UCLA Civil and Environmental Engineering

Executive Summary Along with source and path effects, site response is an essential component of ground motion prediction. Widely used ground motion models (GMMs), also known as ground motion prediction equations, provide an ergodic representation of each component in the sense that observations from global databases during the observation period (generally the last few decades) are taken to apply for a particular site and tectonic setting of interest, following conditioning on relevant parameters (magnitude, distance, time-averaged shear wave velocity in the upper 30 m, VS30). Such models inherently average across effects that may exhibit location-to-location variability, increasing model dispersion. The use of non-ergodic site response has gained increasing attention in recent years as a means by which to increase model accuracy and reduce model dispersion, both of which affect the outcomes of seismic hazard analysis. The analysis of non-ergodic site response can, in general, be undertaken through analysis of recordings at the site of interest, or (in the absence of such data) through the use of geotechnical simulations. The most common simulation approach, known as ground response analyses, simplifies the actual site response problem by assuming horizontal soil layers and vertically propagating waves. The objective of this research was to compile and analyze data from vertical arrays in California for the purpose of evaluating ground response analysis as a method of predicting non-ergodic site response and to estimate epistemic uncertainties associated with its application. More specifically, we investigated three questions: (1) how effective is ground response analysis at predicting observed small strain, essentially visco-elastic, site response as observed at California vertical arrays?; (2) which models for small strain damping are most effective for use in ground response analyses?; and (3) recognizing the imperfect ability of ground response analysis to capture observed site response effects, how should epistemic uncertainty in site response be represented when it is estimated using ground response analysis procedures? We consider a database of 21 California vertical arrays operated by the California Strong Motion Instrumentation Program (CSMIP) and the University of California Santa Barbara. Each of the considered arrays has ≥ four surface and downhole ground motion recordings, and cumulatively our database contains 287 ground motion pairs from 207 earthquakes. Uncorrected (version 1) acceleration time series were processed using standard procedures developed for the Next Generation Attenuation projects. Although this database is considerably smaller than the KiK-net database that has been widely used in prior research, it has two notable benefits: (1) it represents site response for a distinct region (California) with a different geologic history and (2) the available velocity profile data is of higher resolution and quality, and is mostly accompanied by geotechnical logs with detailed information on soil conditions. The processed data were plotted as surface-to-downhole transfer functions and ratios of 5% damped pseudo acceleration response spectra, each of which represents in different ways the frequency-dependent site response for essentially visco-elastic conditions over the depth range of the arrays. The site response is considered visco-elastic because relatively weak ground motions were selected for analysis. Ground response analyses were performed using the measured shear wave velocities, various damping models, and the recorded base motion as input. We find a higher percentage of California sites, as compared to KiK-net sites from Japan, to have a reasonable match of empirical and theoretical transfer function shapes. The empirical transfer functions also have a greater degree of event-to-event consistency than has been found previously in Japan. We were unsuccessful at diagnosing conditions that would indicate, a priori, whether ground response analyses are or are not effective for a particulate site. Three damping models were considered in the ground response analyses – geotechnical models, models for quality factor (Q) based on seismological inversion, and models derived from the site-specific site diminutive parameter (κ0). These models represent, to varying degrees, the attenuation of ground motions from two physical mechanisms – soil intrinsic (hysteretic) damping and wave scattering, both of which would be expected to be present to varying degrees at a given site. Despite the different mechanisms, the principle means by which to incorporate damping in ground response analysis is through the soil hysteretic damping considered in the analysis (D), which was the approach taken here. As expected, the effects of using different damping models are concentrated at high frequencies, specifically those higher that the frequency of the modelled soil column. Ground response analyses based on geotechnical models underestimate site attenuation, which has been observed previously and is expected because scattering effects are neglected. The models based on seismological inversion tend to overestimate site attenuation; this conclusion is likely not fully general, but applies to the considered data inventory. We describe a means by which to adjust geotechnical models for D using observations of κ0, more specifically the change of κ0 across the depth range of vertical arrays. This approach yielded intermediate levels of site attenuation that modestly improved prediction accuracy and reduced the dispersion of residuals relative to the other damping models. We use the residuals of ground response analysis predictions of site response, relative to observation, to quantify epistemic uncertainties. Our proposed methodology partitions prediction residuals into between- and within-site components, and takes the between-site standard deviation as a quantification of epistemic uncertainty. Our results suggest values ranging from 0.35-0.5 in natural log units, which is surprisingly consistent with related prior results from other investigators using Japanese data. We also find levels of event-to-event variability for a given site that are consistent with observations elsewhere, including Japan and Taiwan.

Probabilistic seismic hazard analysis (PSHA) are performed for routine applications using source models and ground motion prediction equations (GMPEs) recommended by a government agency (e.g., US Geological Survey) or an expert panel (e.g., SHARE project). For important projects, site-specific PSHA involves critical analysis of GMPEs and sources for the application region. We adopt the latter approach for a dam site in Calabria (southern Italy), a high seismic hazard region. Following SHARE convention, we consider faults sources, background zones, and area sources. We identify several problems with assigned maximum magnitudes in the SHARE model for fault and in-slab subduction sources. We add two sources not present in some prior inventories – the crustal Lakes fault and the Calabrian arc subduction interface. Following procedures developed for the Global Earthquake Model, we select GMPEs that are much better constrained in the hazard-controlling range of magnitudes and distances than those typically used in prior Italian applications. Short-period spectral accelerations at the 2475 year return period exceed those from prior SHARE studies by about 10-15%. Despite the site being located in a region with finite faults capable of generating large events, the 2475-year hazard is dominated by source zones that allow for earthquakes directly beneath the site.

Soil-structure interaction (SSI) analysis evaluates the collective response anddynamic interplay of three linked systems: the structure, the foundation, and the soil underlying and surrounding the foundation. Problems associated with practical application of SSI for building structures are rooted in a poor understanding of fundamental SSI principles. Implementation in practice is hindered by a literature that is difficult to understand, and codes and standards that contain limited guidance and, in some cases, are proprietary. A recent report published by the National Institute of Standards and Technology (NIST) provides a mechanism for advancing the state of practice in SSI for practicing engineers. It offers a synthesis of the body of SSI literature, distilled into a concise narrative and harmonized under a consistent set ofvariables and units. Techniques are described by which SSI phenomena such as foundation-soil compliance and damping (inertial interaction), and foundation-to-free-field ground motion change (kinematic interaction) can be evaluated in engineering practice. Specific recommendations for modeling these and other seismic soil-structure interaction effects on building structures are provided. The resulting recommendations are illustrated and tested though simulations of two example buildings with earthquake recordings.

A fragility model for seismic deformations of levees was developed in a separate study using case history data from the Shinano River region of Japan (SRJ). In that model, levee fragility was shown to be principally related to ground motion intensity, geomorphology, and ground water level relative to the levee base. Our objective in this manuscript is to demonstrate the applicability of the developed fragility models for geotechnical conditions along urban levees in the Central Valley region of California (CVC). For this purpose, we compare SPT penetration resistance data (in the form of energy- and overburden-corrected blow counts) between regions for common soil types conditional on geology and topography. Among the geologic categories considered, arguably the most important is Holocene flood plain deposits, which comprise 38% of investigated sites in CVC and 97% in the SRJ. Within this geological unit, we find penetration resistance data for coarse-grained soils in the SRJ and CVC study regions to be similar, whereas for fine-grained soils the CVC sediments are stiffer. For two other geological units (Holocene basin and Pleistocene), both coarse- and fine-grained deposits in the CVC are stiffer than Holocene floodplain deposits. We also considered topographical conditions (elevation, ground slope and river gradient) as alternative means for sorting the data, with the general conclusion that such indicators are less capable than geology of describing variations of penetration resistance within the respective regions. The results provide insight into the relative vulnerability of levees in the two regions for given levels of ground motion amplitude.

Remote data monitoring was performed to measure settlement and pore pressure in soft peat beneath a model levee constructed in the Sacramento / San Joaquin Delta. Consolidation and secondary compression behavior was monitored following construction and dynamic testing of the model levee using piezometers embedded in the peat and an in-place horizontal inclinometer at the base of the model levee. A solar powered data acquisition system was used to gather the data, and a modem transmitted the data to a web-based interface. This system enabled us to (i) know when primary consolidation had finished prior to testing, (ii) observe the large influence of secondary compression on observed settlements, and (iii) observe a lack of any significant post-cyclic settlement. The initial change in pore pressure was predicted well using Skempton's pore pressure parameters.

The M_w 9.0 Tohoku-oki Japan earthquake and many of its subsequent aftershocks and triggered events were very well recorded, providing opportunity for detailed study of source, path, and site effects. In previous work, we utilized mainshock data to demonstrate faster attenuation with distance in backarc than in forearc regions, positive event terms for high frequency intensity measures for multiple ground motion prediction models, and minimal scaling of site amplification with V_s30 at high frequencies. In this paper, we consider two subsequent events, one M_w 6.7 on the plate interface that ruptured in the mainshock and one M_w 6.3 inland from the volcanic front, to examine if similar ground motion features are apparent from these data sets. We continue to find evidence for the divergence in attenuation rate in forearc and backarc regions, despite the fact that the events are located on opposite sides of the volcanic front. The site response for these events does not appear to follow previously observed patterns for Japan, being relatively similar to V_s30-scaling in active crustal regions generally.