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This study addresses a significant gap in understanding the features of the south‐central Cascadia subduction zone, a region characterized by complex geologic, tectonic, and seismic transitions both offshore and onshore. Unlike other segments along this margin, this area lacks a 3‐D velocity model to delineate its structural and geological features on a fine scale. To address this void, we developed a high‐resolution 3‐D P‐wave velocity model using active source seismic data from ship‐borne seismic shots recorded on temporary and permanent onshore seismic stations and ocean‐bottom seismometers. Our model shows velocity variations across the region with distinct velocity‐depth profiles for the Siletz, Franciscan, and Klamath terranes in the overlying plate. We identified seaward dipping high‐velocity static backstops associated with the Siletz and Klamath terranes, situated near the shoreline and further inland, respectively. Regions of reduced crustal velocity are associated with crustal faults. Moreover, there is significant along‐strike depth variation in the subducting slab, which is about 4 km deeper near the thick, dense Siletz terrane and becomes shallower near the predominantly less‐dense Franciscan terrane. This highlights a sudden tectonic and geologic transition at the southern boundary of the Siletz terrane. Our velocity model also indicates slightly increased hydration, though still minimal, in both the oceanic crust and the upper mantle of the subducting plate compared to other parts of the margin. 

ABSTRACT We simulate shaking in Tacoma, Washington, and surrounding areas from Mw 6.5 and 7.0 earthquakes on the Tacoma fault. Ground motions are directly modeled up to 2.5 Hz using kinematic, finite-fault sources; a 3D seismic velocity model considering regional geology; and a model mesh with 30 m sampling at the ground surface. In addition, we explore how adjustments to the seismic velocity model affect predicted shaking over a range of periods. These adjustments include the addition of a region-specific geotechnical gradient, surface topography, and a fault damage zone. We find that the simulated shaking tends to be near estimates from empirical ground-motion models (GMMs). However, long-period (T = 5.0 s) shaking within the Tacoma basin is typically underpredicted by the GMMs. The fit between simulated and GMM-derived short-period (T = 0.5 s) shaking is significantly improved with the addition of the geotechnical gradient. From comparing different Mw 6.5 earthquake scenarios, we also find that the response of the Tacoma basin is sensitive to the azimuth of incoming seismic waves. In adding surface topography to the simulation, we find that average ground motion is similar to that produced from the nontopography model. However, shaking is often amplified at topographic highs and deamplified at topographic lows, and the wavefield undergoes extensive scattering. Adding a fault damage zone has the effect of amplifying short-period shaking adjacent to the fault, while reducing far-field shaking. Intermediate-period shaking is amplified within the Tacoma basin, likely due to enhanced surface-wave generation attributable to the fault damage zone waveguide. When applied in the same model, the topography and fault damage zone adjustments often enhance or reduce the effects of one another, adding further complexity to the wavefield. These results emphasize the importance of improving near-surface velocity model resolution as waveform simulations progress toward higher frequencies. 

ABSTRACT We explore the response of ground motions to topography during large crustal fault earthquakes by simulating several magnitude 6.5–7.0 rupture scenarios on the Seattle fault, Washington State. Kinematic simulations are run using a 3D spectral element code and a detailed seismic velocity model for the Puget Sound region. This model includes realistic surface topography and a near-surface low-velocity layer; a mesh spacing of ∼30 m at the surface allows modeling of ground motions up to 3 Hz. We simulate 20 earthquake scenarios using different slip distributions and hypocenter locations on a planar fault surface. Results indicate that average ground motions in simulations with and without topography are similar. However, shaking amplification is common at topographic highs, and more than a quarter of all sites experience short-period (≤2 s) ground-motion amplification greater than 25%–35%, compared with models without topography. Comparisons of peak ground velocity at the top and bottom of topographic features demonstrate that amplification is sensitive to period, with the greatest amplifications typically manifesting near a topographic feature’s estimated resonance frequency and along azimuths perpendicular to its primary axis of elongation. However, interevent variability in topographic response can be significant, particularly at shorter periods (<1 s). We do not observe a clear relationship between source centroid-to-site azimuths and the strength of topographic amplification. Overall, our results suggest that although topographic resonance does influence the average ground motions, other processes (e.g., localized focusing and scattering) also play a significant role in determining topographic response. However, the amount of consistent, significant amplification due to topography suggests that topographic effects should likely be considered in some capacity during seismic hazard studies. 

The US National Seismic Hazard Model (NSHM) was updated in 2023 for all 50 states using new science on seismicity, fault ruptures, ground motions, and probabilistic techniques to produce a standard of practice for public policy and other engineering applications (defined for return periods greater than ∼475 or less than ∼10,000 years). Changes in 2023 time-independent seismic hazard (both increases and decreases compared to previous NSHMs) are substantial because the new model considers more data and updated earthquake rupture forecasts and ground-motion components. In developing the 2023 model, we tried to apply best available or applicable science based on advice of co-authors, more than 50 reviewers, and hundreds of hazard scientists and end-users, who attended public workshops and provided technical inputs. The hazard assessment incorporates new catalogs, declustering algorithms, gridded seismicity models, magnitude-scaling equations, fault-based structural and deformation models, multi-fault earthquake rupture forecast models, semi-empirical and simulation-based ground-motion models, and site amplification models conditioned on shear-wave velocities of the upper 30 m of soil and deeper sedimentary basin structures. Seismic hazard calculations yield hazard curves at hundreds of thousands of sites, ground-motion maps, uniform-hazard response spectra, and disaggregations developed for pseudo-spectral accelerations at 21 oscillator periods and two peak parameters, Modified Mercalli Intensity, and 8 site classes required by building codes and other public policy applications. Tests show the new model is consistent with past ShakeMap intensity observations. Sensitivity and uncertainty assessments ensure resulting ground motions are compatible with known hazard information and highlight the range and causes of variability in ground motions. We produce several impact products including building seismic design criteria, intensity maps, planning scenarios, and engineering risk assessments showing the potential physical and social impacts. These applications provide a basis for assessing, planning, and mitigating the effects of future earthquakes.