Abstract The reflectivity method for computing synthetic seismograms at the ground surface has been extended to include buried receivers while maintaining computational stability. Three-component seismograms are computed for two test cases and compared with results obtained through other methods, confirming the validity of the extension. Another example based on a real situation is used to examine the common assumption that shallow borehole seismograms can be approximated by surface seismograms. The results support this assumption for the vertical component, but not for the horizontal components, where both amplitudes and waveforms differ substantially.
Abstract We present Valutazione dell’Impatto e Degli Effetti Attesi del Terremoto—assessment of the impact and expected effects of the earthquake, a system that provides a time-evolving assessment of earthquake effects in Italy in terms of macroseismic intensity, by combining the most up-to-date information available in real time at predefined intervals after the event. The automated system is designed to integrate intensity values from different sources, namely attenuation models, accelerometric data, crowdsourced reports, and field-assessed intensities from trained technicians. Data integration is performed using a logical decision tree that operates at each timestep to select the best available source as data evolve in both quantity and quality, together with a spatially variable smoothing procedure. Specific weights were assigned to intensities according to the accuracy of each data type, with the highest weights given to observed intensities, namely, values assessed using at least three crowdsourced reports and field-assessed values. After assigning macroseismic intensity to each municipality, the system estimates fatalities and economic losses across Italy, providing stakeholders with information to better guide emergency response procedures. The article analyzes specific earthquakes to show how intensity estimates improve over the hours following the event as instrumental data and citizen-reported and field-assessed intensities become progressively available.
Abstract Computing magnitudes is a key part of characterizing earthquakes. They are especially important for developing the magnitude–frequency distributions that are used to forecast shaking from future large earthquakes. Although dozens of magnitude scales have been proposed, moment magnitude (Mw) is preferred because it does not saturate for large events and it is related to physical source properties such as fault area. However, it is challenging to resolve Mw for small earthquakes (Mw<3.5), which dominate regional earthquake catalogs, and seismic networks typically report regional magnitudes (e.g., mbLg, ML, Mc, and Md). Therefore, it is common to develop simple relations that convert regional magnitudes into Mw before calculating magnitude–frequency distributions. Conversion relations can add bias and variance to seismic hazard analysis, and ideally Mw would be calculated directly for all earthquakes. To address this issue, we applied a spectral modeling framework to estimate Mw from Sg waves recorded at distances of 10–300 km for 25,712 natural tectonic earthquakes that occurred in or near Utah between 1 January 2001 and 30 June 2025. The procedure is simple, fast, and robust, and can be run automatically in near-real time, thus potentially contributing to earthquake response products. Refinements to time windows or waveform selection can be implemented during analyst review, similar to the existing workflow for ML or Mc. We determined high-quality moment magnitudes for 63% of the target earthquakes, increasing the number of Utah events with an Mw by a factor of ∼170. The resulting Mw catalog contains 16,173 values ranging from 0.54 to 5.54 with a median of 2.06. A declustered version has a magnitude of completeness of 2.2 and a b value of 1.14 ± 0.02. Its large size will enable improved resolution of spatial and temporal variations in b value, which may allow inference of crustal stress variations in Utah.
Abstract Laboratory testing of digitizers and seismometers helps ensure that, before deployment, the instrumentation can produce high-quality data and is operating within specifications. In this work, we detail the software package called the Albuquerque Seismological Laboratory digitizer test suite. This Java software package provides several algorithms to verify various performance parameters of digitizers commonly used for recording analog seismic instruments. The goal of these tests is not to be exhaustive, but to identify common failures that could compromise the integrity of seismic data being recorded on the digitizer. For example, Sandia National Laboratories (e.g., Slad and Merchant, 2018) routinely do comprehensive testing of digitizers for various monitoring missions. Although these test reports are valuable for comprehensively characterizing a recording system, it would be resource-intensive to conduct such tests on every seismic recorder used in a network. We focus on tests that include ways to estimate the sensitivity, timing, self-noise, and clip level of the digitizer, as well as the fidelity of the signal being recorded. The software is publicly available and provides a way for the community to verify the integrity of a digitizer using a minimum amount of outside equipment.
Abstract Distributed acoustic sensing (DAS) is rapidly emerging as a powerful complement to conventional seismic monitoring, enabling dense spatial sampling over tens of kilometers using existing fiber-optic telecommunication infrastructures. In this study, we present the design, implementation, and initial performance of a regional-scale DAS seismic monitoring network integrated within the operational framework of the Seismological Research Center of OGS in north-eastern Italy. The system is built upon dark fiber segments of a public telecommunication network and interrogated by multiple DAS units, covering several hundred kilometers of optical fiber and producing tens of thousands of virtual sensing channels. We describe the end-to-end workflow developed to support continuous DAS acquisition in an operational environment, including data conversion from proprietary formats to standard seismological archives, channel aggregation strategies, metadata handling, and real-time streaming through SeedLink and International Federation of Digital Seismograph Networks services. Attention is given to practical constraints related to data volume, latency, channel naming, and integration with existing seismic monitoring infrastructures. Using representative examples, we illustrate the capability of the network to record teleseismic and local seismic signals, to characterize spatially variable seismic noise conditions along fiber routes, and to contribute to earthquake location when combined with conventional seismic stations. These case studies highlight both the opportunities and current limitations of distributed acoustic sensing (DAS) for regional seismic monitoring, including issues related to channel localization, directional sensitivity, and effective sensing range. The presented network represents one of the most extensive operational DAS deployments for seismic monitoring in Europe to date. Our results demonstrate the feasibility of integrating large-scale DAS systems into routine monitoring activities and provide a reference framework for future developments toward multisensor, fiber-optic-based seismic observatories.
Abstract Foreshocks may offer critical insights into the nucleation process of an upcoming mainshock, yet their underlying physical mechanisms remain a topic of ongoing debate. On 5 and 6 May 2018, two significant earthquakes with magnitudes of Ms 4.8 and 5.2, respectively, struck Chengduo, China. Notably, a sequence of foreshocks began three days prior to the Ms 4.8 earthquake. In this study, we conduct a comprehensive analysis of the earthquake sequence, including earthquakes detection and relocation, focal mechanisms inversion of two mainshocks, rupture dimension estimation and stress perturbation analysis of the large foreshocks, and repeating foreshocks identification. A total of 638 earthquakes were detected, which is about 3.4 times that of the catalog. The spatial-temporal evolution of earthquakes shows the foreshocks and the Ms 4.8 mainshock occurred on the same unmapped northeast-east–southwest-west-trending, near-vertical fault. The foreshock activity process can be divided into two stages. The rupture patches of foreshocks within the same stage exhibit significant overlap, and two groups repeating earthquakes identified in foreshock sequence, indicate aseismic slip occurred during the period of foreshock activity. The epicenters of two stages are concentrated in two adjacent regions, and some large foreshocks occurred in the stress increased region, suggest that stress transfer may also play an important role in the triggering process. Our results support a combination effort of aseismic preslip and cascade stress triggering in the nucleation process of the 2018 Chengduo doublet.
Abstract The Longriba fault zone (LRBFZ) represents a key tectonic transition zone where the eastward extrusion of the eastern Tibetan plateau is obstructed by the rigid Yangtze block, making it crucial for understanding regional crustal deformation and seismic hazard. However, the lack of dense seismic arrays has prevented previous studies from resolving its shallow, high-resolution velocity structure. In this study, we deployed 309 short-period nodal seismometers across the middle segment of the fault zone from April to May 2024. Using ambient noise tomography, we constructed a high-resolution 3D anisotropic model of the shallow crust (0–12 km), including both isotropic shear-wave velocity (VS) and azimuthal anisotropy. Our isotropic VS model reveals (1) a pronounced low- to high-velocity contrast across the fault zone in a southeastward direction that is consistent with regional geological outcrops and (2) an enhanced low-velocity anomaly near the fault linkage zone of LRBFZ and the Aba South fault (ASF), likely indicating intense rock fracturing and fluid enrichment. Azimuthal anisotropy further indicates that fast velocity directions are generally northwest-west–southeast-east (NWW-SEE) oriented, in agreement with the regional stress field driven by the eastward extrusion, but exhibit local rotations in the fault linkage zone caused by stress redistribution along ASF. Our study provides new constraints on the structural heterogeneity and deformation processes governing seismic behavior and fault zone dynamics of the LRBFZ.
Abstract Teleseismic records from the Hellenic Unified Seismic Network for 815 events with magnitudes mb≥6, from 2011 to 2021, were thoroughly analyzed to provide the most comprehensive set of arrival-time picks and amplitude measurements. Analysis reveals a significant increase in observations compared with those reported to the International Seismological Centre (ISC), particularly for depth phases, S-type phases, and amplitudes used in broadband magnitude calculations. The short-period body-wave magnitudes (mb) presented in this study exhibit considerable scatter relative to those from the two main agencies, ISC and the National Earthquake Information Center. The magnitudes, especially for larger events, tend to be higher when derived from Greek stations. A key contributing factor to the scatter appears to be the inconsistent measurement of seismic wave periods as observed at ISC. In contrast, broadband surface-wave magnitudes (Ms) obtained from the present analysis show a near one-to-one correlation with the ISC standard Ms_20, indicating greater internal consistency. The use of broadband Ms is therefore recommended over the standard Ms_20 because it avoids the limitations associated with amplitude measurements in the 18–22 s period range, which is employed in approximately 85% of ISC Ms_20 readings. Broadband body-wave magnitudes (mB) are systematically higher than short-period mb values. However, due to the limited availability of comparable studies, no definitive conclusions can be drawn regarding this difference. A global map of surface-wave magnitude residuals Ms calculated per event reveals a consistent pattern that correlates well with the global surface-wave attenuation model, offering further support for its validity. This study shows that detailed teleseismic analysis by local networks improves local catalogues, reveals network-specific features, and contributes valuable data to global seismology. At the same time, it highlights the near absence of broadband magnitude usage, despite the IASPEI definition over 15 yr ago.
Abstract The b-value (i.e., the slope of the Gutenberg–Richter magnitude–frequency distribution) has been reported as a potential seismic precursor, implying that its variation over time can provide diagnostic information about the imminent occurrence of large earthquakes. However, careful data analysis and rigorous statistical approaches are essential, as the estimation of the b-value—and, more importantly, the windowing process—is strongly affected by statistical noise and biases. This study proposes a methodology to optimize b-value time series to effectively unravel seismogenic changes from statistical fluctuations. We implement a time-based moving-window approach and assess the significance of b-value variations (through lag-one autocorrelation analysis) to initially capture a global view (i.e., large-scale view obtained with nonoverlapping windows) of the temporal evolution of the b-value. To reveal the small-scale variations, the time series is subsequently refined by applying window overlapping, which provides higher temporal resolution. The procedure yields a reference configuration (window size and overlapping percentage) for which the resulting b-value variations are statistically robust and physically interpretable, in agreement with empirical expectations.
Abstract In 1984, the National Earthquake Prediction Evaluation Council (NEPEC) recognized that the 40 km long Parkfield section of the San Andreas fault is the best location to trap a significant earthquake and recommended that the U.S. Geological Survey (USGS) establish a focused earthquake prediction experiment at Parkfield. The Parkfield earthquake prediction experiment established in 1985 was designed to determine if short-term earthquake prediction was feasible and to attempt to issue a public warning before the next Parkfield mainshock. Networks of instruments to record all credible earthquake precursors were installed, and Parkfield was soon the best-instrumented earthquake region in the world. A prediction plan was reviewed and approved by NEPEC and by the California Earthquake Prediction Evaluation Council (CEPEC). The Director of the USGS and the Governor of California delegated their authority to issue a prediction if the plan was followed. The Parkfield earthquake prediction remains the only earthquake prediction ever officially recognized by the U.S. government or by the State of California. Data recorded before, during, and after the 28 September 2004 Parkfield earthquake have resulted in fundamental advances in our understanding of earthquake physics (e.g., Harris and Arrowsmith, 2006). Only subtle precursory changes in strain were recorded before the 2004 earthquake, too small to provide a basis for reliable public warnings. Reliable short-term earthquake prediction remains unattainable. Ground motion at 8 sites within 1 km of the 2004 rupture and at 40 sites within 1 and 10 km were recorded, nearly doubling the global data set of strong-motion records at these distances. Large spatial variations in strong-motion amplitudes were observed. Parkfield remains the best place to trap a significant earthquake, and the next Parkfield earthquake will provide another opportunity to advance our understanding of earthquake physics.
Abstract Seismograms from a seismic station installed in the city of Guadalajara during the intense and unusual seismic swarm that occurred from 8 May to late September were recently recovered in the national historical seismograms archive of the Mexican Seismological Service. These seismograms show earthquakes recorded in August and early September 1912 by two Wiechert mechanical instruments installed with the purpose of observing this unusual sequence and as one of the permanent seismic stations of the budding Mexican Seismic Network. The earthquakes recorded in these seismograms range from barely visible signals to larger magnitude earthquakes that were reported as having been felt as moderate or strong. In some cases, it is possible to identify the arrival of the Pand S phases. The difference in arrival times suggests that the epicenter for some of these recorded events was approximately 8–12 km from the site where the Wiechert instruments were installed. This epicentral distance from the seismic station is consistent with the spatial distribution of the more intense shaking observed during the swarm. The shape and duration of the seismic traces also confirm that these events correspond to shallow earthquakes located near the seismic station. A coda-wave duration relation calibrated for crustal earthquakes in the Mexico City basin was used to estimate the magnitude of selected events. The largest estimated magnitude, interpreted as a conservative lower-bound value, was Mw 4.1, for the largest event recorded on 31 August 1912. In a classification of the intensity of shaking for earthquakes of the swarm reported instrumentally, this earthquake is classified as strong. Thus, those events classified as very strong and causing the more intense shaking most probably have a larger magnitude, compatible with the lower bound of the magnitude range inferred from macroseismic data for the largest earthquakes of the sequence.
Abstract Dynamic simulations of earthquake sequences are expensive and time-consuming due to the inherent complexity of natural fault systems and the requirement of fine discretization in time and space for good numerical resolution. Simplified, low-cost earthquake simulations can provide significant benefits, as their shorter simulation times facilitate the exploration of a broader range of parameter space and enable trial-and-error tuning. To streamline the possibility of using such earthquake simulators for comparison with real-case examples, we conducted an earthquake simulation using the pixel-tracing technique to generate 3D nonplanar geometries of the Ridgecrest earthquake, assuming that the fault geometry is the only source of stress distribution. We conduct the simulations using two different grid dimensions: 200 and 500 m. The simulation times required to produce the Ridgecrest earthquake sequence are 20 min for the 200 m grid and just 1 min for the 500 m grid on a standard desktop computer (central processing unit: Intel 13900k). We show that, in some instances, the simulation yields a correct order of foreshock–mainshock sequence similar to the observations and predicts a distribution of slip in close agreement with the measurements. Notably, the slip magnitude at the end of the simulations is comparable for both grid dimensions, suggesting that the 1 min cost-efficient simulations still yield reliable results regarding potential rupture magnitude. In contrast, the timing and rupture sequence are highly sensitive to both physical inputs and numerical schemes, such as the stress field and the grid size. Overall, we conclude that our simplified, low-cost simulation method can be a valuable tool for investigating the mechanisms underlying specific earthquake rupture sequences and simulating potential earthquakes in fault systems with complex fault geometry.
Abstract The Cascadia subduction zone (CSZ) hosts major seismic and tsunami hazards, yet key questions persist about the relationship between margin structure, fluid distribution, episodic tremor and slip, shallow megathrust behavior, shaking and tsunamigenesis, and the resulting hazard estimates. Addressing these problems requires an empirically grounded, three-dimensional seismic velocity model to illuminate subsurface structure and properties and to provide a basis for geophysical studies such as earthquake simulations and ground-motion estimation. In May 2024, the National Science Foundation-funded Cascadia Region Earthquake Science Center (CRESCENT) community velocity model (CVM) working group, with U.S. Geological Survey and regional partners, convened a workshop to identify priorities for such a model. Participants emphasized the features necessary for addressing key science questions, including implementing findability, accessibility, interoperability, and reusability (FAIR) access, capturing along-strike and along-dip structural heterogeneity, resolving shallow offshore–onshore structure, constraining elastic properties and quantifying their uncertainties for numerical wave propagation simulations, their validation benchmarks, and supporting associated accurate earthquake ground-motion simulations and hazard assessments. This article describes the priorities defined in the workshop, and a description of how, guided by these needs, CRESCENT plans to develop multiple generations of a CVM to advance CSZ science and improve seismic and tsunami hazard modeling across the Pacific Northwest. The CVM will span the CSZ from the surface to ∼100 km depth, offshore and east of the Cascades into Idaho (∼132°–110° W) and the southern and northern tectonic regime transitions (∼36°–52° N) to capture the entire tectonic system as well as its surroundings.
Abstract Alexandria in Egypt is reported to have been highly affected in A.D. 320 by an earthquake somewhat similar to the 1955 M 6.4 event, the strongest near-field event in this country since 1900. The A.D. 320 event was later characterized as spurious, reflecting a misunderstanding of a word referring to the impacts of the heresy of Arius on the Alexandria Church by the ninth century historian Theophanes. Shedding light on this earthquake is important because it is included in earthquake catalogs and studies of seismic risk; the latter is high due to the type of structures and foundation soils of Alexandria, a sprawling city of nearly six million people. An analysis of the previously examined and ignored ancient texts, of their sources, of their historical context, as well as of their copying, editorial, and translation history, indicates no earthquake in Alexandria in A.D. 320, nor that this spurious event reflects the misunderstanding of an ancient word. A possibility is that the information on an earthquake in the year A.D. 320, confirmed by another sixth century book, refers not to Alexandria, but to another town, the composite name of which was corrupted. According to this hypothesis, the A.D. 320 earthquake affected Alexandria Troas, on the Aegean coast, near the Homeric Troy, and close to a strand of the North Anatolian fault and the Aegean Sea, or Alexandria Catisson (Alexandria ad Issum), modern Iskenderun, near the East Anatolian fault. Both towns are in highly seismically active areas in Türkiye (Turkey).
Abstract The microtremor horizontal-to-vertical spectral ratio (HVSR) method, as an emerging passive seismic technique, shows strong potential for high-resolution imaging of shallow subsurface structures. By incorporating full-wavefield contributions and enabling access to high-frequency signals (>5 Hz), it resolves detailed near-surface features (<200 m depth) with high lateral resolution using single-station observations. This study develops a complete HVSR methodological framework to address key challenges in metallic ore exploration, such as strong seismic scattering and poor high-frequency signal recovery. Based on diffuse field assumption theory and Markov Chain Monte Carlo inversion, we introduce a “three-step” blind inversion strategy to overcome the common absence of borehole data and prior information in mining areas. Applied to the Tonglüshan Ore District in Hubei Province, the method successfully reconstructs shear-wave velocity (VS) structures down to 200 m depth, revealing velocity anomalies correlated with different mineralization types. Key results include: (1) HVSR curves exhibit superior sensitivity and inversion performance for shallow structures compared with fundamental-mode dispersion curves; (2) shallow low-velocity anomalies (VS≈600 m/s) at 50 m depth align spatially with west-northwest- and north-northeast-trending fault zones and breccia-type ore bodies; (3) high-velocity anomalies (VS>2200 m/s) at 150–200 m depth correspond to skarn-type ore bodies in quartz monzodioritic porphyry contact zones. This work validates the reliability and effectiveness of the HVSR method for structural imaging in complex ore districts, offering an environmentally friendly and cost-effective alternative to active-source seismic surveys and a new technical approach for mineral exploration in covered regions.
Abstract The 2020 southwestern Puerto Rico seismic sequence occurred in an area not known for seismic activity and included multiple normal and strike-slip focal mechanisms. The largest earthquake, Mw 6.4, caused significant damage and permanent shoreline subsidence. The locations of the ruptured faults have been explored extensively using seismological and geodetic evidence, but no consensus has been reached. Here, we report the results of a high-resolution seismic reflection survey that identified a north-verging 20 km long curved normal fault spanning the upper reach of the Guayanilla canyon, which we call the Guayanilla Canyon fault. Part of the fault likely ruptured during the Mw 6.4. The concave-to-the-north fault bounds a south-dipping ∼1 km deep asymmetric basin. This long-lived structure, identified also on sediment isopach and magnetic anomaly maps, appears to control the formation of the Guayanilla canyon system, the only submarine drainage system in southern Puerto Rico, suggesting a relationship among seismic activity, geomorphology, and reef development. Our seismic reflection data do not support the existence of previously published major seismogenic faults in the area, such as the north-northeast–south-southwest Guayanilla fault zone, the Northern Boqueron Bay-Punta Montalva fault, or a northwest–southeast strike-slip fault, suggested to have ruptured as part of the Mw 6.4 earthquake. We identified smaller faults in the survey area, some of which can be correlated with seafloor expressions, and/or with changes in sediment thickness and magnetic anomalies, and/or with moderate-sized earthquakes. Multiple seismic images of Investigator fault, a prominent mid-slope feature parallel to the south coast of Puerto Rico, show buried low-angle thrusts and an abrupt change in the attitude of the overlying sediments, leading us to interpret this feature as the expression of a boundary between the now-inactive upper accretionary prism of Muertos trough and a buried back-arc basin that overlies the arc basement of Puerto Rico.
Abstract Stress drop, defined as the difference in shear stress on a fault before and after an earthquake, is a key parameter for characterizing earthquake source processes and for understanding fault strength, rupture dynamics, and seismic hazard. We present SDpy (stress drop in Python), an open-source Python package for estimating earthquake stress drop based on the spectral fitting and spectral ratio methods. SDpy supports analysis of P waves, S waves, and coda waves with either single- or multi-window approaches, and incorporates multiple theoretical source models, including the Brune and Boatwright models. The package is modular, extensible, and user-friendly, accommodating a variety of processing scenarios, such as single- versus three-component recordings, single- versus multi-station data, and single versus multiple empirical Green’s functions. This flexibility facilitates rapid processing and comparative studies of earthquake source characteristics, providing a practical tool for seismologists investigating rupture behavior in diverse tectonic environments. Such versatility makes SDpy suitable for a wide range of research and operational applications in observational seismology. Application of SDpy to a repeating earthquake sequence targeted by the San Andreas Fault Observatory at Depth yields stress-drop estimates consistent with the previous study, demonstrating the tool’s robustness and practical reliability.
Abstract The Huangtupo Linjiang Number 1 landslide (HTPLJ1) in the Three Gorges Reservoir area poses a significant geological hazard, influenced by periodic fluctuations in reservoir water level and rainfall. This study employs the horizontal-to-vertical spectral ratio (HVSR) method to investigate changes in the subsurface medium within the landslide body, using ambient noise data collected from five three-component nodal seismometers over a one-year observation period (2023). By analyzing temporal variations in peak frequencies and amplification factors, we identify distinct responses linked to shallow and deep sliding zones. Results reveal that reservoir water-level rise induces a notable decrease in peak frequencies and amplification factors for shallow sliding zones, attributed to reduced shear-wave velocities and impedance contrasts caused by increased pore-water pressure and effective stress weakening. In contrast, the deep sliding zone exhibits weaker hydrological sensitivity. Modal decomposition further delineates the dominant control of reservoir water-level fluctuations (low-frequency signals) compared with transient rainfall effects (high-frequency signals). Spatial heterogeneity in landslide response highlights the stronger influence of hydrological factors on the frontal zone relative to the rear. This study underscores HVSR as an effective tool for monitoring landslide stability dynamics and provides critical insights into hydromechanical coupling mechanisms governing landslide evolution in reservoir environments.
