ABSTRACT We compare the on-fault seismicity and probabilistic seismic hazard in Aotearoa New Zealand produced by a synthetic earthquake catalog generated in the physics-based Rate and State Earthquake Simulator (RSQSim) and the inversion fault model (IFM) from the 2022 New Zealand National Seismic Hazard Model (NZ NSHM 2022). Because both models implement the New Zealand Community Fault Model (NZ CFM), the similarities and differences between their magnitude–frequency distributions (MFDs) and seismic hazard estimates can provide insights into how they simulate on-fault seismicity. We find that the NZ NSHM 2022 IFM indicates that faults participate in larger magnitude ruptures than the NZ CFM-RSQSim catalog, although the latter’s MFD is still within the range of the NZ NSHM 2022 IFM’s 5th and 95th percentile uncertainty. For a 10% probability of exceedance in a 50 yr hazard level, the normalized discrepancies in peak ground acceleration between the two models are <50%. However, at 0.5% probability of exceedance in 50 yr, larger differences (>250%) emerge for sites around low-slip-rate faults or for sites along the eastern North Island that lie above the Hikurangi–Kermadec subduction zone interface. Because both models are incomplete representations of seismicity and seismic hazards in New Zealand, we also incorporated the NZ NSHM 2022 distributed seismicity model (DSM) into these comparisons. In low-seismicity regions, the normalized hazard discrepancies are reduced once the DSM is included, but they persist in sites overlying the Hikurangi subduction zone. Cumulatively, our study highlights that although there is an obvious scientific appeal to adding “physics” into earthquake rupture forecasts, there are challenges and opportunities with incorporating uncertainty into earthquake simulators, and developing meaningful tests for them in regions with short historical earthquake catalogs.
ABSTRACT We present a new model of crustal Lg-wave attenuation and site amplification across the central and eastern United States (CEUS), derived from joint tomographic inversions of vertical and horizontal seismic amplitudes. We use more than 75,000 broadband records from over 1,600 regional crustal earthquakes (Mw≥3.5, depth <30 km) recorded between 2007 and 2024 at more than 1,500 seismic stations. Using this dataset, we improved the imaging of the frequency-dependent seismic quality factor (Q), site terms, and source amplitudes across five center frequencies from 0.7 to 11.3 Hz. Our results show significant lateral variations in attenuation even in what has been considered the low attenuating Midcontinental region. Q0 values as low as ∼200 to 230 occur in sediment-rich, tectonically active regions. Stable cratonic interiors exhibit high Q0 values (up to ∼380) but contain lower Q subregions (some newly identified) associated with tectonic features in the crust and mantle. We also resolve the frequency dependence of attenuation via the η parameter, which varies from ∼0.7 in high-Q0 regions to >1.0 in areas with strong scattering. Site amplification terms show higher values (50%–100% increase) in intracratonic basins and coastal plain sediments and lower values (10% decrease to 20% increase) in regions with exposed or shallow bedrock. The horizontal-to-vertical site amplification ratio exhibits a polarity shift across the Coastal Plain boundary. Checkerboard and feature recovery tests demonstrate the model’s spatial resolution down to ∼100 km, and jackknife resampling confirms relative uncertainties below 2% across most of the domain. Compared to previous CEUS attenuation studies, our model benefits from denser ray-path coverage and improved inversion methodology. These findings provide critical input for refining seismic hazard assessments and updating ground-motion prediction models for the next-generation U.S. National Seismic Hazard Model.
ABSTRACT On 29 July 2025 (UTC), an earthquake with a moment magnitude of approximately 8.8 occurred along the plate interface of the Kamchatka subduction zone and generated a trans-Pacific tsunami. Using tsunami waveform data recorded at offshore stations, we conducted tsunami waveform inversion based on a multiple time-window approach to investigate the spatiotemporal distribution of coseismic slips during the earthquake. The inversion results suggest temporally distributed slip evolution, with the largest slip occurring approximately 180–225 s after the earthquake occurrence. The slip distribution indicates that the dominant asperities were located primarily in the shallow-to-intermediate portions of the fault plane, whereas the largest slip was estimated to be in the deeper region at approximately 12 m. Resolution tests of our inversion analysis suggest that slips in the shallow-to-intermediate portions of the fault plane were relatively well constrained, whereas those in the deeper region were less well resolved. This reduced resolution likely reflects the lower sensitivity of far-field tsunami observations to deep fault slip, together with limitations in the azimuthal distribution of offshore stations. The total seismic moment derived from the developed source model was 1.44×1022 N·m, corresponding to a moment magnitude of 8.7. The spatiotemporal characteristics of slips in our source model were comparable with source models developed by previous studies using other observational data.
ABSTRACT Complex fault systems are often surrounded by topography. Although it is known that nearby topography can affect on-fault rupture dynamics, the particulars of its effect on supershear rupture and behavior at fault intersections have not been fully explored. In this study, we conduct 3D dynamic rupture simulations comparing the rupture behavior between a branch fault system with a flat free surface and one with asymmetric fault-adjacent topography. To isolate the effects of topography, we implement simplified uniform on-fault traction and illustrate that the fault-adjacent topography causes the free surface-induced supershear to propagate much more slowly and asymmetrically compared with the corresponding flat free surface case. As an example, for a right-lateral fault system along the x direction, the daughter crack is slightly faster in the positive x direction as opposed to the negative x direction. In addition, depending on the initial shear stress conditions, the normal stress perturbations introduced by the nearby topography can fully suppress the formation of the daughter crack in the slower direction, leading to a unilateral supershear rupture in the other direction. Furthermore, topography can facilitate throughgoing rupture onto secondary fault segments at fault intersections. The results from this simple dynamic modeling study suggest that topography can have significant implications for earthquake dynamics and may explain why some strike-slip earthquakes do not experience supershear rupture.
ABSTRACT Probabilistic seismic hazard assessment (PSHA) increasingly leverages physics-based simulations (PBS) to improve the representation of source, site, and path effects, particularly in regions with limited observational data. Source parameters exert a strong influence on ground-motion intensity, underscoring the need for rigorous quantification of rupture-related uncertainty. By solving the seismic wave equation for predefined kinematic rupture models, PBS provides a physically consistent framework that circumvents the ergodic assumption inherent in traditional ground-motion estimation approaches. We evaluate the Castro-Cruz and Mai (2025; CM2025) rupture generator for its ability to produce statistically representative rupture scenarios and capture realistic median values of ground-motion intensity. In this article, the ground-motion variability produced by CM2025 ruptures is benchmarked against empirical ground-motion models (GMMs) across multiple earthquake scenarios. After validating CM2025’s capability to capture ground-motion variability, we address a fundamental question motivated by the substantial computational cost of PBS: How many simulations are required to robustly characterize the ground-motion variability from kinematic rupture models? To answer this question, we perform over 2000 simulations of Mw 6.5 strike-slip and dip-slip earthquakes and quantify confidence intervals for the median and logarithmic standard deviation of peak ground velocity (PGV). To improve efficiency, we introduce a new directivity parameter into the statistical characterization of ground motions. This innovation reduces the number of simulations required to achieve a target confidence interval by up to 80% for sites with strong rupture-directivity sensitivity. For example, stations aligned with strike-slip faults initially require over 200 simulations for accurate PGV estimation; our new method achieves equivalent confidence with only 35 simulations. Our findings are a step toward a scalable framework for optimizing computational resources, enabling more efficient and accurate seismic hazard assessments.
ABSTRACT Recent advances in Probabilistic Seismic Hazard Analysis (PSHA) leverage physics-based ground-motion simulations to estimate seismic hazard, such as the CyberShake project. However, computational costs quickly escalate when performing PSHA for numerous faults or sites and can become prohibitively expensive. To reduce computational demands, CyberShake uses reciprocity and interpolates physics-informed corrections from simulations conducted at fewer locations, but the accuracy of these interpolations remains poorly quantified. To quantify the interpolation accuracy, we derive high-resolution, frequency-dependent site terms for southern California and compare them with interpolated site terms using the CyberShake approach. We accomplish this by performing a set of earthquake point-source simulations distributed along the nonplanar fault geometry for the southern San Andreas fault (SSAF) extending from Bombay Beach to Lake Hughes. Using SeisSol, we simulate three minutes of viscoelastic seismic wave propagation for these sources and store the horizontal-component Green’s functions for 480,000 sites. We then use a scientific machine learning approach based on interpolated proper orthogonal decomposition to construct an accurate reduced-order model of the Green’s functions to efficiently predict effective amplitude spectra (EAS) for finite-source rupture models of SSAF earthquakes. Using minimum curvature interpolation with tension, as used in CyberShake, we compare the interpolated site terms against our high-resolution site terms. We identify local discrepancies with EAS differing by up to a factor of approximately three. Furthermore, we identify locations where unexpectedly high or low ground motions are missed when using the interpolated dataset for these earthquakes. We estimate that our approach may be used within CyberShake to reduce the time-to-solution by a factor of 336 for the entire earthquake rupture forecast. Our analysis of physics-based site terms provides more insight into the seismic hazard due to SSAF ruptures and guides future developments by combining high-performance computing and reduced-order modeling techniques for PSHA.
ABSTRACT The 2023 Mw 6.0 Jishishan earthquake in China caused severe damage and fatalities. Waveform analyses indicate that using a 1 Hz cutoff frequency underestimates broadband ground motions by about 70% for peak ground velocity (PGV) and up to 90% for peak ground acceleration. Physics-based simulations up to 3.5 Hz using an inverted finite-fault rupture model still underestimate PGV due to missing high-frequency components, emphasizing the need for broadband rupture models. In this study, we develop a broadband hybrid source model incorporating deterministic features of the inverted finite-fault model. Simulations with this hybrid source model produce PGV values that better match observations. Further analyses demonstrate that, for the 2023 Jishishan earthquake, ground motions simulated using the hybrid source model are jointly controlled by large-scale rupture characteristics (e.g., slip distribution and rupture velocity), which are generally well constrained by finite-fault inversion, and rise-time distributions, which are typically poorly constrained. These findings emphasize the importance of broadband hybrid source models for improving physics-based ground-motion simulations and in turn, seismic hazard assessment.
ABSTRACT Accurate modeling of slip-rate time functions in the shallow portion above the seismogenic zone on a fault is essential for ground-motion prediction. Recent observations, such as those from the 2016 Kumamoto, central Kyushu, Japan, earthquake (MJMA 7.3), have revealed long-period ground motions and permanent displacements with long rise times near the surface fault, indicating that contributions of the shallow fault portion must be considered in addition to strong-motion generation areas within the deep seismogenic zone. In this study, we propose a friction law applicable to fault surfaces in an elastic medium, in which a slip-rate strengthening mechanism operates below a characteristic slip rate Vc, supplementing the standard slip-weakening law. Pure slip weakening is applied within the seismogenic zone, whereas the slip-rate strengthening branch is applied to the shallow fault portion above the seismogenic zone. Dynamic rupture simulations using a 3D elastic model with a shallow stress drop of 0 MPa demonstrate that the model incorporating slip-rate strengthening suppresses slip rates on the shallow layer and reduces peak ground velocity amplitudes within approximately 1 km of the surface fault compared with a purely slip-weakening model, despite producing nearly identical overall rupture propagation and seismic moments. Sensitivity analyses further show that δ, which is the incremental parameter of the rate-strengthening friction law, primarily controls peak slip rate through its influence on the effective fracture energy, whereas Vc mainly affects rupture duration, particularly the slip-termination phase. Although long Dc or negative stress drop has traditionally been used to mimic suppression of shallow slip rates, the proposed law more efficiently reproduces a gradual slip-rate onset, prolonged slip duration, and peak suppression, compared to friction laws without slip-rate dependence that have a similar effective fracture energy. The proposed friction formulation is simple to implement, computationally efficient, and useful for large-scale source-model exploration and applications in near-fault ground-motion prediction.
ABSTRACT We evaluate a recently developed 3D crustal velocity model for northern Israel using numerical simulations of seismic wave propagation for eighteen earthquakes (3≤Mw≤4.5). Synthetic seismograms were compared with observed recordings from the Israel Seismic Network in the frequency range 0.1–0.5 Hz in the time domain through cross correlation, and in the frequency domain through a spectral-misfit analysis. Cross correlations of observed records with synthetics show higher similarities to those generated by the 3D model. In addition, the time lags between synthetic and observed waveforms are concentrated around zero. By contrast, synthetics generated using the 1D regional model show increasing time lags with increasing distance. The 3D model also substantially reduces the median spectral misfit relative to the 1D model across all components, with Gitt1D median misfit values of 0.72 (Z), 1.03 (R), and 1.02 (T) reduced to −0.19 (Z), 0.23 (R), and 0.38 (T) in Be3D, yielding a more balanced reproduction of observed amplitudes. Spatially, the 3D model performs best in regions of relatively simple crustal structure but underestimates spectral amplitudes within narrow sedimentary basins, particularly along the Kinneret–Kinarot and Hula basins, likely due to unresolved near-surface low-velocity zones. This study represents an intermediate step in an ongoing model development cycle. Although the 3D model demonstrates clear improvements over the 1D reference, the evaluation also identifies key limitations that must be addressed. These findings directly inform planned future modeling efforts, including the incorporation of finer-resolution sedimentary basin structures constrained by additional geophysical datasets.
ABSTRACT The Statewide California Earthquake Center has developed the CyberShake platform to perform 3D simulation-based probabilistic seismic hazard analysis. Using earthquake event rates, kinematic ruptures, and a 3D seismic velocity model, all constrained by observations, CyberShake utilizes wave propagation simulations and a reciprocity-based approach to generate hundreds of thousands of seismograms per site of interest. These simulation results are used to produce hazard data products including uniform hazard spectra, significant durations, site-specific hazard curves, and regional hazard maps. Previous CyberShake modeling efforts have computed seismic hazard models in southern California and Central California. In this article, we present CyberShake Study 24.8, which implements the CyberShake workflow for 315 sites in the greater San Francisco Bay area, extending physics-based hazard estimates to the entire San Andreas fault system. We construct the velocity model by tiling together three 3D and 1D models and applying a near-surface VS30-based merged taper to the top 700 m. We used the AWP—ODC wave propagation code to perform the ground-motion simulations, and the Graves–Pitarka kinematic rupture generator to create slip time histories for individual events. CyberShake Study 24.8 produces overall slightly reduced hazard compared to the NGA-West2 ground-motion models (GMMs). Specifically, CyberShake produces lower hazard immediately surrounding the San Francisco Bay likely due to use of a minimum VS of 400 m/s, lower hazard in near-fault regions, and higher hazard at long periods near sedimentary basins in the velocity model. We find larger directivity effects for CyberShake in northern California than in southern California, likely due to increased uniformity in fault characteristics and orientation in northern California. Validation results using the 1989 M 6.9 Loma Prieta event show that CyberShake reproduces observed ground motions with reduced bias compared with GMMs and 1D simulations.
ABSTRACT The characteristic magnitude distribution hypothesis predicts a higher rate of large earthquakes than a Gutenberg–Richter extrapolation of the small-earthquake rate would imply. Characteristic magnitude distributions have been commonly applied to faults in probabilistic seismic hazard analysis (PSHA), and in modern models they can emerge from the way short-term seismicity constraints are combined with long-term geologic and geodetic constraints. We test the characteristic magnitude distribution hypothesis by comparing the fault-based magnitude distributions from the 2023 update to the National Seismic Hazard Model (NSHM23) in the Western United States with observed seismicity over the past 93 yr. We find that observed magnitude distributions fall outside the model-predicted confidence bounds in regions where NSHM23 produces characteristic magnitude distributions: in these regions, the model predicts higher rates of large earthquakes than are observed. An analysis of the earlier California model (Uniform California Earthquake Rupture Forecast, version 3) also reveals discrepancies between the modeled and observed magnitude distributions. In addition, we find that observed magnitude distributions near modeled faults are not significantly different from those in background regions. These results challenge the prevalence of characteristic magnitude distributions in fault-based seismic hazard models and call for a reassessment of how disparate data sets are integrated in PSHA.
ABSTRACT Using two aftershock groups with 1 km diameter zones and three groups with 2 km diameter zones, we investigated three research issues on the prediction of high-frequency earthquake motions using empirical Green’s tensor spatial derivatives (EGTD): determination of the cutoff frequency on the high-frequency side with reference to the size of aftershock zones, conditions for accurate prediction, and selection of the source time function giving the best prediction accuracy from among the currently available source time functions. Our results indicate that: (1) the EGTD derived from the aftershock zones of 1 km diameter can predict high-frequency earthquake motions accurately up to the cutoff frequency of 8 Hz. Although the EGTD derived from 2 km aftershock zones cannot predict ground motions accurately up to the cutoff frequency of 4 Hz, they can still predict high-frequency motions at observation sites with similar earthquake records. (2) Two conditions are imposed on the estimation of the EGTD having such good prediction performance: earthquake motion records at individual observation sites are similar in waveform to each other, and the aftershocks have various focal mechanisms. (3) The selection is difficult because there are no significant differences in the prediction accuracy between the source time functions considered. Conclusions (1) and (2) mean that after confirming the similarities in waveforms of earthquake motion records, radiated from aftershocks with various focal mechanisms, observed at individual sites, we can estimate the EGTDs that can predict high-frequency earthquake motions accurately. Therefore, we think that the EGTDs are promising tools for the prediction of high-frequency earthquake motions and for the kinematic source inversion. Moreover, our results can be used to identify high-frequency earthquake motion radiation areas on fault surfaces and to estimate dynamic fault rupture parameters.
ABSTRACT A borehole fiber-optic strainmeter and pore-pressure gauges were installed at the C9038B deep-seafloor borehole observatory at the Nankai trough subduction zone for the precise measurement of seafloor crustal deformation. To monitor activities associated with subduction processes, such as slow-slip events, through continuous observations at the borehole observatory, the characteristics of these observations must be clarified. In the first two years of observations after installation, clear tidal and tsunami responses were recorded. To understand the physical origins of these observational characteristics, we performed finite-element method (FEM) simulations. The response characteristics of observations were modeled by considering both the crustal structure around the observatory and the instrument used for observations. The simulation sequentially solved two elastic models comprising the crustal structure around the observatory and sensor structure. The sedimentary layer was modeled as V-shaped, with the major components of the sensor units being included in the simulations. The elastic properties assigned to these models were based on drilling parameters and seismic surveys. Consequently, the simulation reproduced the tidal response of strain and pore pressure observations. The loading response coefficient derived from the FEM-based tidal analysis also successfully explained the signals associated with the tsunami generated by the Kamchatka earthquake on 29 July 2025. This demonstrated that the numerical results accurately represented the response of the observatory to external loading events. Furthermore, to investigate the general tendencies of the borehole observations, similar simulations were performed using models that considered various elastic parameters and sedimentary layer geometries. Regarding changes in the elastic parameters, the strain and pore pressure observations were sensitive to Young’s modulus and Poisson’s ratio, respectively. Changes in the shape of the sedimentary layer caused perturbations in the strain observations; however, these effects were relatively smaller than those associated with variations in the elastic parameters within realistic ranges.
ABSTRACT How does a source rupture propagate in a geometrically complex multifault system? Reconstructing the spatiotemporal evolution process of such complex geometric ruptures is a crucial way to advance understanding of earthquake source physics, but sufficient observations of such events are difficult to obtain due to their rare occurrence. On 6 February 2023, two destructive earthquakes successively struck Kahramanmaraş, Türkiye, within 9 hr, forming the largest continental earthquake doublet ever recorded by modern seismological instruments. Here, we integrate the teleseismic backprojection imaging and the joint finite-fault inversion to investigate the rupture kinematics of the two earthquakes. We find that the two earthquakes exhibit distinct rupture propagation patterns, with transient and persistent supershear ruptures occurring in the first Mw 7.8 event and the subsequent Mw 7.7 one, respectively. The Mw 7.8 earthquake originates on a splay fault branching from the east Anatolian fault (EAF), and propagates to the main strand of the EAF at a subshear velocity. After passing through fault bends, the rupture generates three transient transitions from subshear to supershear, with two located in the Amanos segment and one in the Erkenek segment. In contrast, the rupture of the Mw 7.7 earthquake initiates in the middle section of the northern strand of the EAF, and propagates bilaterally at a persistent supershear velocity. Based on the analysis of a series of potential causes contributing to the supershear rupture, we argue that the difference in the geometric complexity of the seismogenic faults directly controls the variation in rupture patterns between the two earthquakes. This highlights that the geometric complexity is a key factor capable of modulating the kinematic characteristics of large-scale source ruptures.
ABSTRACT The 5 December 2024 Mw 7.0 Offshore Cape Mendocino earthquake was a challenging test of the U.S. West Coast ShakeAlert earthquake early warning system due to its offshore epicenter and limited near-source station coverage. We analyzed real-time performance of all components of the ShakeAlert system, including the seismic algorithms (earthquake point-source integrated code [EPIC] and Finite-fault rupture Detector [FinDer]), the geodetic algorithm (Geodetic First Approximation of Size and Time–peak ground displacement [GFAST-PGD]), and network telemetry during the event. EPIC created the first solution for this earthquake 15 s after origin time with an initial magnitude estimate of M 5.6 and location error of 10 km from the Advanced National Seismic System epicenter. An early spurious trigger from station CE.89101 fortuitously maintained location accuracy and, correspondingly, magnitude accuracy. FinDer contributed its first solution at 18 s with a location estimate closer to the seismic network and produced two distinct rupture geometries, leading to minor fluctuations in estimated intensity contours. GFAST-PGD did not meet alerting thresholds but otherwise performed as expected. Network latencies were <2 s for most stations, supporting the rapid detection of this earthquake by the system. Roughly five million alerts were delivered to cell phone devices in California and Oregon during this event. This was also the first instance of a school district-wide ShakeAlert-powered system being activated. Comparisons to recorded seismograms demonstrate that the maximum warning times before potentially damaging shaking (intensity 6+) were in the range of 5–55 s. Although the ShakeAlert system provided accurate solutions and useful alert delivery, this earthquake raised awareness of potential issues within the system, including the need for improved offshore location estimates, a combination of solutions from ShakeAlert servers, and handling of spurious triggers.
ABSTRACT In the framework of site-specific seismic hazard assessment, the definition of reference motion is a crucial step. Reference motion is generally associated with hard-rock conditions, characterized by S-wave velocity (VS) exceeding 1500 m/s. However, ground motion recorded at sites with such conditions is underrepresented in existing strong-motion databases. Consequently, the validity domains of most empirical ground-motion models (GMMs) are not representative of reference hard-rock conditions. To address this limitation, we consider the empirical approach to retrieve and model reference ground motion for shallow crustal earthquakes in a seismically active region, proposed by Shible et al. (2023) using the Japanese Kiban–Kyoshin network data. Following this approach, we apply a deconvolution of site responses from strong-motion recordings to estimate ground motion at reference conditions in the Euro-Mediterranean region. The first step involves compiling a large database by merging the Engineering Strong-Motion Database (ESM database, Luzi et al., 2020) with additional data from the Greek, French, and Spanish networks. The second step involves estimating site response at network stations using both parametric and nonparametric generalized inversion techniques (GITs). A careful selection of reference hard-rock stations is made to constrain the inversions. The database signals are then deconvolved using the site terms obtained by GIT, allowing us to virtually bring the sites of the entire database to very high VS values, similar to outcropping bedrock conditions. GMMs are then determined for reference conditions, and the results are discussed. The derived GMMs show limited site-to-site variability (0.1–0.2 in natural log), indicating effective removal of site responses from data. In addition, an average amplification model, based on VS30, is proposed to complement the reference GMM prediction in cases where a site-specific soil response estimate is unavailable.
ABSTRACT The nature of seismic sources for moderately large (moment magnitude, Mw 5.0–6.5) events are commonly characterized by their moment tensor (MT) solutions and obtained by inversion of regional distance (200–1600 km) long-period (20–50 s) waveforms. Regional MT estimates are often calculated from average plane-layered, one-dimensional (1D) velocity models. However, 1D model calculations can produce misfits in the arrival times and waveform shapes that introduce errors, particularly at longer distances or for shorter periods, which are necessary for analyzing lower magnitude events. Approximate Earth models (e.g., 1D) representing broad areas may be inadequate, particularly in the crust and uppermost mantle of tectonically complex regions. In this study, we show how a three-dimensional (3D) Earth model obtained from full waveform inversion tomography can improve waveform fits and decrease phase errors. We developed a platform and workflow to perform routine 3D MT inversions and inverted MTs for 25 earthquakes in the western United States and seven nuclear explosions using an average 1D and a recent 3D Earth model, WUS256 (Rodgers et al., 2022). Using the 3D model improves waveform fits (variance reduction and phase time shifts) compared with the 1D model, and the 3D MT solutions are stable across large distances. This study shows that 3D models obtained from full waveform tomography can improve MTs and source characterization especially at far regional distances (>800 km).
ABSTRACT We develop a model to predict the amplification of ground motion for 5% damped pseudospectral accelerations for periods ranging from 0.01 to 0.5 s while considering the effects of varying topography. We use 487 ground motions recorded at densely deployed 118 temporary stations and one regular seismic station in Pohang, South Korea. The proposed model depends on the magnitude of detected motions and the distance from the epicenter. In addition to the relative elevation, we newly introduce the azimuth angle from the station to the epicenter relative to slope aspect as an important topography parameter, which we denote as α. We observe that ground motions are deamplified when the relative elevation is lower than −18 m, and the angle α is smaller than 90° (the slope faces the epicenter). When relative elevation is higher than 14 m, ground motions are amplified for α<90° and deamplified for α>90° (the slope is facing away from the epicenter). The ground motions are not affected by the angle α when the relative elevation ranges from −18 to 14 m. Finally, we propose a predictive model as a function of earthquake magnitude, epicentral distance, relative elevation, and α.
ABSTRACT Full moment tensor source mechanisms of seismic events during fault reactivation of induced earthquakes provide critical information for microseismic monitoring, and the uncertainty quantification of these full moment tensors (MTs), particularly when station coverage is uneven and limited, is fundamentally important for understanding triggering mechanisms of these earthquakes. In this study, utilizing the open-source package MTUQ (Moment Tensor Uncertainty Quantification), we invert high-frequency P-phase amplitude ratios recorded by a dense 198-geophone surface monitoring array for source mechanisms of (micro)seismic events that occurred during hydraulic fracturing operations between August and September 2015 in the northern Montney formation, British Columbia. Specifically, we obtain full MTs and their uncertainties for 567 (micro)seismic events with Mw≥0.4 in the vicinity of the main fault plane along the Blueberry fault that resulted in an Mw∼4.6 induced earthquake on 17 August and another Mw∼3.17 event ∼3 hr and 28 min later. The full MTs of these two events show a dominant double-couple component, whereas in comparison, MTs of seven foreshocks starting from three and a half days before the Mw∼4.6 event show a noticeable positive compensated linear vector dipole component (+CLVD), suggesting a tensile opening component in the mechanisms possibly associated with poroelastic stress transfer and/or aseismic slip resulting from fluid injections. The aftershocks exhibit positive isotropic components in addition to the significant +CLVD, which may be attributed to the combination of aseismic slip and fluid migration through newly formed pathways after fault reactivation.
ABSTRACT Peak ground acceleration (PGA) data from accelerometers and broadband instruments in two networks in New Caledonia and one in Vanuatu are compared with ground-motion prediction equations (GMPEs). New Caledonia is located on the outer rise of the downgoing Australian plate, which is a rare location for land and islands, and the Vanuatu volcanic arc is on the overriding Pacific plate. Deep subduction events (>50 km depth) recorded in Grande Terre (the main island of New Caledonia) fit GMPE predictions better than at Loyalty Islands, which are closer to the subduction zone, whereas shallow events have anomalously low PGAs in both regions. Source effects are ruled out because PGAs at a station on the overriding plate fit GMPE predictions. Mapped geology and horizontal-to-vertical spectral ratios indicate site amplification at some places in Grande Terre and the Isle of Pines that explains part of the observed variability. Different distance-related regional attenuation for shallow and deep earthquakes is observed, but a localized effect (barrier to energy) beneath the shallow (<50 km) outer slope of the subduction trench is required to explain our observations. We propose that a shallow barrier of fractured hydrated lithosphere is responsible for reducing ground shaking from shallow earthquakes, and this may be a common phenomenon in other subduction zones. We suspect there are local barriers beneath Grande Terre, but we do not have sufficient data to evaluate the effect. A GMPE for subduction outer rise settings is required, or some correction terms need to be considered when using existing GMPEs to evaluate ground motions for such tectonic regions. Seismic hazard in New Caledonia is lower than predicted by GMPEs, but local magnitudes of shallow earthquakes and hence real-time tsunami hazard may be underestimated using local observations unless this shallow trench path effect is accounted for.
