduce that the pre-earthquake anomalies are caused by the diffusion of strain energy near the epicenter during the accumulation process, which can be used as a signal of pre-seismic anomalies, whereas the post-earthquake anomalies are caused by the frequent occurrence of aftershocks.

Exploring the Gravitational Impact of Spring Tides on Earthquake Occurrence in Central Asia

Abstract In this study, we analyzed variations in seismic activity in relation to the syzygy of the Sun, Moon, and Earth, corresponding to the semilunar or spring tide (ST) cycle. We focused on two regions within the Central Asian Orogenic Belt: the North Tien Shan region (NTSR) and the Xinjiang Uygur tectonic province (XUTP), using data from two regional earthquake catalogs. For the NTSR, we used data from the Seismological Experimental and Methodical Expedition in Almaty, Kazakhstan, covering 1 January 1970 to 31 July 2022. For the XUTP, we utilized data from the Data Management Centre of China National Seismic Network at the Institute of Geophysics China Earthquake Administration, spanning 1 January 1987 to 30 June 2019. We applied the nonparametric Kuiper’s test to evaluate cyclic variations in seismic activity, focusing on different magnitude thresholds in the regional catalog. For both regions, the original catalog as well as the catalogs of mainshocks and aftershocks were analyzed. The nearest-neighbor declustering method was employed to decompose the seismic catalogs. Kuiper’s test statistics revealed varying trends in the occurrence times for mainshocks and aftershocks during ST cycles. The test showed “the good-to-fit” to a uniform distribution for the timing of mainshocks within the ST cycle. Kuiper’s test revealed notably consistent trends in aftershock occurrence times during ST cycles with a significance level α of 0.001 or less in both regions analyzed. These results suggest a potential correlation between aftershock activity and the gravitational effects of the ST cycle. The most substantial decrease in aftershocks occurs during the micromoon ST cycle, when the distance between the Moon and the Earth exceeds 400,000 km.

Smoothing-Based Aftershock Probabilistic Seismic Hazard Assessment Using the Spatiotemporal ETAS Model

ABSTRACT Probabilistic seismic hazard analysis (PSHA) is generally based on computing time-invariant occurrence rates of mainshocks using the Poisson process. However, aftershock probabilistic seismic hazard analysis (APSHA) allows for assessing time-varying aftershock occurrence rates within a short-term seismic hazard mitigation framework. Our proposed methodology of APSHA develops a smoothing-based analytical formulation to capture the spatial distribution and temporal evolution of aftershock sequences using the spatiotemporal epidemic-type aftershock sequence model. This approach is tested on case studies of the 2013 Bushehr, 2021 and 2022 Hormozgan seismic events, and characterizes the aftershocks’ hierarchical structure to improve the reliability of aftershock hazard assessments. Then, the results of APSHA (aftershock ground-motion hazard at specific sites) based on smoothing are compared with conventional PSHA (pre-mainshock ground-motion hazard at specific sites). This comparative analysis highlights the importance of considering aftershock effects when assessing ground-motion hazards because PSHA does not fully account for aftershock hazard increases following major earthquakes.

Post-Natural Disasters Emergency Response Scheme Selection: An Integrated Application of Probabilistic T-Spherical Hesitant Fuzzy Set, Penalty-Incentive Dynamic Attribute Weights, and Non-Compensation Approach

This paper presents an innovative methodology for the dynamic emergency response scheme selection (ERSS) problem in post-major natural disasters. It employs a combination of subjective and objective composite weights and the integrated ELECTRE-score approach. The study aims to provide a practical approach for continuously determining optimal decision schemes at various time points during the decision period in the aftermath of significant natural disasters while accommodating evolving real-world scenarios. Firstly, the probabilistic T-spherical hesitant fuzzy set (Pt-SHFS) captures decision-makers’ ambivalence and hesitation regarding diverse evaluation attributes of different schemes. Subsequently, Pt-SHFS is integrated with the best–worst method (BWM) to determine subjective weights, followed by the structured CRITIC method to amalgamate subjective weights and derive the final combination weights of criteria. Additionally, this paper proposes applying a penalty-incentive mechanism to establish dynamic attribute weights during scenario evolution. Furthermore, the ELECTRE-score method, which may fully exploit the advantages of non-compensation situations, is adopted to obtain more reliable dynamic optimal decision outcomes. Consequently, based on these foundations, an integrated dynamic ERSS approach is formulated to determine optimal dynamic emergency response schemes. Finally, a case study on the Gansu Jishishan earthquake, sensitivity analysis, comparative analysis, and continuous analysis are conducted to verify the practicality, stability, and effectiveness of the proposed approach. The result shows that the proposed comprehensive approach can depict variances among experts’ information, dynamically adjust attribute weights in response to evolving scenarios, and assign a score range and a representative score to each scheme at each decision state. Sensitivity and comparative analyses show this model has strong stability and dynamics. Furthermore, the proposed approach can effectively deal with the complex dynamic situation in the earthquake rescue process, such as the secondary collapse of buildings after the earthquake, the damage of materials caused by heavy rain, and the occurrence of aftershocks. The model can continuously optimize decision-making and provide scientific and reliable support for emergency decision-making.

Evolution and spatiotemporal analysis of earthquake public opinion based on social media data

As critical conduits for the dissemination of online public opinion, social media platforms offer a timely and effective means for managing emergencies during major disasters, such as earthquakes. This study focuses on the analysis of online public opinions following the Maduo M7.4 earthquake in Qinghai Province and the Yangbi M6.4 earthquake in Yunnan Province. By collecting, cleaning, and organizing post-earthquake Sina Weibo (short for Weibo) data, we employed the Latent Dirichlet Allocation (LDA) model to extract information pertinent to public opinion on these earthquakes. This analysis included a comparison of the nature and temporal evolution of online public opinions related to both events. An emotion analysis, utilizing an emotion dictionary, categorized the emotional content of post-earthquake Weibo posts, facilitating a comparative study of the characteristics and temporal trends of online public emotions following the earthquakes. The findings were visualized using Geographic Information System (GIS) techniques. The analysis revealed certain commonalities in online public opinion following both earthquakes. Notably, the peak of online engagement occurred within the first 24 hours post-earthquake, with a rapid decline observed between 24 to 48 hours thereafter. The variation in popularity of online public opinion was linked to aftershock occurrences. Adjusted for population factors, online engagement in areas surrounding the earthquake sites and in Sichuan Province was significantly high. Initially dominated by feelings of “fear” and “surprise”, the public sentiment shifted towards a more positive outlook with the onset of rescue operations. However, distinctions in the online public response to each earthquake were also noted. Following the Yangbi earthquake, Yunnan Province reported the highest number of Weibo posts nationwide; in contrast, Qinghai Province ranked third post-Maduo earthquake, attributable to its smaller population size and extensive damage to communication infrastructure. This research offers a methodological approach for the analysis of online public opinion related to earthquakes, providing insights for the enhancement of post-disaster emergency management and public mental health support.

Rapid early afterslip characteristics of the 2010 moment magnitude (Mw) 8.8 Maule earthquake determined with sub-daily GPS solutions

Ground surface deformations can be observed during the coseismic and postseismic periods. The accurate determination of displacements is of paramount importance for the assessment of the destructive power of large earthquakes and the characterization of fault behaviors. Therefore, we employ the sub-daily Global Positioning System (GPS) solutions at 19 GPS stations to determine the coseismic and postseismic deformations of the 2010 moment magnitude (Mw) 8.8 Maule earthquake. Using sub-daily GPS data, we can accurately measure both coseismic and early postseismic deformation signals, which can precisely identify the distribution of coseismic slip and the spatiotemporal evolution of early afterslip within the first 36 h. In particular, the sub-daily solution can provide more accurate and quicker results, nearly 10% smaller than those with the daily solution. Furthermore, there is significant ground motion in the immediate postseismic period, which decreases rapidly thereafter. The largest postseismic deformation observed during the first 2 h occurred at station CONZ and amounted to 3.6 cm. During the immediate postseismic period of the 2010 Maule earthquake, afterslip is the dominant mechanism, while poroelasticity plays a negligible role within the first 36 h. Meanwhile, early aftershocks tend to occur in the boundary and the inner part of the afterslip, indicating that the afterslip has the potential to drive the occurrence of aftershocks in the initial stages of postseismic activity.

Evaluation of the b Maps on the Faults of the Major (M > 7) South California Earthquakes

AbstractWe use the Godano et al. (2022, https://doi.org/10.1029/2021ea002205) method for evaluating the b maps of the faults associated with the largest earthquakes M ≥ 7.0 that occurred in California. The method allows an independent evaluation of the b parameter, avoiding the overlap of the cells and the omission of some earthquakes, while keeping all the available information in the catalog. We analyzed four large earthquakes: Landers, Hector Mine, Baja California, and Searles Valley. The maps obtained confirm that the b value can be considered as a strain meter and allow us to elucidate the presence of barriers, such as obstacles to the propagation of the fracture, on the fault of the analyzed earthquakes. A further estimated parameter is the time window during which aftershocks occur in the cell, Δt. This quantity is very useful for a better definition of the aftershock generation mechanism. It reveals where the stress is released in a short time interval and how the complexity of the faulting process controls the occurrence of aftershocks on the fault, and also the duration of the entire sequence.

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.

Analysis of the Fractal Dimension, b-value, Slip Ratio, and Decay Rate of Aftershock Seismicity Following the 6 February 2023 (Mw 7.8 and 7.5) Türkiye Earthquakes

On 6 February 2023, Türkiye experienced a pair of consecutive earthquakes with magnitudes of Mw 7.8 and 7.5, and accompanied by an intense aftershock sequence. These seismic events were particularly impactful on the segments of the East Anatolian Fault Zone (EAFZ), causing extensive damage to both human life and urban centers in Türkiye and Syria. This study explores the analysis of a dataset spanning almost one year following the Turkiye mainshocks, including 471 events with a magnitude of completeness (Mc) ≥ 4.4. We employed the maximum likelihood approach to estimate the b-value and Omori-Utsu parameters (K, c, and p-values). The estimated b-value is 1.21 ± 0.1, indicating that the mainshocks occurred in a region characterized by elevated stress levels, leading to a sequence of aftershocks of larger magnitudes due to notable irregularities in the rupture zone. The aftershock decay rate (p-value = 1.1 ± 0.04) indicates a rapid decrease in stress levels following the main shocks. However, the c-value of 0.204 ± 0.058 would indicate a relatively moderate or low initial productivity of aftershocks. Furthermore, the k-value of 76.75 ± 8.84 suggests that the decay of aftershock activity commenced within a range of approximately 68 to 86 days following the mainshocks. The fractal dimension (Dc) was assessed using the correlation integral method, yielding a value of 0.99 ± 0.03. This implies a tendency toward clustering in the aftershock seismicity and a linear configuration of the epicenters. The slip ratio during the aftershock activity was determined to be 0.75, signifying that 75% of the total slip occurred in the primary rupture, with the remaining fraction distributed among secondary faults. The methodologies and insights acquired in this research can be extended to assist in forecasting aftershock occurrences for future earthquakes, thus offering crucial data for future risk assessment.

Temporal Characteristics of the February 6, 2023 MW7.8, Turkey Earthquake Aftershock Sequence

The devastating earthquake with moment magnitude MW7.8, (local magnitude ML=7.5) which occurred in the East Anatolian Fault Zone, westwards of Gaziantep, Turkey on February 6, 2023 was followed by an intensive aftershock activity. Studying time distribution of aftershocks is important for understanding physics of the earthquake generation process.
 In the present study temporal pattern of aftershock sequence of the 2023 MW7.8 Turkey earthquake is analyzed. Because of the factors such as location and radiation pattern and the cumulative nature of building damage, aftershocks can cause more damage than the main shock. The temporal clustering of aftershocks is a dominant non-random element of the seismicity, so when the clusters are removed, the remaining activity can be modelled (as first approximation) as a Poisson process.
 On the assumption that aftershocks are distributed in time as a non-stationary Poisson process, we use the maximum likelihood method for estimating the parameters (K, c and p) of the modified Omori formula. For testing the goodness of fit between the aftershock occurrence and different statistical models, a transformation from the time scale t to a frequency-linearized time scale τ is applied. Akaike's information criteria is used to select the best model for the temporal distribution of aftershocks.
 It has been found that temporal distribution of the 2023 Turkey earthquake aftershocks is dominated by the classic power law decay in time and suggests the existence of secondary aftershock activity.

InSAR, Gravity, and Geomagnetic Observations Suggest the 2020 Mw 6.5 Stanley Earthquake Occurred on a Blind Strike-Slip and a Normal Fault

ABSTRACT The 2020 Mw 6.5 Stanley earthquake occurred in western North America and is the largest earthquake in Idaho state in the past ∼40 yr. The hypocenter is located at the western margin of the Centennial Tectonic Belt (CTB), which is a subprovince of the Basin and Range and characterized by a series of normal faults. In this study, the coseismic Interferometric Synthetic Aperture Radar observations, geophysical data, and aftershocks are jointly collected to estimate the seismogenic fault of the 2020 Stanley earthquake. The best-fitting fault models suggest the earthquake occurred on two blind faults. Slip on the main, west-dipping, seismogenic fault was sinistral and on the second fault predominantly normal slip with a minor component of right-lateral strike slip. Geophysical observations including gravity and geomagnetic data are consistent with the fault interpretations. The Coulomb failure stress (CFS) change indicates that the rupture of the main seismogenic fault had a positive triggering effect on the slip of the second fault and may have increased the rupture risk of the Sawtooth fault. Moreover, a potential seismic hazard risk zone with a notably low occurrence of aftershocks is found in the west of the central segment of the main seismogenic fault.

Investigating the sensitivity of losses to time-dependent components of seismic risk modeling

Conventional earthquake risk modeling involves several notable simplifications, which neglect: (1) the effects on seismicity of interactions between adjacent faults and the long-term elastic rebound behavior of faults; (2) short-term hazard increases associated with aftershocks; and (3) the accumulation of damage in assets due to the occurrence of multiple earthquakes in a short time window, without repairs. Several recent earthquake events (e.g. 2010–2011 Canterbury earthquakes, New Zealand; 2019 Ridgecrest earthquakes, USA; and 2023 Turkey–Syria earthquakes) have emphasized the need for risk models to account for the aforementioned short- and long-term time-dependent characteristics of earthquake risk. This study specifically investigates the sensitivity of monetary loss (i.e. a possible earthquake-risk-model output) to these time dependencies, for a case-study portfolio in Central Italy. The investigation is intended to provide important insights for the catastrophe risk insurance and reinsurance industry. In addition to salient catastrophe risk insurance features, the end-to-end approach for time-dependent earthquake risk modeling used in this study incorporates recent updates in long-term time-dependent fault modeling, aftershock forecasting, and vulnerability modeling that accounts for damage accumulation. The sensitivity analysis approach presented may provide valuable guidance on the importance and appropriate treatment of time dependencies in regional (i.e. portfolio) earthquake risk models. We find that the long-term fault and aftershock occurrence models are the most crucial features of a time-dependent seismic risk model to constrain, at least for the monetary loss metrics examined in this study. Accounting for damage accumulation is also found to be important, if there is a high insurance deductible associated with portfolio assets.

Elevated collapse risk based on decaying aftershock hazard and damaged building fragilities

This article proposes a framework to support postearthquake building safety and reoccupancy decisions by quantifying the change in building collapse risk following a mainshock earthquake event. This risk may be exacerbated by both an increase in seismic hazard due to aftershock activity and a reduction in building collapse resistance due to structural damage. To address these factors, the framework is based on a hazard that includes (1) both the steady-state and the aftershock occurrence rates, that is, the elevated hazard that accounts for the dependence on the mainshock magnitude and the aftershock rate that decays over time, and (2) revised collapse fragility functions that account for structural damage sustained during the mainshock. The framework is capable of addressing region-specific questions such as (1) What are the mainshock magnitudes for which aftershocks pose a life-safety concern? (2) How long does it take for the elevated risk due to aftershocks to dissipate? and (3) What gaps in current knowledge deserve further attention from the earthquake engineering and seismology communities? The framework addresses these questions for a 20-story building in San Francisco, assuming three different, hypothetical mainshock events of magnitudes 7,7.5, and 8 M W on the San Andreas fault. This is followed by a parametric study that considers a range of buildings and provides a graphical representation of the elevated risk to inform building evaluation (tagging) decisions, based on the intact building’s collapse capacity, the amount of structural damage, and the length of time after the mainshock.

Including stress relaxation in point-process model for seismic occurrence

SUMMARY Physics-based and statistic-based models for describing seismic occurrence are two sides of the same coin. In this paper, we compare the temporal organization of events obtained in a spring-block model for the seismic fault with the one predicted by probabilistic models for seismic occurrence. Thanks to the optimization of the parameters, by means of a Maximum Likelihood Estimation, it is possible to identify the statistical model which fits better the physical one. The results show that the best statistical model must take into account the non-trivial interplay between temporal clustering, related to aftershock occurrence, and the stress discharge following the occurrence of high magnitude main shocks. The two mechanisms contribute in different ways according to the minimum magnitude considered in the data fitting catalogue.

Multiple segmentation and seismogenic evolution of the 6th February 2023 (Mw 7.8 and 7.7) consecutive earthquake ruptures and aftershock deformation in the Maras triple junction region of SE-Anatolia, Turkey

On 6th February 2023 (UTC), two consecutive and catastrophic earthquakes with moment magnitudes (Mw) 7.8 and 7.7 struck the Maras Triple Junction (MTJ) region in SE Anatolia along with dozens of aftershocks, causing numerous casualties and significant building damage, and generating the most complex and longest surface ruptures ever observed in Turkey. The main driving mechanisms of this complex double event are still unresolved and remain controversial, even though they are likely linked with conventional fault activations, recurrence intervals and seismic gaps. Here, the aim was to gain insight into the source regimes and rupture processes of both events and their relationship with resolved fault focal solutions for the observed aftershocks, and to present an interpretation that accounts for the most puzzling aspects of the fault rupture models. In line with this, the co-seismic slip distributions of these two events were examined by joint analyses of centroid moment tensor (CMT) and finite-fault source inversions using regional and teleseismic broadband observations. Inversion results indicate that both earthquakes were left-lateral strike-slip events, and the main ruptures extended mainly from close to NNE to SSW and E to W, with maximum slips of ∼6.5–10 m, mostly confined to a shallow depth range of ≤ ∼10–15 km and extending to the surface, indicating bilateral source processes with an average rupture velocity of ∼3.5–5.5 km/s. The estimated total seismic moment range was 4.94–8.22 × 1020 N m, associated with ∼352–152 km long (along strike) and ∼ 25 km wide (along dip) fault planes at focal depth of ∼10 km. Regional CMT results indicate nearly pure normal-slip and left-lateral normal oblique-slip focal mechanisms and shallow centroid depths (≤ ∼15 km) for the early aftershock distribution that are obviously complementary with the co-seismic bilateral rupture propagations. This result highlights that double pull-apart branching of focal mechanisms for aftershock occurrence implies interacting fault ruptures embedded in the MTJ area, where two sub−/supershear-rupturing faults meet, thus explaining multiple segmentation and seismogenic evolutions of two interrelated mainshocks, i.e. “triple junction earthquakes”. The results reveal that the MTJ tends to migrate to the SSW and likely drives the SSW-stepping of the left-lateral strike-slip shear (∼136 km). This accounts for the peak slips, long co-seismic fault ruptures and the associated faulting styles. Hence, the co-seismic faulting apparently distributed across the MTJ may reflect triple junction migration, and thus large extension at the core of the Anatolian-Arabian plates, leading to very high seismic hazard in similar junction regions of the country.

Incorporating Foreshocks in an Epidemic-like Description of Seismic Occurrence in Italy

The Epidemic Type Aftershock Sequence (ETAS) model is a widely used tool for cluster analysis and forecasting, owing to its ability to accurately predict aftershock occurrences. However, its capacity to explain the increase in seismic activity prior to large earthquakes—known as foreshocks—has been called into question due to inconsistencies between simulated and experimental catalogs. To address this issue, we introduce a generalization of the ETAS model, called the Epidemic Type Aftershock Foreshock Sequence (ETAFS) model. This model has been shown to accurately describe seismicity in Southern California. In this study, we demonstrate that the ETAFS model is also effective in the Italian catalog, providing good agreement with the instrumental Italian catalogue (ISIDE) in terms of not only the number of aftershocks, but also the number of foreshocks—where the ETAS model fails. These findings suggest that foreshocks cannot be solely explained by cascades of triggered events, but can be reasonably considered as precursory phenomena reflecting the nucleation process of the main event.

Flexible spatio-temporal Hawkes process models for earthquake occurrences

Hawkes process is one of the most commonly used models for investigating the self-exciting nature of earthquake occurrences. However, seismicity patterns have complicated characteristics due to heterogeneous geology and stresses, for which existing methods with Hawkes process cannot fully capture. This study introduces novel nonparametric Hawkes process models that are flexible in three distinct ways. First, we incorporate the spatial inhomogeneity of the self-excitation earthquake productivity. Second, we consider the anisotropy in aftershock occurrences. Third, we reflect the space–time interactions between aftershocks with a non-separable spatio-temporal triggering structure. For model estimation, we extend the model-independent stochastic declustering (MISD) algorithm and suggest substituting its histogram-based estimators with kernel methods. We demonstrate the utility of the proposed methods by applying them to the seismicity data in regions with active seismic activities.

Classification of transient triggering mechanisms of aftershocks in the post-seismic phase of the 2017 Pohang earthquake, South Korea

SUMMARY The 2017 Mw 5.5 Pohang earthquake occurred near an enhanced geothermal system site and generated thousands of aftershocks, the largest of which, a Mw 4.6 earthquake, occurred 87 d after the mainshock. Redistribution of the groundwater pressure perturbed by the mainshock has been suggested as a cause of the post-seismic stress changes triggering several aftershocks, including the time-delayed event. However, to date, possible effects of variations in pore pressure on the aftershock occurrence have not been quantified in this region. Therefore, we conducted poroelastic modelling to evaluate this contribution to spatiotemporal distribution of the aftershocks, including the delayed event, using a fully coupled hydromechanical code. To construct a poroelastic model, a segmented fault geometry and a layered lithological structure were used. In addition, we utilized a kinematic slip model, a split-node algorithm and in-situ properties to simulate reliable coseismic and post-seismic behaviour. Our reference model successfully reproduced coseismic surface deformation in a line-of-sight direction, comparable to the corresponding observation from interferometric synthetic aperture radar, and was calibrated using groundwater measurement in a well. In addition to constructing the reference model, a series of numerical simulations were conducted to explore the effects and sensitivities of various hydraulic conductivities. Finally, the modelled Coulomb stress changes and spatiotemporal distribution of the aftershocks were analysed to elucidate the transient triggering mechanisms based on conditional statements to classify the mechanisms into several subsets. The classification showed that the poroelastic effect driven by depth/conductivity-dependent fluid diffusion is more critical to aftershock occurrence than the diffusion in the entire simulation time, and we propose that the delayed earthquake of Mw 4.6 could be correlated with poroelastic triggering rather than diffusion triggering. Furthermore, we inferred that this poroelastic effect could contribute to decay of aftershocks, particularly relatively small-magnitude aftershocks, as well as slow this decay in bedrocks. However, the proposed model does not explain all of the observed aftershocks, and other driving forces or triggering mechanisms need to be considered.

Question-Driven Ensembles of Flexible ETAS Models

AbstractThe development of new earthquake forecasting models is often motivated by one of the following complementary goals: to gain new insights into the governing physics and to produce improved forecasts quantified by objective metrics. Often, one comes at the cost of the other. Here, we propose a question-driven ensemble (QDE) modeling approach to address both goals. We first describe flexible epidemic-type aftershock sequence (ETAS) models in which we relax the assumptions of parametrically defined aftershock productivity and background earthquake rates during model calibration. Instead, both productivity and background rates are calibrated with data such that their variability is optimally represented by the model. Then we consider 64 QDE models in pseudoprospective forecasting experiments for southern California and Italy. QDE models are constructed by combining model parameters of different ingredient models, in which the rules for how to combine parameters are defined by questions about the future seismicity. The QDE models can be interpreted as models that address different questions with different ingredient models. We find that certain models best address the same issues in both regions, and that QDE models can substantially outperform the standard ETAS and all ingredient models. The best performing QDE model is obtained through the combination of models allowing flexible background seismicity and flexible aftershock productivity, respectively, in which the former parameterizes the spatial distribution of background earthquakes and the partitioning of seismicity into background events and aftershocks, and the latter is used to parameterize the spatiotemporal occurrence of aftershocks.

An evaluation of the characteristic aftershock parameters following the 24 January 2020 Mw = 6.8 Elazığ-Sivrice (Türkiye) earthquake

A comprehensive evaluation of region-time-magnitude behaviours of aftershocks following the 24 January 2020 (Mw = 6.8) Elazığ-Sivrice (Türkiye) earthquake was achieved by using the characteristic parameters such as b-value, p-value, Dc-value and Mamax value of aftershock occurrences. The b-value was calculated as 0.82 ± 0.02 by considering the magnitude of the completeness value as Mcomp = 1.9, and it is relatively small compared to typical b ≈ 1 for the magnitude-frequency relationship of aftershocks. This low b-value may also be caused by the abundance of aftershocks with ML ≥ 4.0. The p-value was computed as 0.80 ± 0.02 with c-value = 0.279 ± 0.098 and is smaller than the global value of p ≈ 1. This low p-value may be due to a relatively slow decay rate of aftershock activity, and the modified Omori model seems appropriate for the estimation of decay parameters. The Dc-value was estimated as 1.87 ± 0.07. This large value shows that aftershocks are homogeneously distributed and more clustered at larger scales/in smaller areas. The temporal variation of b-value indicates that decreases in b-value may result from the gradual increase in the effective stress following the larger aftershocks. The lowest b-values and Mamax values greater than 5.0 were observed in the north, south and southwest parts of the mainshock including Pütürge and Erkenek segments. These results show that there is an apparent relation between the smallest b-values and the largest Mamax values. The largest p-values were estimated in and around the main shock including Pütürge segment. The regions with the smallest b-value and the largest p-value have high stress and coseismic deformation, respectively. Stress variations and coseismic deformation are extremely effective on the changes of b- and p-values. As a remarkable result, aftershock hazard following the mainshock may be considered extremely related to aftershock parameters, and detailed analyses of the region-time-magnitude characteristics of aftershocks are recommended for a preliminary evaluation following the mainshock.