Reduction of Bias and Uncertainty in Regional Seismic Site Amplification Factors for Seismic Hazard and Risk Analysis

Abstract

Site amplification factors in National Building Codes are typically specified as a function of the average shear wave velocity over the first 30 m (Vs30) or site class (A, B, C, D and E) for defined ranges of Vs30 and/or ranges of depth to bedrock. However, a single set of amplification factors may not be representative of site conditions across the country, introducing a bias in seismic hazard and seismic risk analyses. This is exemplified by significant differences in geological settings between East and West coast locations in North America. Western sites are typically characterized by lower impedance contrasts between recent surface deposits and bedrock in comparison to Eastern sites. In North America, site amplification factors have been derived from a combination of field data on ground motions recorded during West Coast earthquakes and numerical models of site responses that are meant to be representative of a wide variety of soil profiles and ground motions. The bias on amplifications and their impact on seismic hazards is investigated for the Montreal area, which ranks second for seismic risks in Canada in terms of population and hazard (PGA of 0.25 g for a 2475 years return period). Representative soil profiles at several locations in Montreal are analyzed with 1-D site response models for natural and synthetic ground motions scaled between 0.1 to 0.5 g. Since bedrock depths are typically shallow (<30 m) across the island, bedrock shear wave velocities have a significant influence on the impedance contrast and amplifications. Bedrock shear wave velocity is usually very variable due to the differences in rock formations, level of weathering and fracturing. The level of this uncertainty is shown to be greatly decreased when rock quality designation (RQD) data, common information when bore hole data are logged, is available since it is highly correlated with both shear and compression wave velocities. The results are used to derive region-specific site amplification factors as a function of both Vs30 and site fundamental frequency and compared to those of the National Building Code of Canada (2015). The results of the study indicate that there are large uncertainties associated with these parameters due to variability in soil profiles, soil properties and input seismic ground motions. Average and confidence intervals for the mean and for predictions of amplification factors are calculated for each site class to quantify this uncertainty. Amplifications normalized relative to class C are obtained by accounting for the correlation between site class amplifications for given ground motions. Non-linearity in the analysis of equivalent linear 1-D site response is taken into account by introducing the non-linear G/Gmax and damping ratios curves. In this method, it is assumed that the shear strain compatible shear modulus and damping ratio values remains constant throughout the duration of the seismic excitation. This assumption is not fully applicable to a case when loose saturated soil profile undergo heavy shaking (PGA > 0.3 g). In this study, all simulations with input motion PGA >0.3 g have been performed by using the EL method instead of the NL method considering that cohesive soils (clay and silt) at Montreal sites are stiff and cohesionless soils (sand and gravel) are considerably dense. In addition, the field and laboratory data required to perform NL analyses are not currently available and may be investigated in future works.

Recent Advances of Deep Learning in Geological Hazard Forecasting

“…As mentioned previously in the introduction, unlike other fields where deep learning is common, the occurrence of geological disasters is full of uncertainties [80], restricting the practical applications of basic neural networks and common numerical methods. To quantify the impact of various uncertainty factors on the occurrence of geological hazards, deep learning needs to be employed in a unified probabilistic model.…”

Vs30 Mapping of the Greater Montreal Region Using Multiple Data Sources

“…The Rock Quality Designation (RQD) is a parameter defining the degree of jointing in the rock and ranging from zero to 100%, the latter being the best quality rock. In order to consider this information when estimating Vs for rock, Talukder et al [25] proposed a relationship as follows:…”

Effect of Base Conditions in One-Dimensional Numerical Simulation of Seismic Site Response: A Technical Note for Best Practice

“…Numerical or analytical approaches require a definition of the local subsoil conditions and of the so-called reference site [1][2][3][4]. Local subsoil conditions are defined in terms of mechanical and geometrical properties [5][6][7][8][9]: geological subsoil models are characterized by the geometry of buried morphologies and topographical surfaces, combined with the profile of shear wave velocity with the depth [V S (z)], unit weight of soil (γ t ), plasticity index (PI), relative density (D R ), nonlinear curves referring to the decay of secant shear stiffness [G S (γ)/G 0 ] and the increment of damping ratio [D(γ)] with shear strain (γ). On the other hand, the reference site is required to define the input motion (i.e., the time history of acceleration or velocity or displacement or stress) for the LSSR.…”

“…The rigid base condition can be adopted (i) if the input motion is recorded at the base of the deposit of interest (for example, see point B in Figure 1), disregarding the impedance contrast between such a deposit and the underlying strata, or (ii) if the input motion is recorded at the outcrop of a stiff material (see point O in Figure 1) and the impedance contrast (briefly, the ratio between the stiffness of the half-space and of the deposit) is theoretically equal to infinite. Truthfully, if the input motion is recorded at the ground surface of the outcropping half-space (i.e., point O in Figure 1), referring to most Italian diffuse case studies, the so-called elastic base condition should be adopted since the impedance contrast between the half-space material and the deposit of interest is in the range 1.5-6 [9]: hence, lower than infinite. In this context, it is worth noting that the terms rigid and elastic refer to the numerical base conditions, rather than to the stiffness of the soil.…”