UCLA Civil and Environmental Engineering

Traditional ergodic models are derived based on time-averaged shear-wave velocity in the upper 30 m of the site. These models are not able to account for site resonances, the presence and frequency of which can be established from microtremor HVSR surveys. Not all California sites exhibit such resonances, but knowledge that peaks are or are not present affects site response over a wide range of frequencies, with the former producing a response spectral peak near the HVSR peak. Research is underway to develop a model using microtremor HVSR data, which will be novel relative to previous models that are based on earthquake HVSR data. Our model is being formulated as modification to a global VS30 and z1.0 relationship. This paper explains the model development approach and findings of a systematic assessment of how HVSR curves relate to features of site-specific (or non-ergodic) response, which is informing model development.

The 2023 Türkiye/Syria earthquake sequence includes the February 6 M7.8 mainshock followed approximately nine hours later by a M7.7 aftershock, and many smaller aftershocks including a M6.8 and M6.3 on February 6 and 20, respectively. These events occurred in a region near the plate boundary of the East Anatolian Fault, in the proximity of which numerous ground motion recordings sites had been installed north of the Türkiye/Syria border. As a result, the events were well recorded both near the fault and at rupture distances up to 570 km. We describe the available recordings and component-specific data processing performed with the aim of optimizing usable bandwidth. The resulting database includes 310, 351, 291, and 229 usable three-component recordings from the M7.8, M7.7, M6.8, and M6.3 events, respectively. We also present source, path, and site metadata that was compiled according to uniform protocols. Comparisons to a global ground motion model (GMM) for active tectonic regions and a local, Türkiye-specific model demonstrate the existence of complex path effects that result in relatively poor fits between the GMMs and observed data at large distances (generally RJB > 200 km). Under-predictions at some stations may be influenced by directivity and/or basin effects that affect the ground motions but that are not accounted for directly in the GMMs. We also present analysis of spatial variability of peak ground acceleration for the M7.8 mainshock. A residual map produced from this analysis demonstrates that the global GMM over-predicts on the Anatolian block and under-predicts on the Arabian block, further supporting the existence of complex attenuation features in the region.

Seismic-hazard analysis (SHA) is typically performed using ergodic ground-motion models (GMMs), wherein the site response component is derived from global data and conditioned on the time-averaged shear-wave velocity in the upper 30 meters (VS30) and a “basin depth” term (e.g., Z1.0 or Z2.5). In the ergodic GMMs, for a given VS30, there is an implicit shear-wave velocity (VS) profile associated with the site response prediction that has smooth increases of velocity with depth. When a site-specific VS profile is characterized by abrupt velocity contrasts, for example at the rock-soil interface, the site response is likely to differ significantly from ergodic model predictions. This limitation of the ergodic models can be overcome by incorporating non-ergodic site response in the SHA. This approach involves customizing the site response for site-specific conditions, which has the effect of decreasing overall model uncertainty. In this paper, we describe results from ergodic SHA and SHA that incorporates non-ergodic site response at two sites in the San Francisco Bay Area. Both sites are characterized by a strong impedance contrast at the top of competent bedrock. Depth to bedrock at these sites varies, ranging from 75 meters to more than 400 meters. At each of the sites, nearby ground-motion records indicate that the ergodic GMMs tend to underestimate spectral accelerations at oscillator periods that are close to the fundamental site period. Conversely, there are typically broad period ranges where the ergodic GMMs overestimate spectral acceleration. Since the non-ergodic site response considers these local ground-motion data, these differences are reflected in the non-ergodic results. The findings from these two sites underscore the importance of estimating the fundamental site period, the limitations of ergodic models at sites with strong impedance contrasts, and the benefits of implementing non-ergodic site response into SHA.

Architected materials design across orders of magnitude length scale intrigues exceptional mechanical responses nonexistent in their natural bulk state. However, the so-termed mechanical metamaterials, when scaling bottom down to the atomistic or microparticle level, remain largely unexplored and conventionally fall out of their coarse-resolution, ordered-pattern design space. Here, combining high-throughput molecular dynamics (MD) simulations and machine learning (ML) strategies, some intriguing atomistic families of disordered mechanical metamaterials are discovered, as fabricated by melt quenching and exemplified herein by lightweight-yet-stiff cellular materials featuring a theoretical limit of linear stiffness-density scaling, whose structural disorder-rather than order-is key to reduce the scaling exponent and is simply controlled by the bonding interactions and their directionality that enable flexible tunability experimentally. Importantly, a systematic navigation in the forcefield landscape reveals that, in-between directional and non-directional bonding such as covalent and ionic bonds, modest bond directionality is most likely to promotes disordered packing of polyhedral, stretching-dominated structures responsible for the formation of metamaterials. This work pioneers a bottom-down atomistic scheme to design mechanical metamaterials formatted disorderly, unlocking a largely untapped field in leveraging structural disorder in devising metamaterials atomistically and, potentially, generic to conventional upscaled designs.

As part of the next generation liquefaction (NGL) project, we are developing probabilistic triggering and manifestation models using laboratory data and cone penetration test (CPT) case histories in the NGL database. The case histories are used to develop probabilistic models for surface manifestation conditional on susceptibility, liquefaction triggering, soil properties, stratigraphic details, and other features. Susceptibility is interpreted as a sole function of soil composition and is expressed as a probabilistic function of soil behavior type index, Ic, obtained from CPT. A triggering model is derived based on laboratory tests on high-quality specimens from literature; this model captures mean responses and uncertainty reflective of data dispersion and is considered as a Bayesian prior that will subsequently be updated by field observation data. A manifestation model is then regressed from field case histories where surface manifestation was or was not observed, information on soil conditions that enables identification of layers likely to liquefy, and ground shaking conditions. We describe the approach applied to develop our manifestation model; for a given layer this model considers layer depth, thickness, CPT tip resistance, and Ic. The result of this process is a logistic function in which manifestation probability decreases with increasing depth, decreasing thickness, increasing tip resistance, and increasing Ic. Profile manifestation is then derived by aggregating individual layer manifestation probabilities.

The pore structures of hardened Portland/slag cement pastes (>75 wt% slag content), and the initial capillary absorption of moisture through these pores, were monitored using ex situ synchrotron X-ray computerised microtomography and in situ quantitative neutron radiography. The pore structure becomes more constricted as the cement hydrates and its microstructure develops. This mechanism was effective even at a slag content as high as 90 wt% in the cementitious blend, where the lowest total porosity and a significant pore refinement were identified at extended curing ages (360 d). By combining this information with neutron radiographic imaging, and directly quantifying both depth and mass of water uptake, it was observed that 90 wt% slag cement outperformed the 75 wt% slag blend at 90 days in terms of resistance to capillary water uptake, although the higher-slag blend had not yet developed such a refined microstructure at 28 days of curing. The assumptions associated with the sharp front model for water ingress do not hold true for highly substituted slag cement pastes. Testing transport properties at 28 days may not give a true indication of the performance of these materials in service in the long term.

Layered double hydroxide (LDH) phases that form during cement hydration can incorporate a variety of interlayer anions in their interlayer positions. Here, a range of phases of general formula [MII(1−x)MIII(x)(OH)2][An−]x/n·zH2O were synthesized, where MII = Mg2+ (hydrotalcite) or Ca2+ (AFm), MIII = Al3+ such that [MII/Al] = 2 (Ca and Mg, atomic units) or 3 (Mg only), and A = Cl−, Br−, or I−. All the synthesized phases were characterized to assess their composition, density, and crystal structure. By approach from undersaturation, the solubility data of these compounds was measured at 5, 25, and 60°C. This thermochemical data was used to successfully model their formation using thermodynamic modeling and to infer the fields of stability of these compounds for conditions of relevance to cementitious systems. It is seen that halide-containing hydrotalcite phases strongly compete with hydroxide-containing hydrotalcite, with the latter prevailing at high pH. In contrast, halide-containing AFm compounds are more stable compared with hydroxide-containing AFm compositions.