Albert R. Kottke

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.

Precariously balanced rocks (PBRs) provide natural geological indicators for constraining the upper bounds of earthquake ground motions over long timescales. However, translating these constraints into fragility models remains challenging because computationally expensive simulations limit inverse analysis and calibration of contact physics, reducing confidence in the fragility model predictions. To address the challenges, we present a simulated shake-table platform built on a physics engine. Using physical large-scale shake-table experiments on a natural PBR and 582 recorded earthquake displacement histories, we calibrate the contact parameters and benchmark overturning predictions against experimental results and a state-of-the-art discrete element method (DEM). The physics-engine approach reproduces overturning with predictive reliability comparable to DEM, while reducing wall-clock cost by approximately 102 to 105 times. This efficiency enables large ensemble analyses and allows us to evaluate how uncertainty in contact parameters propagates into inferred fragility boundaries. Among the contact parameters, lateral friction exerted the strongest influence on PBR fragility, whereas restitution and spinning friction had comparatively minor effects, with contact damping and stiffness exhibiting more complex behavior. Our study establish a practical pathway for using PBRs as quantitative constraints in seismic hazard assessment.

Mapping of potentially active faults is critical to characterizing earthquake processes and surface rupture hazard. Geologists map tectonic faults from remote sensing datasets by interpreting tectonic landforms formed during past earthquakes. However, evidence for faulting is often ambiguous, incomplete, and challenging to interpret. Conventional wisdom suggests that mapping skill improves with experience. In contrast, prior work has shown that experienced geologists can still miss faults and disagree on fault location. To test whether, when, and to what extent mapping improves with experience, we designed a repeat mapping experiment involving 23 participants, ranging from undergraduate students to senior professionals. Each participant created regional-scale fault maps using pre-earthquake remote sensing data and a systematic, geomorphology-based approach. While professionals slightly outperformed students, some graduate students produced similar quality maps, and all participants missed identifying tectonic landforms indicative of future ruptures. Using participant mapped fault data, we quantify inter-mapper epistemic uncertainty in fault location to be between 55 and 117 m. Our work gives insight into approaches to improve fault mapping and suggests that experience beyond a graduate student level does not significantly improve mapping results.

Accurate identification of sedimentary basins and modeling of their amplification effects are essential for seismic hazard analysis, particularly in regions where ground motions are strongly influenced by basin structures. Traditional basin identification methods often rely on manual interpretation, which is limited by data coverage and by uncertainty from subsurface velocity models or subjective judgment. We present an automated method for identifying sedimentary basins using digital elevation models (DEMs) and estimating their horizontal extent. Topography often reflects subsurface structure, with basins typically appearing as low, flat terrains formed by sediment accumulation in natural depressions. We apply the continuous wavelet transform to decompose topography into multiple length scales. Sedimentary basins exhibit low-amplitude coefficients at short scales due to minimal surface roughness, and negative coefficients at larger scales corresponding to their lateral extent. Our automated approach identifies basins across California and Nevada, producing results consistent with traditionally mapped basins in California and revealing basin areas in Nevada. This technique provides spatially continuous and reliable basin identification and estimates of basin size, which serves as a horizontal metric independent of traditional depth metrics such as Z1.0 and Z2.5. This approach can be readily applied to sedimentary basins in other regions worldwide that exhibit clear topographic expression, given the broad availability of DEMs. These basin terms can be used as predictors in ground-motion models to include basin effects.

An indispensable component of any site‐specific probabilistic seismic hazard analysis (PSHA) is incorporating local site effects through site response analyses, rather than relying on generic amplification factors in ground‐motion models (GMMs). Traditionally, the seismic hazard is calculated at a buried rock horizon and then convolved with amplification factors for the overlying layers, which requires the definition and characterization of the buried rock profile (without which neither the model for rock motions nor the site response analyses can be developed). An alternative approach is to retain the host reference rock profile in the GMM for the hazard calculations and then perform the host‐to‐target site response adjustment for the full profiles in a single step. This approach is illustrated by its application to a site‐specific Senior Seismic Hazard Analysis Committee Level 3 PSHA for Idaho National Laboratory. The site adjustment factors are calculated using a logic‐tree approach that captures the full epistemic uncertainty in these factors. This is not necessarily achieved when the logic tree only captures uncertainty in the near‐surface shear‐wave velocity profile. The application illustrates the use of borehole and noninvasive shear‐wave velocity measurements, in conjunction with local recordings of weak earthquake motions, to constrain the logic‐tree branches and their associated weights.

This study utilizes recorded ground motions from West Texas to evaluate the source, path, and site effects in this region. We analyze the Fourier amplitude spectra from 354 earthquakes in the Delaware basin recorded by the TexNet seismic network at 49 sites. The magnitudes of the earthquakes range from ∼3.0 to 5.0, with hypocentral distances up to 300 km. Using a Bayesian parametric inversion on 6479 records, we simulate the spectra as a combination of source, path, and site effects from a stochastic point-source model. The inverted moment magnitudes were comparable to the catalog moment magnitudes, and the stress parameter (Δσ) ranged from 5 to 134 bar, increasing with magnitude. The geometrical spreading was characterized by a smooth bilinear function, yielding a near-field γ of 1.15 and a frequency-dependent quality factor of Q=837·f0.59. The inverted site amplification functions revealed six distinct clusters of response, with three clusters located in the western part of the study area and the other three scattered to the east. The inverted high-frequency decay parameter (κ0) ranged from 0.005 to 0.056 s, with no clear relationship to shear-wave velocity but systematic variation with cluster and location. Correlation between parameters was observed, but only the prior distribution of Δσ strongly influenced the other inverted parameters.

Existing ground-motion models (GMMs) for Türkiye have primarily relied on ergodic or partially nonergodic frameworks, which are limited in capturing systematic ground-motion characteristics and the trade-off between aleatory and epistemic uncertainties. This study uses a comprehensive Turkish database of over 35,000 shallow crustal ground-motion recordings spanning 1983–2023 to develop a fully nonergodic GMM that accounts for systematic source, site, and path effects. The backbone of the nonergodic model is a regionalized ergodic GMM developed as part of this study, which incorporates an event-specific attenuation term. The nonergodic systematic effects are modeled using Gaussian processes (GPs) based on the residuals of the ergodic backbone GMM. GP parameters are constrained through a variogram-based approach, which is shown to provide advantages compared to a direct GP fitting. Path effects are evaluated using cell-specific attenuation and spatially correlated GPs, accounting for variations in the quality factor and velocity structure. The evaluated source, site, and path terms exhibit different spatial patterns and correlation lengths across spectral periods. Potential factors influencing the patterns are discussed. In particular, we find that the correlation lengths of kappa and the site term at short periods are comparable, suggesting dominating kappa effects at short periods. In addition, we observe parallels between the estimated path effects and available quality factor maps for Türkiye. The spatial distribution of short-period source effects and regional stress drop in western Türkiye also share similarities. The developed nonergodic model reduces aleatory variability by 35%–50% depending on the spectral period. This reduction, once considered in the context of epistemic uncertainties, is key for seismic hazard quantification and performance-based earthquake engineering applications in Türkiye.

Seismic slope displacement analyses are crucial for assessing the performance of earth systems and natural slopes under earthquake loading. Input ground motions are important components of such analyses and represent one of the main sources of variability in estimated values of slope displacement. Most studies on seismic slope displacement analyses have been conducted using shallow crustal earthquake motions. However, ground motions from subduction zones are known to have comparatively longer durations. This study proposes a framework to quantify the effects of ground-motion duration on slope displacements using short- and long-duration ground-motion suites from subduction zone earthquakes. Challenges in isolating duration effects from those associated with the amplitude and frequency content of ground motions are overcome by creating sets of ground motions with the same amplitude and similar spectral shape, but with different significant durations. These equivalent short- and long-duration suites of motions are then utilized in nonlinear finite-element analyses to evaluate their impact on slope displacement. The finite-element model is developed to numerically simulate the strength and stiffness degradation of soils under cyclic loading from strong ground shaking. We find that the long-duration motions in our dataset can cause larger permanent displacements compared to their short-duration counterparts (with similar spectral shape but different duration), especially at higher ground shaking intensity levels that can trigger nonlinear soil behavior. Comparisons with simplified analyses show that the simplified methods can result in biased estimates of permanent displacements from the long-duration motions in our study. This study provides a framework to develop ground motion sets that can enable more in-depth investigations of the role of duration on seismic slope displacements and the damage potential of ground motions, particularly from subduction events.

Following the 2019 Mw 7.1 Ridgecrest, California, earthquake, 15 dense 1D and 2D arrays (461 sites) were deployed around the main ruptures, including four 1D arrays across the surface ruptures of the mainshock. The dense arrays captured numerous aftershocks and provided an unprecedented dataset for studying near-fault ground motions. We combine the near-fault ground motions measured from the dense arrays with a regional ground-motion dataset and develop a near-fault partially nonergodic ground-motion model (GMM). The partially nonergodic GMM results are used to investigate several aspects of the near-fault ground motions that are related to source and fault zone properties. We find that shallow events tend to produce weaker short-period energy due to smaller stress drop or stronger attenuation but stronger long-period energy from surface-wave excitation. Significant ground-motion variations across the fault zone are observed, which include amplifications within the damage zones due to both reduced seismic velocities and trapped waves at certain periods. We also observe double-couple radiation pattern effects only in long-period ground motions, agreeing with a period-dependent earthquake rupture process. Between 3 and 15 days following the mainshock, the ground motions of aftershocks decrease with time, likely reflecting decreasing stress drops. No temporal evolution of site response and wave propagation effects within the fault zone are observed between 10 and 35 days following the mainshock. Our study demonstrates the capability of dense array data to improve our understanding of earthquake processes, fault zone properties, and near-fault seismic hazard.