Journal of Seismology最新文献

This study analyzes the characteristics of the January 23 and 24, 2022, Tabriz earthquakes in northwestern Iran. These events, with magnitudes of (M_w) 4.2 and (M_w) 4.0, occurred within the Tabriz pull-apart basin and represent the largest seismic occurrences in the region over the past two centuries. Focal mechanism solutions indicate strike-slip faulting for the first event and normal faulting with a strike-slip component for the second event. Both events occurred on NW-trending faults. Based on recorded ground motion data, the first event ruptured in the NW direction, while the second event ruptured in the opposite direction (SE). Coulomb stress changes analysis suggests that stress changes imparted by the first event, estimated about 0.1 bar at the hypocenter of the second event, likely promoted its triggering. These findings highlight the active tectonics of the Tabriz Basin and the significant seismic hazards posed by its active faults, particularly for the city of Tabriz, which has a population over 2 million and is situated within the basin.

The shallow seismic response to earthquakes is important for ground motion prediction and is controlled by the major structural heterogeneities including topography. In this study, constrained inversion of microtremor horizontal-to-vertical spectral ratios (HVSRs) and Rayleigh phase velocity dispersion curves, along with well-constrained single station HVSRs inversion, produce the first 2D near-surface shear wave velocity (({V}_{s})) model of Garhwal Himalaya, India. We analyse passive source seismic data from 158 sites, along with two-sided active seismic array records from 96 locations, to evaluate the 1D ({V}_{s}) structure for different geo-tectonic units in the studied area. To explain the observed HVSR response, we calculate the HVSR curves using both theoretical (modified Haskell matrix) and numerical (modal summation) approaches. The simulated HVSR curves agree well with the observations in ~ 1.0–25.0 Hz. We examine the feasibility of the obtained 1D ({V}_{s}) profiles through extensive synthetics. The resultant 1D ({V}_{s}) profiles were then compiled to create the 2D near-surface ({V}_{s}) models for various litho-tectonic units. High ({V}_{s}) anomalies correlate well with the major tectonic features, such as the Kaliyasaur fault, North Almora thrust, and the anticline structures, while the syncline structures, Singtali thrust, and the depression fault zones exhibit low ({V}_{s}) anomalies. The 1D ({V}_{s}) profiles of ten known stratigraphic sections clearly delineate the interface boundaries between various rock strata. For the upper 30 m depth, the ({V}_{s}) 30 value ranges from 280 m/s to 600 m/s. Our velocity model demonstrates intense rock folding and faulting beneath the region, which can be used to evaluate the local site response for improved seismic hazard assessment of the region.

In this study, we investigated the Moho reflected phases and used them to estimate the spatial variation of the Moho depth in the northwestern Iran. The Moho reflected phases are secondary phases, which can be observed at the distance range between 20 and 185 km. We used 109 earthquakes whit depth shallower than 35 km occurred from 1996 to 2017 and collected the approximate travel time of 188 PmP and 140 SmS high quality phases recorded by 36 seismic stations. We used the differential travel time of direct and the Moho reflected phases to estimate the depth of Moho. The results of the reflected phases PmP and SmS are very similar in character. Although differences are also observed, especially in the northern part of the studied area where reflection points are not well distributed. The results of the inversion of P data are more reliable owing to the accuracy of the picking of P- compared to S- phases. According to the results, the average Moho depth is about 45 km in the north and south of the North Tabriz Fault, decreasing to 43.5 km towards the eastern and northwestern parts of the study area.

We analyse the background seismicity, including mainshocks and isolated events, from a distinct clustered component using the nearest-neighbour declustering method. After declustering the seismic catalog, two components were identified: background and clustered. The clustered component includes isolated networks, and for mainshock selection within each network, we applied outdegree and closeness centrality measures from network theory. This approach differs from the conventional method, which selects mainshocks from individual clusters network based on the highest magnitude. The background events dataset was obtained using the nearest-neighbour method and network analysis. This methodology was applied to the Southern California region, encompassing four significant events with magnitudes greater than 7, over the period 1981–2021. The primary objective is to assess the relationship between background seismicity and the Poisson process, as well as to identify the magnitude threshold at which it aligns with the Poisson model. To accomplish this, the background dataset was divided into specified magnitude ranges from 3 to 4.2, with intervals of 0.2. Temporal statistical tests, including the conditional chi-square test, Brown-Zhao test, and Kolmogorov–Smirnov test, were performed, while the Luen and Stark statistical test was applied for space–time analysis. For nearly all magnitude cut-offs, the temporal statistical tests reject the null hypothesis. The exception is at a magnitude of 3.4, where the temporal test is satisfied; however, the space–time statistical test still rejects the null hypothesis. However, the background dataset for the study region does not conform to the Poisson process in either the temporal or space–time tests across all magnitude thresholds. This inconsistency may be attributed to a limited number of data points at certain magnitude cutoffs, the declustering method used, or the potential need for an alternative conditional model for analysing background events.

In earthquake engineering, the precise characterization of long-period ground motion in the form of surface waves (Love and Rayleigh type) is crucial for designing resilient structures, particularly in complex environments such as sedimentary basins. This study evaluates the efficacy of the Normalized Inner Product (NIP) method for estimating surface wave parameters using limited input data within seismic analyses conducted based on numerical simulations. The method is benchmarked against two established techniques–Six Degrees-of-Freedom Polarization Analysis (6C-Pol) and Multiple Signal Classification (MUSIC)–to evaluate its precision in parameter identification. As an example, the methodologies are first applied to analyze surface waves from synthetically generated signals and then from basin-induced surface waves coming from a simplified basin with known characteristics, employing the the spectral element code SEM3D for 3D wave propagation simulation. The results revealed that the NIP method efficiently estimated surface wave characteristics using minimal information, demonstrating its efficiency. Furthermore, due to its capacity to rapidly process large datasets, the NIP method effectively quantified basin-induced surface waves across the basin surface, offering a robust framework for a more comprehensive understanding of 3D basin effects.

We investigated stress change parameters during ruptures for earthquakes globally with M_w ≥ 6.0 from 1991 to 2023. Employing a formulation and alternative graphical method, we analyzed variations between initial and final stresses as compared to frictional stress during rupture. Our goal was to assess the validity of Orowan’s model (final stress equals frictional stress) across different environments, crucial for recurrent source studies. Our findings reveal significant deviations among event types: reverse-type events diverge slightly from Orowan’s model, while normal events show even larger discrepancy. Strike-slip events exhibit a blend of stress difference mechanisms, with around 30% displaying overshoot (final stress smaller than average frictional stress). The partial stress drop (final stress larger than average frictional stress) percentage for reverse and normal types indicates that approximately 21–23% of the available stress for rupture was not relieved. Our results suggest that partial stress drop is a widespread phenomenon across all event types. This observation implies higher energy at higher frequencies than expected for an ω² frequency decay in the source spectra (Brune, 1976), potentially leading to underestimation of expected damaging accelerations. Our observations underscore the complexity of stress dynamics during earthquakes, with potential implications for energy release and damaging effects.

Two earthquakes from the first half of the twentieth century in the Peloponnese region (southern Greece) are analyzed here. These earthquakes occurred 1 year apart in the summers of 1926 and 1927; epicentral locations in the literature show them close to each other (~ 50 km apart). The earthquakes are of particular importance for at least two reasons: they appear to be of intermediate depth and macroseismic data associated with them has been available until now in paper format. Intermediate depth earthquakes are associated with the subduction zone along the Hellenic arc and do not occur as frequently as crustal earthquakes, therefore making the revisiting of such events even more important. Historical European databases which hold macroseismic data, such as AHEAD ((Albini et al. Istituto Nazionale Di Geofisica e Vulcanologia (INGV), 2013), are essential for disseminating this data type and making the different studies for each seismic event in the catalogue available. However, AHEAD includes earthquakes that occurred prior to 1900. Paradoxically, it is easier to find the macroseismic field of an earthquake from the 1500s than one from the early twentieth century. Therefore, disseminating and analyzing the macroseismic data of the two Peloponnese events from this period, previously available only in paper format, is crucial. In line with this goal, newly collected eyewitness accounts with existing observations for these events are presented, while macroseismic intensities are re-assigned in EMS98. Re-estimated epicentres suggest that the 1926 earthquake occurred further offshore, while the 1927 earthquake was on land at the southernmost tip of the Peloponnese peninsula. Isoseismal maps were created for both events using the Rossi-Forel and EMS98 scales. The use of modern unified European macroseismic scale (EMS98) in the study of historical earthquakes is essential for harmonizing the present and past with future hazard and risk analyses. The use of EMS98 with additional IDPs in the construction of isoseismal maps further refines the macroseismic field of the 2 events, while suggesting a different fault rupture orientation.

The development of a comprehensive Probabilistic Seismic Hazard Analysis (PSHA) framework for Egypt marks a pivotal advancement in seismic hazard assessment, with significant implications for critical infrastructure, large-scale developments, and the revision of the Egyptian Building Code. This study generates Peak Ground Acceleration (PGA) and Spectral Acceleration (SA) maps, addressing the inherent complexities and uncertainties of PSHA through robust quantitative methodology. The research utilizes Kullback-Leibler Divergence (KLD) to assess the importance of logic-tree branches and evaluate the performance of the implemented models. By incorporating updated seismicity catalogs, refined seismotectonic models, and advanced ground motion prediction equations (GMPEs), the study optimizes logic-tree branch weights through rigorous statistical evaluation and sensitivity analyses. The results obtained using KLD show that the most effective seismic hazard model integrates recent seismotectonic models, GMPEs designed for shallow active crustal seismic sources, and those suited for seismic sources within the subduction zones of the Mediterranean Sea. This data-driven approach, leveraging the KLD-weighting scheme, effectively minimizes uncertainties in PSHA and enhances the reliability of parameter selection for site-specific seismic hazard analysis. The results obtained using the KLD exhibit a strong alignment with findings from previous PSHA studies conducted for Egypt. This concordance underscores the robustness and reliability of the KLD-based approach in evaluating and ranking seismic hazard models. By effectively capturing the statistical similarities and divergences among logic-tree branches, the KLD methodology not only validates the current framework against established studies but also demonstrates its capacity to refine and enhance the understanding of seismic hazard distributions in Egypt.

Effective discrimination between earthquakes and explosions is pivotal, particularly in the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification regime. This paper introduces the usage of a Support Vector Machine (SVM) algorithm tailored to discern seismic records produced by natural earthquakes from those caused by underground nuclear tests, wherein the registered values of mb and Ms magnitudes (body-wave and surface-wave magnitudes respectively) of each event are selected as feature vectors. These magnitude values are directly provided in official bulletins for each seismic event, therefore, no preliminary calculations were necessary, making our method easy to implement. By harnessing a diverse dataset and employing state-of-the-art machine learning algorithms, our approach demonstrates remarkable accuracy in discriminating these events. Also, we provide a posterior probability that estimates the correctness of the prediction performed by the classification algorithm. This work represents a significant stride towards enhancing the capabilities of seismic monitoring systems, thereby reinforcing international efforts towards nuclear non-proliferation and global stability.