Comprehensive Earthquake Site Amplification Assessment For Greater Vancouver

Jamal Assaf, Sheri Molnar, Hesham M. El Naggar

In the proceedings of: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Soil Mechanics and Foundations (Seismic Analysis)

ABSTRACT: Soft thick sediments of the Fraser Delta in Vancouver are well known to amplify seismic waves at frequencies related to their characteristics. In this study, all available strong-motion recordings from 7 moderate earthquakes of magnitude (M) > 4.3 between 1976 and 2015 are utilized to provide a comprehensive assessment of observed site amplification in Greater Vancouver. Microtremor measurements conducted near these strong motion stations are used to calculate horizontal-to-vertical (H/V) spectral ratios. H/V ratios from earthquake recordings and microtremor measurements in the Fraser delta consistently demonstrate an amplification of about 6-8 at lower frequencies (0.3 Hz). Upper-to-lower and horizontal-to-vertical spectral ratios from borehole recordings from the 2015 earthquake are compared to theoretical amplification of the 1D soil model. The fundemental frequencies are well captured by the 1D model, however peak amplitudes are slightly overpredicted.

RÉSUMÉ: Les sédiments mous et épais du delta du Fraser au Vancouver, sont connus par leurs amplification des ondes sismiques à des fréquences liées à leurs caractéristiques. Dans cette étude, tous les enregistrements de mouvements forts disponibles provenant de 7 séismes de magnitude modérée (M) > 4,3 entre 1976 et 2015 sont utilisés pour fournir une évaluation complète de l'amplification du site observé dans Vancouver. ces stations de mouvement fort sont utilisées pour calculer les rapports spectraux horizontal-vertical (H/V). Les rapports systématiquement une amplification d'environ 6 à 8 pour des fréquences plus basses (0,3 Hz). Les rapports spectraux supérieurs à inférieurs et horizontaux à verticaux provenant des enregistrements de forage du séisme de 2015 sont comparés à l'amplification théorique du modèle de sol 1D. Les fréquences fondamentales sont bien captées par le modèle 1D, par contre, les amplitudes maximales sont légèrement surestimées. 1. INTRODUCTION Amplification of seismic waves due to the local geology, specifically soft soils, has been confirmed in many previous earthquakes, 1985 Mexico earthquake and the 1994 Northridge earthquakes (Rayhani and El Naggar 2007). It is well known that amplification at a specific site can occur at certain frequencies related to the stiffness and depth of geologic structure beneath. Severe ground shaking resulting from amplified waves can induce significant damage to structures and trigger landslides and liquefaction of soils. Greater Vancouver, the largest metropolitan area in British Columbia (BC), is located in one of the most seismically active regions in Canada (Rogers et al. 1998). The Fraser river delta, in southern Vancouver, has a substantial and increasing population and a number of vital economic facilities, including port facilities, the Vancouver International Airport, a ferry terminal and hydroelectric transmission cables (Cassidy and Rogers 1999). The Fraser River delta is mainly composed of alluvial sediments that modify the amplitude and frequency of seismic waves (Cassidy and Rogers 2004). Amplification hazard in Vancouver is associated with the presence of deep soft soils in the Fraser River delta area. Cassidy and Rogers (2004) observed a peak amplification at frequencies 1.5 to 4 Hz in the Fraser delta from 4 previous earthquakes. Molnar et al. (2013) demonstrated that earthquake recordings at 3 thick delta sites exhibit a consistent low fundamental frequency (0.3 thickest Holocene delta sediments exhibit microtremor horizontal-to-vertical (H/V) spectral ratio peaks around 0.2 Hz (5 sec) (Onur et al. 2004). Agreement between low-level earthquake and microtremor H/V spectral ratios at Victoria sites was demonstrated by Molnar et al. (2006). Other studies have tried to assess the applicability of theoretical site response to represent observed amplification at the surface in Fraser River delta. Using two 300 m deep boreholes in Fraser delta as site models, Harris et al. (1998) predicted a significant 1D amplification at longer periods (3.5 to 5s) in contrast to the observed amplification. Further, Finn et al. (2003) compared theoretical site response analysis in Fraser delta to the 1996 earthquake recordings. They concluded that 1D analysis predicted the recorded response only at deep sites, while neither 1D nor 2D predicted that of shallow sites. Understanding and assessing the amplification hazard in Vancouver is a first step to effectively reduce its effects on communities. This paper provides a comprehensive site response analysis from the largest earthquake data sets recorded in Vancouver, with 2011 and 2014 earthquake results presented for the first time. The acceleration spectral amplitudes at different stations for earthquakes between

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