This study examines systematic site effects from the prediction residuals of physics-based ground motion simulations using a dataset of small magnitude (3.5 ≤ M_W ≤ 5.0) active shallow crustal earthquakes recorded in New Zealand (NZ). A significant amount of total uncertainty in ground-motion modelling comes from within-event residuals, highlighting the need for a comprehensive study of the site characteristics that contribute to this uncertainty. A diverse range of sedimentary basins and sites in distinct geomorphic categories are considered in this study with the primary objective of improving physics-based ground motion simulations in NZ. Advancing ground-motion predictability through ground motion simulations is a continually iterative process and requires addressing fundamental questions like: Which geographic regions have predictions that significantly deviate from observations and why? Which sites exhibit systematic prediction residuals and how can the attributes influencing them be identified? Which predictor variables show dependence with the site-to-site residuals? This study examines these questions by classifying 212 NZ sites using Nweke et al. (2022) geomorphic categories and hierarchical clustering of site-to-site residuals. Using these categories and data-driven approaches, this study explores the geospatial variation of site-to-site residuals. Trends in the site-to-site residuals for each geomorphic category indicate apparent differences between the four categories, for example, residuals for valley sites illustrate a clear dependence with the inferred fundamental site period. The utilization of hierarchical clustering of site-to-site residuals in conjunction with geomorphic categorization of sites has facilitated the understanding of diverse shapes of site-to-site residuals throughout the country. Site-to-site residuals for the hill sites within the selected clusters of the country were primarily influenced by the relative elevations of the ground motion recording sites. Presently, an iterative process of utilizing site characterization and clustering is underway to gain insights into the underlying causes of site-specific biases and inaccuracies in ground-motion modelling.

Large and long-period velocity pulses observed in the parallel direction at near-fault stations are important in earthquake engineering. A previous study has indicated that the existence of the ground surface and sedimentary layers affects fault rupture. This study conducts dynamic rupture simulations and discusses the dynamic interaction between fault rupture and seismic wave field induced by the sedimentary layer. The dynamic model of fault rupture is implemented using the finite element method to consider the interaction. Longer τh (pulse width) and shorter τr (rise time) of the near-surface slip rates are observed when the sedimentary layer exists, and τr is related to the travel time of reflected waves at the interface between the sedimentary layer and the basement.

The influence of local site conditions on the significant duration of strong ground motions for shallow crustal earthquakes was evaluated using a database of 2,610 strong ground motions recorded in the United States and Taiwan. Motions recorded with the closest distance to the causative fault, R_RUP, between 10 and 100 km with moment magnitude, M_W, between 4 and 7.6 were considered in the analysis. Values for the significant duration, D_5-95, the time between the arrival of 5% and 95% of the strong motion energy at the site, were downloaded from the strong motion database maintained by the Pacific Earthquake Engineering Research Center in the U.S. The associated values of M_W, R_RUP, and the average shear wave velocity for the top 30 m of the site, (V_S)₃₀, were also downloaded from this database. The data was separated into Site Classes A (hard rock) through E (soft soil) as defined based upon (V_S)₃₀. A functional form relating D_5-95 to M_W developed in previous studies was employed in the analysis. The data set was analyzed in aggregate and then separately for the recordings from the U.S. and Taiwan. The results of the analysis show the importance of considering the site class and tectonic setting when evaluating the duration of strong ground motion.

Shallow site effects are usually not explicitly modelled in hybrid broadband ground-motion simulations, and their proper incorporation may be key to improving prediction at soil sites. This paper examines five methods to adjust hybrid simulations to account for these effects. These methods require different levels of site-characterization data: Methods 1 and 2 only use proxy parameters (e.g., Vs30, Z1.0) to describe the site conditions, with Method 1 relying solely on proposed site adjustments in existing ground-motion models and Method 2 incorporating a host-to-target adjustment; Methods 3 and 4 use a shear-wave velocity profile along with two different frequency-domain approaches to predict the linear site response, coupled with the nonlinear component of Method 1; and Method 5 uses time-domain wave propagation analysis, and generally requires additional data to constrain nonlinear constitutive-model input parameters. Preliminary results of a validation study using 1000+ ground motions recorded at multiple strong-motion stations in the Canterbury Region (New Zealand) are provided. The results show that the incorporation of shallow site effects can significantly improve the accuracy of simulations. However, Method 1 tends to produce overamplification at relatively long vibration periods. An explanation for this phenomenon is provided in the paper and different strategies to mitigate this issue are discussed.

The quantitative evaluation of site amplification is an essential component of strong ground motion prediction because a site amplification factor (SAF) significantly differs from site to site, reflecting its specific geophysical underground profile. For the large-sized sedimentary basins, we investigated the spatial properties of the SAFs based on the generalized inversion technique using Fourier spectra. We found that the spatial distribution of the SAFs for the Swave portion with 5 to 15 seconds in duration (sSAF) was relatively similar in a small region for a low-frequency range from 0.12 to 0.5 Hz. We have also confirmed that SAF for the whole duration of motion (wSAF) is quite similar in a neighborhood area, although its amplitude is much larger than sSAF inside a basin, which reflects the inclusion of basin-induced surface waves. We proposed the prediction procedure for wSAF at an arbitrary site, using the ratio

between them named WSR. It is interesting to see similar WSRs in both horizontal and vertical components, even though their sSAFs are quite different. We confirmed these observed facts by a numerical simulation of Osaka Basin.

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.

In seismic engineering problems, artificial boundaries are often introduced in finite-volume models within an infinitely large earth. The reflection and refraction of seismic waves at these boundaries can distort waveforms within the model, making it an important issue to mitigate their effects. Absorbing Boundary Condition (ABC) has been widely used for this purpose since Berenger introduced it in 1994. However, the Lysmer-Kuhlemeyer (LK) viscous boundary, which employs dimensionless parameters a and b, is often recommended with values of 1.0 for both parameters, which is not always optimal for absorbing energy at different incident angles. This study aims to find the optimal a and b values for different incident angles. First, the discontinuous deformation analysis (DDA) method is used to simulate the usual artificial boundaries, such as free and fixed boundaries, demonstrating the waveform disturbance phenomenon caused by reflections inside the model. Next, it is shown that the LK boundary with a = 1.0 and b = 1.0 can effectively absorb reflection interference when the incident wave is perpendicular to the artificial boundary. However, for cases with incident angles, the LK boundary with a = 1.0 and b = 1.0 fails to provide effective absorption. Consequently, the Monte Carlo simulation method is employed to randomly select a and b values, using DDA to simulate seismic wave propagation within the model. The optimal a and b values corresponding to the incident angle are found based on the maximum absorption rate. A large number of calculations are performed for different incident angles, and the empirical formulas for a and b are derived.