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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.

Incremental slip rates of the Clarence fault, a dextral fault in the Marlborough fault system of South Island, New Zealand, varied by a factor of 4–5 during Holocene–latest Pleistocene time, as revealed by geomorphic mapping and luminescence dating of faulted fluvial landforms at the Tophouse Road site. We used high‐resolution lidar microtopographic data and field surveys to map the fine‐scale geomorphology and precisely restore the offset features. We dated the offsets using a stratigraphically informed protocol for infrared stimulated luminescence dating. These data show that incremental slip rates varied from ~2.0 to 9.6 mm/year, averaged over multiple earthquakes and millennial timescales. Comparison to incremental slip rates of the nearby Awatere fault suggests that these faults may behave in coordinated (and anticorrelated) fashion. This study adds to a growing body of evidence suggesting that incremental slip rate variation spanning multiple earthquake cycles may be more common than previously recognized. Faults are commonly assumed to accommodate relative tectonic motions (slip) at constant rates when averaged over several large earthquakes. Testing this assumption requires documenting how far a fault has slipped at several different points in time over the past several millennia. In this study, we examine a rare instance in which four independent markers of fault slip over time are recorded in the landscape along the Clarence fault ‐ a major fault in the Marlborough fault system of South Island, New Zealand. We use high‐resolution digital topographic laser scans (lidar) to document the fine‐scale topography of the site, and luminescence dating to determine the ages at which the sediments constituting the landforms were last exposed to sunlight. Together, these markers show that, far from being constant over time, the Clarence fault at Tophouse Road has sped up and slowed by by a factor of four or five over the past eleven thousand years or so. This type of study needs to be conducted on other faults to examine whether they have exhibited similar behavior. Such phenomena can provide insights into the processes controlling tectonic plate behavior over timescales of hundreds to thousands of years. Geomorphic analysis of lidar data and a luminescence dating protocol accurately constrain the offset history of a faulted terrace flightClarence fault incremental slip rates, spanning millennia and multiple earthquakes, varied by a factor of 4–5 since latest Pleistocene timeComparison of incremental slip rate variability for the Clarence fault and Awatere fault may suggest coordinated system‐level slip behavior