Journal of Seismology最新文献

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.

In this study, we assess the state of the stress field in eastern Türkiye and discuss whether the stress field in the region has changed over time. To assess the governing stress field, we applied stress tensor inversion techniques using focal mechanisms from earthquake catalogs. The focal mechanisms were grouped into subregions based on spatial and temporal factors. We employed a damped regional-scale stress tensor inversion algorithm and an iterative joint inversion technique to determine the stress field for eastern Türkiye. Near the North Anatolian Fault Zone, the stress field exhibits strike-slip faulting with a slight counterclockwise rotation of the principal stress axes. From 1983 to 2016, reverse faulting was observed near Erzincan, transitioning to strike-slip faulting after 2017 due to changes in Smid and Smin axes. Near the East Anatolian Fault Zone, the stress field remains strike-slip, but after two major earthquakes in 2023, normal faulting emerged in areas with longitude less than 37 (^circ text{E}), likely due to stress perturbations. In the easternmost region near the Zagros Fold Belt, reverse faulting dominates the north and south, while strike-slip faulting characterizes the central part. High R-values suggest frequent interchange of Smid and Smin axes. The damped inversion method effectively smoothens stress axis transitions in data-sparse regions but introduces errors in isolated areas, where the iterative method proves more reliable. These findings illuminate eastern Türkiye’s evolving tectonic stress field.

Building 3-D regional velocity models of complex seismotectonics settings remains challenging; strong heterogeneities, resulting from plate interactions, are difficult to resolve and the lack of well constrained moment tensor solutions for the set of earthquakes used as sources makes model convergence difficult or altogether impossible. We propose a joint velocity and moment tensor elastic full waveform inversion framework in the time domain, to find a solution for P-wave and the complete Moment Tensor (MT) of the seismic sources. We synthetically validate our approach using an upscaled version of the SEAM 3-D velocity model, a standard validation test in exploration seismology known for its strong heterogeneities and velocity contrasts. Our approach shows promising results when applied to the tectonic setting of the Middle Magdalena Valley (MMV) basin, a region known for the poorly understood geometry of the underlying subducting plates. As sources we use intermediate depth earthquakes from the Bucaramanga Seismic Nest (BSN), one of the most compact and active seismicity volumes in the world. As a result, we obtain a 3-D velocity model for the region and a set of well constrained MT solutions for moderate BSN earthquakes. Our framework can be readily applied to other regions and it will be of special importance in applications dominated by moderate seismicity (Mw<5.5), where very few global MT solutions are available.

The continuous occurrence of destructive earthquakes in the Philippine Archipelago, generated by both mapped and unmapped faults, highlights the shortcomings of traditional fault-based hazard assessment techniques. The earthquakes caused by unmapped faults, in particular, emphasize the necessity of adopting area-based hazard evaluation approaches. In view of this, the present study implements an area–based earthquake nowcasting approach to statistically compute the current level of seismic hazards in 26 densely populated cities across Philippines. We utilize the concept of natural time, the inter–event counts of small earthquakes occurring between successive large earthquakes, to calculate Earthquake Potential Score (EPS) for the defined city regions. To derive the natural time statistics, we incorporate a diverse range of reference probability distributions, including heavy–tailed, time–dependent, time–independent, and exponentiated group of distributions. Statistical inference for observed natural times reveals that (1) the Weibull distribution provides the best representation; (2) as on August 15, 2024, the EPS values (%), corresponding to M (ge ) 6.5 earthquakes for 26 cities range from 09% to 71%, with Tacloban (71%), Cagayan de Oro (69%), Dasmarinas (64%), Bacoor (63%), Las Pinas (63%), Manila (62%), Paranaque (61%), Taguig (60%), Valenzuela (60%), Makati (60%), Quezon City (58%), Pasig (58%), Caloocan (56%), Antipolo (55%), Marawi (55%), Zamboanga (54%), San Jose Del Monte (53%), Legazpi (44%), Cebu (39%), San Carlos (31%), Bacolod (28%), General Santos (27%), and Davao (09%), and (3) the nowcast scores are consistent despite some variations in threshold magnitude and city regions. These EPS values provide a unique measure to determine the ongoing progression of the earthquake cycle of large sized events of the target regions, enabling a consistent city ranking based on their current level of seismic progression. The nowcasting approach and emanated results offer valuable insights for informed decision–making to enhance preparedness and risk management strategies across the Philippine Archipelago.

The volcanic region of Mt. Etna (Italy) has a well-documented historical seismic activity, with records of seismic and volcanic events on the volcano dating back to late 1633. This historical data, covering a time span longer than that recorded by instrumental seismological data, is a testament to the reliability of the intensity-magnitude relations, the only means to obtain macroseismic information, the sole indicator of the energy released by earthquakes. Previous studies in the literature have proposed various methods for converting epicentral intensity into macroseismic magnitude for the Etna region. Still, these methods were based on older datasets with limited instrumental data. The updated relationship proposed in the paper significantly improves the accuracy of macroseismic magnitude estimates, aligning them more closely with local magnitudes calculated for recent earthquakes. The study uses a dataset of 58 volcano-tectonic events from 1997 to 2018, with magnitudes between 2.5 and 4.8 and intensities ranging from IV to VIII on the EMS scale. The instrumental magnitudes were obtained from the Mt. Etna seismic catalogue and the Italian seismological database, while macroseismic data were sourced from the macroseismic catalogue of Etnean earthquakes. In the volcanic area of ​​Etna, macroseismic epicenters are often located very close to the sites where the maximum intensity is observed, this is due to the strong attenuation of seismic energy and the shallowness of the epicenters. For this reason, the epicentral intensity is generally assumed to be equal to the maximum intensity. The new relationship is tailored explicitly for shallow earthquakes (H ≤ 3 km), which are the most recurrent. It includes a correction factor for depth, making it applicable to deeper events and enhancing its relevance in real-world scenarios.

The evolution of the study of macroseismic effects of earthquakes in Catalonia is presented. From the first attempts to develop historical earthquake catalogues for the region to the recently introduced automatic analysis of questionnaires, a long way has been covered and new projects are on way. Macroseismic studies had been extremely important for the seismic hazard and risk assessment of Catalonia as the most important damaging earthquakes of the region are historical earthquakes. The most recent seismic hazard and risk assessments are based mainly on the catalogue of historical earthquakes from the “Atles Sísmic de Catalunya” (Susagna and Goula 1999) compiled 25 years ago. Having a better understanding of the damaging earthquakes of the past through improving the data and quality of the location and intensity determination also contributes to generate damage scenarios that can be used to build public awareness about the seismic risk affecting the region and promote the creation of seismic crisis action plans like the Special seismic emergency plan for Catalonia (SISMICAT 2003–2020). In the recent decades several new macroseismic studies had been completed that indicate that the Catalonia historical earthquake catalogue must be updated. This future updated catalogue is of key importance to develop the seismic hazard and risk scenarios to be included in the next update SISMICAT action plan.

The ground-motion duration, the time window in which the ground motion time history is considered to be strong, may be classified into three main groups, i.e., uniform, bracketed, and significant duration. In the current study, the three definitions were employed to argue why "significant duration" is the preferred model to calculate shear wave duration and how it was derived using a modified approach. Using the modified approach, for the first time, the path-dependent part was estimated in Zagros region. The Simulated Annealing inversion method was employed to regress a piecewise-linear function to the medians of path duration versus rupture distance. The data used includes acceleration waveforms obtained from the Iranian strong motion network operated by the Building and Housing Research Center of Iran (BHRC). The events occurred between May 1997 and July 2022 with magnitudes between ({text{M}}_{text{W}}) 5.0 and ({text{M}}_{text{W}}) 7.3. Using the modified model shows a reduction in duration in the distance range between 100 and 160 km and its effect on ground motion intensity measures (GMIMs). Moreover, the region-specific model predicts a longer duration than those provided by both the traditional model and the model proposed for western North America. Additionally, the contribution of source- and path-dependent duration to the total duration across a wide range of magnitudes and distances was analyzed. The analysis indicated that an accurate estimation of path-dependent duration, even for short distances and large magnitudes, is essential. To demonstrate the reasonableness of the modified model, the EXSIM program (enabling dynamic frequency mode) was employed to simulate peak ground acceleration (PGA) values for earthquakes greater than ({text{M}}_{text{W}}) 6.0. Additionally, residual analysis illustrated that the modified model predicted PGA with accuracy and negligible trends. We propose that for other regions with different crustal thicknesses, a new region-specific model must be developed.

The slip models of the Oaxaca, Mexico earthquakes of 29 November 1978 (M_w 7.6) and 23 June 2020 (M_w 7.4) were estimated by inverting P and SH teleseismic velocity waveforms. The inversion of the 1978 event used broadband and long-period data. In the case of the 2020 event, broadband data were available. In both cases, the rupture zones lie down dip of the hypocenter. It has been suggested that the events of 1978 and 2020 are quasi-repeater earthquakes breaking similar asperities of previous events. Based on this, the slip of the seismic rupture obtained in recent events is used to characterize the slip of previous events, and to calculate the slip deficiency in the four rupture zones defined by the 1928 events. The largest slip deficiency is where the large 7.6 event occurred in October 1928, between the ruptures of 1978 and the Mw 7.2 earthquake of June 2018. Here, no great earthquakes have occurred in the last 96 years, suggesting high accumulation of elastic strain that may generate potentially an earthquake Mw 7.8. This gap separates two regions with different seismic behavior, suggesting a complex rupture process in the Oaxaca subduction zone. The other three regions, where the 1978, 2018, and 2020 earthquakes took place, show average slip deficiencies of 500 cm. The great earthquake of 1787 broke the four rupture areas defined by the 1928 events in a single Mw 8.6 earthquake, consistent with a variable rupture mode that has been observed in other subduction zones of the world. In conclusion, the Oaxaca subduction zone suggests a high seismic potential.

The Isthmus of Panama experiences high seismic activity, having the potential for destructive earthquakes and serious risks to the population. Here, we present a new 1-D P-wave velocity model for Panama, which could be used for routine and accurate determination of earthquake locations, since Panama currently relies on a global velocity model. We used 23,178 P-wave arrival times from 1,672 selected seismic events between 2013 and 2022, recorded by 128 seismic stations across the country. To perform the analysis of P-wave arrival times, we utilized the Particle Swarm Optimization (PSO) method, which propagates multiple particles that explore the solution space to find the best possible velocity model. The new 1-D P-wave velocity model was obtained after multiple PSO runs, using the results of the previous run as a starting model until we find a model that best fits the seismic data. This model consists of 10 layers extending to a depth of 70 km, where the velocities range from 5.76 km/s at depths of 0-5 km to 8.27 km/s in the deepest layer. The station corrections, consistent with the geology of the Isthmus, allowed accurate relocation of earthquakes, achieving an epicentral distance error of ±3 km and a hypocentral distance error of ±6 km. These results are not only relevant for 3-D seismic tomography, but also valuable for seismic hazard and risk assessments in the Isthmus of Panama.