Earthquake Observations Research Articles

Subsurface S-wave velocity structure inversion using particle swarm optimization based on horizontal site response extracted using generalized inversion technique

A new methodology, including a mature algorithm with adjustments and a new target for the subsurface sedimentary inversion problem was introduced. This study adopted the particle swarm optimization (PSO) algorithm as a global optimization algorithm. PSO incorporates the patterns embedded in natural bird foraging behaviors to invert the subsurface S-wave velocity structure. Using the concept of particle velocity, PSO involves particle individual inertia, individual experience, and social experience of the swarm, pursuing the global optimum solution in a multidimensional abstract space. The inversion target was the horizontal site amplification factor (HSAF), which was extracted using the generalized inversion technique for the observed strong ground motions. HSAF is an appropriate target in the S-wave velocity structure inversion problem as it directly represents the site response to the incident S-wave at the seismological bedrock. We validated the convergence ability of the PSO and applied it to three field earthquake observation stations. From the perspective of the misfit between the target and estimation velocity profile, improvements exceeding 95 % and 70 % can be confirmed in the validation case and practical applications, respectively. The use of HSAF as the target and PSO as the algorithm is currently limited but is demonstrably effective. This study introduces a methodology that has significant potential for solving velocity structure inversion problems at subsurface levels.

Inclined bending seismic reflection layer in the crust illuminated by distributed fibre-optic-sensing measurements in western Japan

Distributed acoustic sensing (DAS), which enables a single fibre-optic cable to function as multiple sensors, is a technique to measure the strain rate distributed along the cable. This technique is applied to record ground motions in western Japan via a 50 km-long fibre-optic cable beneath a national road. The measured values are strain changes along the cable every 5 m, corresponding to 9788 sensor deployments. This high-density measurement along the long cable successfully recorded the 2021 M2.8 and M3.2 earthquakes that occurred in the crust within the distance of the cable in southern Kyoto. The direct S waves were followed by seismic waves approximately 8–14 s later, which were reflected by lower crustal structures. These waveforms were previously reported by observing many earthquakes via multiple seismometers, but the DAS observations clearly illuminate reflected wavefields from single earthquake observations for the first time. The numerical simulation of the strain-rate wavefields of these earthquakes reveals the existence of a north-dipping thin layer with a slow seismic velocity in the lower crust, which becomes steeper in the shallower part. This layer might represent the path of slab-derived fluid to the shallow fault zone.

Rapid Urban-Scale Building Collapse Assessment Based on Nonlinear Dynamic Analysis and Earthquake Observations

Rapid damage assessment after an earthquake is crucial for allocating and prioritizing emergency actions. Building damage due to an earthquake depends on the seismic hazard and the building’s strength. While it is now possible to promptly access acceleration data as seismic input through online strong motion networks in urban areas, good models are necessary to evaluate the damage in different zones of the affected area. This study aims to present a rapid method for such an urban-scale building collapse evaluation by conducting a nonlinear dynamic analysis of modeled buildings. Based on the Nagato and Kawase model, this study estimates the yield shear strength of 3-story steel buildings, 3-story reinforced concrete buildings, and 1-story masonry buildings in Sarpol-e-Zahab City after the 2017 Mw7.3 earthquake. The damage ratio is calculated through nonlinear dynamic analyses using estimated records from the main earthquake shock in different city zones. The research found that the seismic yield shear strength of steel and reinforced concrete buildings might be weaker than that of the Iranian seismic code’s standard value. Conversely, masonry-building resistance is stronger than the guidelines assumed. The constructed numerical models can be used for the rapid building damage assessment immediately after a damaging earthquake.

Use of tsunami observations for resolving coseismic slip of recent and historic large submarine earthquakes

Tsunamis generated by seafloor displacements accompanying large submarine earthquakes provide sensitivity to absolute slip position and distribution for offshore faulting analogous to that of geodetic observations for landward faulting. Tsunami recordings at deep‐water and near‐shore ocean bottom pressure sensors and tide gauges, along with runup and inundation measurements, can now be reliably modeled using detailed bathymetric structures and robust numerical codes. As a result, tsunami observations now play an important role in quantifying coseismic slip distributions for large submarine earthquakes in subduction zones and other tectonic environments. Applications of joint modeling or inversion of seismic, geodetic and tsunami observations for recent major earthquakes are described, highlighting the specific contributions of the tsunami observations to source model resolution. Tsunami observations provide unique information on the up‐dip extent of earthquake coseismic slip on subduction zone megathrust faults and occurrence of near‐trench slip, which are usually not well constrained by seismic and land‐based geodetic signals. Tsunami signals also help to detect offshore slow slip that is not evident in seismic or land‐based geodetic data and to balance geophysical constraints on ruptures that extend from on‐shore to off‐shore. Tsunami runup measurements and stratigraphic deposits further provide unique constraints on large earthquake ruptures that occurred prior to modern geophysical instrumentation.

A Quantitative Comparison and Validation of Finite‐Fault Models: The 2011 Tohoku‐Oki Earthquake

AbstractLarge earthquakes rupture faults over hundreds of kilometers within minutes. Finite‐fault models image these processes and provide observational constraints for understanding earthquake physics. However, finite‐fault inversions are subject to non‐uniqueness and uncertainties. The diverse range of published models for the well‐recorded 2011 9.0 Tohoku‐Oki earthquake illustrates this challenge, and its rupture process remains under debate. Here, we comprehensively compare 32 published finite‐fault models of the Tohoku‐Oki earthquake. We aim to identify the most coherent slip features of the Tohoku‐Oki earthquake from these slip models and develop a new method for quantitatively analyzing their variations. We find that the models correlate poorly at 1‐km subfault size, irrespective of the data type. In contrast, model agreement improves significantly with increasing subfault sizes, consistently showing that the largest slip occurs up‐dip of the hypocenter near the trench. We use the set of models to test the sensitivity of available teleseismic, regional seismic, and geodetic observations. For the large Tohoku‐Oki earthquake, we find that the analyzed finite‐fault models are less sensitive to slip features smaller than 64 km. When we use the models to compute synthetic seafloor deformation, we observe strong variations in the synthetics, suggesting their sensitivity to small‐scale slip features. Our newly developed approach offers a quantitative framework to identify common features in distinct finite‐fault slip models and to analyze their robustness using regional and global geophysical observations for megathrust earthquakes. Our results indicate that dense offshore instrumentation is critical for resolving the rupture complexities of megathrust earthquakes.

Deep Underground Earthquake Observation: Small to Moderate Earthquakes and Microearthquakes Identification

Deep Underground Earthquake Observation: Small to Moderate Earthquakes and Microearthquakes Identification

Spatiotemporal dominance of afterslip and viscoelastic relaxation revealed by four decades of post-1973 Luhuo earthquake observations

Measuring surface displacements driven by afterslip and viscoelastic relaxation following large earthquakes enables inferring frictional and rheological properties of Earth's outermost layers where hazardous earthquakes occur. However, the two concurrent mechanisms generate similar and intertwined surface deformations, complicating the extraction of physical information from the measurements. Here, we quantify the spatiotemporal dominance of co-evolving afterslip and viscoelastic relaxation following the 1973 Ms 7.6 Luhuo earthquake in eastern Tibet, using 42 years (1976‒2018) of fault-crossing short-baseline observations. The finite-fault slip of this M7+ earthquake was constructed from triangulation data measured in 1961‒1975. Based on the model incorporating two postseismic relaxation mechanisms and calibrated by the measurements over four decades, we show that, in the temporal domain, afterslip may produce geodetically measurable deformations (>1.5 mm/yr) in local near-field regions even 20 years after the mainshock, with spatially broader impact within ∼10 years. Viscoelastic relaxation may last for nearly six decades, imposing a broad influence for three decades. In the spatial domain, afterslip may produce deformations within ∼2 times the seismogenic depth (SD; 20 km) from the fault, but dominantly in the near-field of ∼1 times the SD. In contrast, viscoelastic relaxation may affect a much wider region reaching 200 km. While afterslip and viscoelastic relaxation collectively affected the region ∼2 times the SD from the fault in the early postseismic period, their resulting deformations gradually separated in space with time, with afterslip-induced deformation shrinking toward the fault, and surface deformation caused by viscoelastic relaxation concentrating in the mid-field, ∼2–3 times the SD from the fault. Their spatiotemporal partitioning helps better learn fault frictional properties and lithospheric rheology based on geodetic data acquired from different domains.

A Detailed Understanding of Slow Self‐Arresting Rupture

AbstractRecent numerical simulation studies suggest the existence of a seismic type that is distinct from regular earthquakes—the slow self‐arresting rupture (SSAR). Unlike regular earthquakes that propagate dynamically following the initiation, The SSARs automatically arrest within the nucleation zone without interference. Additionally, numerical simulations indicate that SSARs exhibit a significantly lower energy release compared to regular earthquakes, while also exhibiting a relatively long source duration. Given these distinctive properties, comprehending the source processes of SSARs assumes great strategic importance. However, our current understanding of SSARs, particularly regarding their response to different frictional conditions and their correlation with natural phenomena, remains limited in scope. To further explore the intricacies of SSARs, we employ a three‐dimensional fully dynamic source model to simulate SSARs under various slip‐weakening frictional conditions. The findings indicate that SSARs occur in frictional environments characterized by large normalized critical slip distances, with the seismic source process being primarily influenced by this parameter. Apart from displaying significantly smaller average slip and stress drop, which are two to three orders of magnitude lower than those of regular earthquakes of comparable magnitude, SSARs also showcase a decrease in duration, seismic moment, slip rate, and stress drop as the normalized critical slip distance increases. The moment‐duration scaling law of SSARs exhibits a linear pattern. Moreover, the observation of slow earthquakes offers further implications for the presence of SSARs, indicating their potential association with a wider range of intricate seismic phenomena.

Similarities and Differences Between Natural and Simulated Slow Earthquakes

AbstractWe investigate similarities and differences between natural and simulated slow earthquakes using nonlinear dynamical system tools. We use spatio‐temporal slip potency rate data derived from Global Navigation Satellite System (GNSS) position time series in the Cascadia subduction zone and numerical simulations intended to reproduce their pulse‐like behavior and scaling laws. We provide metrics to evaluate the accuracy of simulations in mimicking slow earthquake dynamics. We investigate the influence of spatio‐temporal coarsening as well as observational noise. Despite the use of many degrees of freedom, numerical simulations display a surprisingly low average dimension, akin to natural slow earthquakes. Instantaneous dynamical indices can reach large values (>10) instead, and differences persist between numerical simulations and natural observations. We propose to use the suggested metrics as an additional tool to narrow the divergence between slow earthquake observations and dynamical simulations.

Fast Bayesian modal identification with known seismic excitations

AbstractFast and accurate identification of structural modal parameters after an earthquake is crucial for assessing structural conditions and facilitating repair. With the development of modern earthquake observation techniques, the recorded ground motion can be leveraged as extra input information for modal identification, enabling the experimental modal analysis applicable. This study develops a Bayesian modal identification algorithm that aims at estimating the most probable value (MPV) of modal parameters and their identification uncertainty. Incorporating the recorded seismic input, the algorithm utilizes with the structural equation of motion in the frequency domain to formulate the likelihood function and adopts a constrained Laplace method for Bayesian posterior approximation of modal parameters. With the aid of complex matrix calculus, an iterative scheme is developed, allowing a fast search of the MPV of modal parameters and an analytical evaluation of the posterior covariance matrix. The performance of the proposed algorithm is validated by examples with synthetic, laboratory and field data, respectively. In addition, its effectiveness on predicting structural responses under a future earthquake is illustrated, showing its potential for various downstream applications in seismic structural health monitoring.

Stress spatial distributions, the Gutenberg–Richter and Omori–Utsu laws

We investigate several earthquake models in one and two dimensions of space and analyze in these models the stress spatial distribution. We show that the statistical properties of stress distribution are responsible for the distribution of earthquake magnitudes, as described by the Gutenberg–Richter (GR) law. A series of predictions is made based on the analogies between the stress profile and one-dimensional random curves or two-dimensional random surfaces. These predictions include the b-value, which determines the ratio of small to large seismic events and, in two-dimensional models, we predict the existence of aftershocks and their temporal distribution, known as the Omori–Utsu law. Both the GR and Omori–Utsu law are properties which have been extensively validated by earthquake observations in nature.

Investigation of seismic damage to existing buildings by using remotely observed images

A prolonged and highly destructive seismic sequence struck Central Italy, spanning from August 24, 2016, to January 2017. This series of earthquakes resulted in widespread structural failures and extensive damage across four regions along the Apennines: Lazio, Umbria, Marche, and Abruzzo. In the context of this study, based on post-earthquake remotely observed damages data, we conducted an in-depth empirical examination of the vulnerability of various types of buildings, including reinforced concrete, masonry, and steel structures. These valuable data, accessible through a Google Street View application, furnished critical insights into building locations, characteristics, and spatially observed patterns of damage before and after the seismic events. We undertook a meticulous review and comparison of this extensive dataset to ensure its consistency with prior earthquake observations, fragility functions, and relevant scientific literature. Our research involved a comprehensive examination of selected structures by combining post-earthquake inspections and visual assessments using Google Street View. This comprehensive study allowed us to compare similar buildings with and without damage, generating significant statistical insights into the types of structural pathologies that lead to localized and widespread damage, as well as instances of structural collapses or domino effects. Furthermore, we propose a preliminary glimpse at the power of Machine Learning techniques into earthquake risk assessment. We suggest that this approach could enhance the robustness and accuracy of modeling procedures, which are essential when addressing such complex challenges.

An approach for predicting surface strong motion using borehole seismometers

South Korea has implemented borehole-type seismometers for reliable earthquake observations and earthquake early-warning systems, with approximately 85% of seismometers being replaced by borehole-type seismometers after the Gyeongju earthquake. Although these seismometers are more effective at detecting earthquakes owing to the reduced artificial ambient noise, they do not record surface-level shaking. Therefore, it is necessary to estimate ground surface shaking directly associated with potential damage when using borehole-type seismometers without surface sensors. This study investigated and compared various methods, including the stochastic point-source ground-motion model, transfer function based on ambient noise, and one-dimensional site response, to estimate horizontal seismograms of the ground surface. We assessed the accuracy of these methods by comparing the waveforms generated in event cases (magnitude from 2.5 to 5.8, with epicentral distances spanning 22 km–209 km) in terms of Fourier spectra, intensity, and spectral acceleration. Among the methods assessed, the transfer function approach, which does not account for the geophysical characteristics such as VS30, proved to be the most appropriate for correcting ground-surface effects.

Mesh size effect on finite source inversion with 3-D finite-element modelling

SUMMARY Three-dimensional finite-element models, which can handle the stress perturbations caused by subsurface mechanical heterogeneities and fault interactions, have been combined with the finite source inversion to estimate the coseismic slip distribution over the fault plane. However, the mesh grid for discretizing the governing equations in the finite-element model significantly affects the numerical accuracy. In this study, we performed kinematic finite source inversion with idealized (regular observation point array; M1A–M1D) and regional (GEONET, GPS Earthquake Observation Network System stations in Japan; M2A–M2H) models with different mesh sizes to quantitatively analyse the effect of the mesh grid size around the fault plane on the inverted fault slip distribution. Synthetic observation data vectors obtained from the finest models (M1A and M2A) are compared with those from the coarser models (M1B–M1D and M2B–M2H), which were adopted to construct Green's function matrix. We found that the coarser mesh models derived a smaller surface displacement, leading to a decrease in the norm of Green's function matrix, which in turn influences the fault slip magnitude from the finite source inversion. Finally, we performed the source inversion for the fault slip distribution of the 2011 Mw 9.0 Tohoku–Oki earthquake using the coseismic surface displacements recorded at the GEONET and seafloor stations and finite-element modelling. By reducing the mesh size on the fault, we confirmed that the estimated magnitude of fault slip converged to approximately 80 m, which is consistent with the range of fault slip amounts from previous studies based on the Okada model. At least 0.88 million total domain elements and a 6.7 km2 mesh size on the fault plane with an area of 240 × 720 km2 are required for the convergence of the fault slip. Furthermore, we found that the location of the maximum fault slip is less sensitive to the mesh size, implying that source inversion based on a coarse mesh model (i.e. less than 0.5 million elements and > ∼60 km2 mesh size) can quickly provide the rough fault slip distribution.

History and activities of the European-Mediterranean Seismological Centre

The European-Mediterranean Seismological Centre (EMSC) provides rapid information on earthquakes and their effects, but does not operate seismic stations. It collects and merges parametric earthquake data from seismological agencies and networks around the world and collects earthquake observations from global earthquake eyewitnesses. Since its creation in 1975, it has developed strategies to complement earthquake monitoring activities of national agencies and coordinated its activities in Europe with its sister organisations ORFEUS and EFEHR as well as with global actors, while being part of the transformative EPOS initiative. The purpose of this article is to give a brief history of the EMSC and describe its activities, services and coordination mechanisms.

Analysis of Strong Ground Motion Characteristics for the Jishishan MS6.2 Earthquake December 18, 2023

The MS6.2 earthquake occurred in Jishishan, Gansu Province, China, on December 18, 2023. Based on some strong seismic acceleration time histories captured by some stations of China National Strong Earthquake Observation Network and National Earthquake Early Warning Network, the spatial distribution characteristics, rupture directivity effect, hanging wall and footwall effect, response spectrum characteristics, and attenuation of the peak ground acceleration (PGA) were analyzed in this paper. It can be used as reference for the site determination of post‐earthquake reconstruction and seismic design of engineering structures. The analysis shows that the spatial distribution of strong ground motion of the Jishishan MS6.2 earthquake is mainly controlled by the distribution of causative fault. Near the fault, the effect of rupture directivity is obvious. The effect of hanging wall and footwall significantly affects the distribution of PGAs for this earthquake, causing a pronounced asymmetric pattern about PGAs relative to the fault, with slower attenuation on the hanging wall compared to the footwall. The type of site obviously affects the characteristics of response spectrum. The ground motion intensity observed at the epicenter of this earthquake significantly exceeded the levels of rare and extremely rare seismic events in the current seismic design codes. The attenuation relationship of PGA obtained by regression analysis shows that ones in the 5th generation Chinese ground motion parameter zoning map for the Qinghai‐Tibet region underestimate the horizontal PGA.

New Magnitude–Area Scaling Relations for the New Zealand National Seismic Hazard Model 2022

ABSTRACT We develop new magnitude–area scaling relations for application in the New Zealand National Seismic Hazard Model 2022 (NZ NSHM 2022) and future applications. A total of 18 published relations are selected, comprising the following tectonic and slip types: crustal strike-slip (seven relations), reverse (two relations), normal (two relations), subduction interface (five relations), and two dip-slip relations to augment the small number of available reverse and normal relations. The scaling relations are evaluated against an instrumental earthquake database flatfile, and scores are provided for each relation. Equations of the form Mw=logA+C are then used to develop mean and bounding relations for the suite of scaling relations. The final set of relations used in NZ NSHM 2022 is adjusted to be consistent with observations of major historical New Zealand earthquakes and U.S. Geological Survey practice. We also provide a second set of Mw=logA+C relations that are absent of these adjustments and so more directly reflect the results of our scoring of the published relations.

Classification Of Earthquake Observation Stations Using Multiple Input Convolutional Neural Network Method

Analysis of the data quality of earthquake observation stations becomes very important as quality control. However, determining the quality of earthquake observation stations is done manually by analyzing several existing parameters by an expert. This study proposes a deep learning method approach to classify the quality of earthquake observation stations based on the expert’s ability to analyze the quality of earthquake observation station data. The method used is Multiple Input Convolutional Neural Network (MICNN). The proposed MICNN architecture development is a combination of previous research. The dataset used in this study results from recording 411 seismometers for 12 months with a data set containing three horizontal components (East and North) and the vertical component (Z) with a recording length of 30 days. Each component is transformed from the time domain into the frequency domain form to obtain a spectrogram image used as input in the MICNN model. In addition, each component has a CNN block that functions as an extraction feature on the spectrogram image. Classification in this study used three classes: Classified, Usable, and Unusable. Tests and experiments on the model were carried out to obtain the best accuracy by tuning hyperparameters for model validation and the accuracy of this MICNN model was 99%.

Directionality and polarization of response spectral ordinates in the 2023 Kahramanmaras, Türkiye earthquake doublet

Until recently, the orientation of maximum horizontal spectral response was generally believed to not have a predominant orientation at rupture distances greater than 5 km. However, a recent study found that the orientation of maximum spectral response for strike-slip earthquakes in the NGA-West2 database tends to occur close to the epicentral transverse orientation, that is, an orientation perpendicular to a line connecting the epicenter to the station. This article investigates directionality in the 6 February 2023 Türkiye doublet earthquakes ( Mw 7.8 and 7.5) with strike-slip faulting. The orientation of the maximum response of 5%-damped linear elastic oscillators was studied. The spatial distribution of the level of polarization, which in this article refers to the amount of directionality, and intensities at specific orientations were also studied. The maximum spectral response was found to occur systematically close to the epicentral transverse orientation, consistent with previous observations for other strike-slip earthquakes. For the Mw 7.8 event where the location of maximum slip was relatively far from the epicenter, it was found that the orientation of maximum spectral response is, on average, closer to the maximum slip transverse orientation (i.e. perpendicular to a line connecting the station to the surface projection of the point of maximum slip) when compared to the epicentral transverse orientation over most period ranges. This suggests that the maximum slip transverse orientation may be a better estimator for determining the orientation of maximum spectral response in large-magnitude strike-slip earthquakes, although further study using more events is warranted. Polarized motions were observed over large geographical areas, and the orientation of maximum spectral response was found to be close to the epicentral or maximum slip transverse for Joyner–Boore distances up to the farthest studied (400 km). These findings further support the case for the development of orientation-dependent ground motion models for strike-slip earthquakes.

Seismological Indicators of Geologically Inferred Fault Maturity

AbstractVariations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. We evaluate the degree of empirical correlation between geological and geometric measurements commonly invoked as indicative of fault zone maturity and remotely measured seismological source parameters of 34MW ≥ 6.0 shallow strike‐slip events. Metrics based on surface rupture segmentation, such as number of segments and surface rupture azimuth changes, correlate best with seismic source attributes while the correlations with cumulative fault slip are weaker. Average rupture velocity shows the strongest correlation with metrics of maturity, followed by relative aftershock productivity. Mature faults have relatively lower aftershock productivity and higher rupture velocity. A more complex relation is found with moment‐scaled radiated energy. There appears to be distinct behavior of very immature events which radiate modest seismic energy, while intermediate mature faults have events with higher moment‐scaled radiated energy and very mature faults with increasing cumulative slip tend to have events with reduced moment‐scaled radiated energy. These empirical comparisons establish that there are relationships between remote seismological observations and fault system maturity that can help to understand variations in seismic hazard among different fault environments and to assess the relative maturity of inaccessible or blind fault systems for which direct observations of maturity are very limited.