Department of Civil and Environmental Engineering, UCLA

We appraised factors and processes related to human activities and high water, subsidence, and seismicity. Farming and drainage of peat soils caused subsidence, which contributed to levee internal failures. Subsidence rates decreased with time, but still contributed to levee instability. Modeling changes in seepage and static slope instability suggests an increased probability of failure with decreasing peat thickness. Additional data is needed to assess the spatial and temporal effects of subsidence from peat thinning and deformation. Large-scale, state investment in levee upgrades (> $700 million since the mid-1970s) has increased conformance with applicable standards; however, accounts conflict about corresponding reductions in the number of failures.

Modeling and history suggest that projected increases in high-flow frequency associated with climate change will increase the rate of levee failures. Quantifying this increased threat requires further research. A reappraisal of seismic threats resulted in updated ground motion estimates for multiple faults and earthquake-occurrence frequencies. Estimated ground motions are large enough to induce failure. The immediate seismic threat, liquefaction, is the sudden loss of strength from an increase in the pressure of the pore fluid and the corresponding loss of inter-particle contact forces. However, levees damaged during an earthquake that do not immediately fail may eventually breach. Key sources of uncertainty include occurrence frequencies and magnitudes, localized ground motions, and data for liquefaction potential.

Estimates of the consequences of future levee failure range up to multiple billions of dollars. Analysis of future risks will benefit from improved description of levee upgrades and strength as well as consideration of subsidence, the effects of climate change, and earthquake threats. Levee habitat ecosystem benefits in this highly altered system are few. Better recognition and coordination is needed among the creation of high-value habitat, levee needs, and costs and benefits of levee improvements and breaches.

Quantifying groundwater depletion and aquifer–stream interactions in intensively irrigated basins remains a critical challenge for sustainable water management. This study numerically examines these issues in the San Joaquin River Basin (SJRB) using the coupled SWAT+ gwflow framework. To manage computational demands across the 28,435 km2 basin, upland contributing areas were represented as inlet point sources at five gauging stations at the outlets of each of the upstream subbasins, reducing computational cost by ~45% while preserving hydrological fidelity. A fine-resolution (300 m × 300 m) model was developed for the downstream valley floor, with release from reservoirs and mountain-block runoff incorporated as a surface boundary condition. We applied the water allocation module gwflow to withdraw water from the aquifer to satisfy crop water stress based on user-defined crop water coefficient values and compared the simulated and observed spatially distributed groundwater head values. Results show a persistent decline in groundwater storage and water levels from 2006 to 2022, with extraction exceeding recharge by ~40%, resulting in an average annual storage loss of 450,175 acre-feet. Intensive pumping also caused hydraulic disconnection between groundwater and some streams, where pronounced cones of depression developed in Turlock, Merced, and western Madera subregions. These findings indicate that targeted managed aquifer recharge and reductions in groundwater extraction in the most severely depleted subregions, are essential to restore aquifer–stream connectivity and ensure long-term water security in the SJRB. Overall, integrating gwflow with SWAT+ provides critical insights into groundwater dynamics in California’s Central Valley and supports sustainable groundwater management.

This study presents physics‐based, 3D simulations using the EQSIM framework for several earthquakes in the Los Angeles region. The primary objective was to assess the ability of deterministic physics‐based ground motion simulations to reproduce the observed motions from historical events. The selected events included the 5.4 2008 Chino Hills, the 4.4 2024 Highland Park, and the 4.3 2021 Carson events. The simulated motions were evaluated by comparing the recorded and simulated seismograms, as well as the Fourier amplitude spectra, across multiple seismic stations. The SCEC 3D velocity model, CVM‐S4.26.M01, was used to represent the regional geology, and ground motion simulations were carried out with a resolution of up to 5 Hz. The results indicate that the simulated motions captured the recorded motions up to approximately 4 Hz. While careful iterations regarding source parameters and corner frequencies were required, and, for the case of the Highland Park event, some of the near‐source stations had relatively low accuracy, the present study established a positive step toward the utilization of physics‐based simulations in practical applications. The computational efficiencies exhibited by EQSIM, especially on GPU clusters, further supported this assertion, as wall‐clock times of simulations involving more than 10 billion grid points were as low as minutes. This permits ensemble simulations for a considered scenario event so that modeling uncertainties (e.g., source and geology) can be bracketed.

We present the development of ground-motion models (GMMs) to predict 5% damped pseudo-spectral accelerations (PSA) and effective amplitude spectra (EAS) as part of the NGA-West3 project. Our approach utilizes the project dataset and shared functional forms for the PSA and EAS models, which are being developed in a coordinated manner to ensure internal consistency. This is different from other approaches that focus mainly on EAS models and then convert them to PSA using random vibration theory and duration models. Both our EAS and PSA models are conditioned on the same predictor variables and include broad regional adjustments to capture tectonic variability. The PSA model is being developed through a traditional period-by-period mixed-effects framework, whereas the EAS model utilizes a seismologically-based source parameterization and frequency-dependent path and site models. Ongoing work aims to refine regionalization, anelastic attenuation, and site response models.

The Next Generation Attenuation (NGA)-West3 Program database builds upon that of NGA-West2 for shallow crustal earthquakes in active tectonic regimes to provide a robust dataset to develop the next iteration of NGA ground motion models (GMMs). Researchers from Italy, Japan, New Zealand, Taiwan, United Arab Emirates, and the United States, amongst others, have collaborated to develop consistently processed data with uniform metadata from the respective regions, with data for other regions drawn from literature (e.g., Greece, Japan, Türkiye). Over 150,000 three-component ground motions (mostly in California and Japan) with magnitudes generally greater than 3.0 have been newly added and the total database size is over 175,000 ground motions (generally three-component except for two-component records from KiK-Net stations in Japan). The database is being used by several teams of NGA GMM developers.

Microtremor-based horizontal-to-vertical spectral ratios (mHVSR) offer insights into subsurface seismic velocity structure, primarily through resonant features revealed by peaks in mHVSR curves. Typically derived from three-component broadband seismometers with high-pass corner frequencies of 0.0083 Hz over relatively short observation periods (1-2 hrs), mHVSR is a cost-effective and time-efficient method of site characterization. This approach has been shown to correlate with site-specific ground motion characteristics and recent studies suggest that mHVSR holds significant potential for improving ergodic ground motion models. This study produced mHVSR curves at 1,127 seismic stations with permanently installed broadband seismometers across California and neighboring states. Each mHVSR curve was uniformly processed using the Python library hvsrprocpy following standard procedures with minor modification. These data were added to the Community Shear-Wave Velocity (VS) Profile Database (VSPDB; https://vspdb.org) and are being used in ongoing work seeking to develop mHVSR-conditioned site terms in GMMs for California.

We are working on the development of an earthquake ground-motion model (GMM) for effective amplitude spectra (EAS, the smoothed quadratic mean of two horizontal-component Fourier amplitude spectra) as part of the NGA-West3 project. κ0 is a model parameter that controls the shape of the high frequency portion of the EAS spectrum by determining how far it deviates from an ω^{-2} source model. Here, we focus on development of κ0 models within the GMM, with increasing levels of complexity: regional constants, regional relationships with VS30, and a modular model based on the depth to the 1 km/s shear wave velocity isosurface (z1). We estimate average values of κ0 range from 0.023 – 0.065 s. Mexico, southern California, and Japan have lower than average κ0 (less attenuation) whereas Taiwan and northern California have higher than average values (more attenuation). We find that sites with low time-averaged shear wave velocity in the upper 30 m (VS30) have larger κ0, whereas VS30 has no control on κ0 for stiff sites (VS30 ≥ 300 m/s). Variations in depth to the 1 km/s isosurface (z1) have a stronger control on κ0, up to a factor of three in some regions; we parameterize a model using z1 such that it can be combined with our κ0(V_{S30}) model.

The National Seismic Hazard Model Program Steering Committee (NSHM-SC) conducts participatory peer-review during the development of NSHM products, working with the NSHM development team in the U.S. Geological Survey (USGS). This report documents the review processes and outcomes related to the development of NSHM products for application to Puerto Rico and the Virgin Islands (PRVI). The committee consists of nine members who were selected by the USGS, based on expertise and experience. The USGS requested a written report from the NSHM-SC answering the following questions relevant to publication of the 2023 NSHM for PRVI: • Was the National Seismic Hazard Model adequately reviewed? • Did the USGS respond appropriately to the review comments and recommendations? • Is the model suitable for release and to serve as the basis for hazard mitigation? The SC responses to the three main questions are: Was the National Seismic Hazard Model adequately reviewed? The NSHM-SC, and earthquake rupture forecast (ERF) and ground motion characterization (GMC) expert panels called by the NSHM-SC, engaged in several meetings to review components and the complete model between August 2024 and December 2025. The NSHM-SC consensus is that the level of review of the materials provided has been adequate. Did the USGS respond appropriately to the review comments and recommendations? The USGS responded to all NSHM-SC and panel comments and recommendations. USGS adopted most of our recommendations. When not adopted, USGS provided explanations that often indicated that a suggested modification could not be made due to limited time or because adopting the suggestion would break long-standing precedent. Because the panels and the NSHM-SC are advisory in nature, this is acceptable. Is the model suitable for release and to serve as the basis for hazard mitigation? The NSHM-SC considers the 2025 NSHM for PRVI to be suitable for use in building code and similar applications at return periods of 475 years (i.e., corresponding to exceedance probabilities of 10% in 50 years) or longer. The 2025 NSHM for PRVI represents a substantial improvement over the previous NSHM for PRVI developed in 2003, because of the use of better input source characterization models, better ground-motion models, and overall improved computational techniques.

In August 2024, the National Seismic Hazard Model (NSHM) Steering Committee appointed a 14-member panel (herein referred to as “The Panel” or “Panel”) to review the time-independent earthquake rupture forecast (ERF) for the 2025 update of the Puerto Rico and U.S. Virgin Islands (PRVI) component of the NSHM (herein referred to as “PRVI25-ERF”). This report summarizes the Panel’s findings and recommendations. The primary materials for Panel review were nine papers documenting the PRVI25-ERF draft model. The Panel was also informed about the process to update the ERF in three briefings by the U.S. Geological Survey (USGS) development team (herein referred to as the “USGS Team” or “Team”). The PRVI25-ERF model is a substantial improvement over the current model, which was released in 2003 (PRVI03-ERF; Mueller et al., 2003, 2010). The USGS Team has incorporated substantial new information obtained about the region and its tectonic environment over the past twenty years, and they have combined this information with state-of-the-art probabilistic seismic hazard modeling techniques to produce a complete probabilistic ERF that, for the most part, reflects the best available earthquake science. The publications and reports on the model development are generally excellent and comprehensive. State-of-the-art techniques have been employed in the neotectonic and paleoseismic evaluations of fault activity, in the analysis of earthquake catalogs, and in the incorporation of slip-rate estimates from paleoseismic and geodetically constrained tectonic block models. The model of fault slip rates has been generalized to a probabilistic representation, and the epistemic uncertainties of these rates have been incorporated into the logic-tree formulation using a novel stochastic sampling method. The fault system inversion techniques developed for the CONUS23-ERF for the western U.S. (Field et al., 2023; Milner and Field, 2023) have been successfully applied to the PRVI fault systems. The Panel raised questions about the adequacy of the PRVI seismicity data, which are compromised by magnitude inconsistencies; the structure of the crustal fault model and its accommodation of strain partitioning; the weakness of the geodetic constraints on seismic coupling factors; and the low amplitudes of the epistemic uncertainties in the hazard derived from the ERF logic tree. Several of the Panel’s 16 actionable recommendations are directed towards a more complete assessment of the ERF uncertainties. Of particular concern is the bias and uncertainty related to the seismic coupling factors. The Panel also offers 19 aspirational recommendations for model improvements that could be implemented in future PRVI updates. The Panel recommends that, after suitable responses to the actionable recommendations in this review, the PRVI25-ERF model be adopted as a component of the NSHM.