Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada

Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada

Journal Pre-proof Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada Sheri Molnar, ...

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Journal Pre-proof Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada

Sheri Molnar, Jamal Assaf, Aamna Sirohey, Sujan Raj Adhikari PII:

S0013-7952(19)31273-6

DOI:

https://doi.org/10.1016/j.enggeo.2020.105568

Reference:

ENGEO 105568

To appear in:

Engineering Geology

Received date:

30 June 2019

Revised date:

19 February 2020

Accepted date:

26 February 2020

Please cite this article as: S. Molnar, J. Assaf, A. Sirohey, et al., Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada, Engineering Geology (2019), https://doi.org/10.1016/j.enggeo.2020.105568

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier.

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Overview of local site effects and seismic microzonation mapping in Metropolitan Vancouver, British Columbia, Canada Sheri Molnar1, Jamal Assaf2, Aamna Sirohey3, Sujan Raj Adhikari 4 1

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Assistant Professor, University of Western Ontario, Dept. Earth Sciences, [email protected] PhD Candidate, University of Western Ontario, Dept. Civil & Env. Engineering, [email protected] 3 MSc Student, University of Western Ontario, Dept. Earth Sciences, [email protected] 4 PhD Student, University of Western Ontario, Dept. Earth Sciences, [email protected]

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Corresponding Author: Sheri Molnar Assistant Professor University of Western Ontario Dept. Earth Sciences 1151 Richmond St. N London, Ontario, N6A 5B7 [email protected]

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Comprehensive seismic microzonation mapping is underway for Metro Vancouver Multiple non-invasive seismic methods are employed Vs profiles are developed near strong motion stations Site amplification is evaluated using microtremor and earthquake recordings Few well-recorded earthquakes present a challenge in validating methodologies

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Highlights

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Abstract

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A multi-year seismic microzonation project to map local variations of site response across Metropolitan (Metro) Vancouver, British Columbia, Canada, is currently underway. This paper presents our progress in one-dimensional site amplification hazard mapping of Metro Vancouver following our first field campaign. We initiated our microzonation study by assembling relevant geological, geophysical and geotechnical datasets from online and private sources to develop a regional 3D geodatabase. We perform site characterization near 20 strong-motion stations located on soft and stiff sediments using multiple non-invasive in situ seismic methods, including surface wave array and microtremor horizontal to-vertical spectral ratio (MHVSR) techniques, to retrieve shear-wave velocity (Vs) depth profiles. We also compile all available earthquake recordings from 6 events of moment magnitude (M) > 3.9 to assess earthquake site amplification across Metro Vancouver. Comparison of the limited earthquake horizontal-to-vertical spectral ratio (EHVSR) data with MHVSRs demonstrates consistent empirical site amplification, supporting the use of MHVSRs as a valid proxy for earthquake site amplification in British Columbia. Vertical array recordings in soft sediments from a M 4.7 earthquake further demonstrate consistency between these amplification measures. To achieve spatial coverage for regional microzonation mapping, the same Vs profiling methods are applied in available open areas and more than 1100 MHVSR measurements are collected in a 600-m resolution grid. Three additional field campaigns will target validation of our non-invasive seismic methods with co-located invasive testing and fill in any spatial gaps. This paper summarizes the developed and applied methodologies and challenges encountered in seismic amplification hazard mapping for the Metro Vancouver region.

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Keywords: seismic hazard, site effects, microzonation, hazard mapping, microtremor, Vancouver

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1. Introduction

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The influence of near surface geology in the upper 100s of meters on the amplitude and duration of earthquake shaking is a well known earthquake site effect phenomenon (Borcherdt 1970). Amplification may result from reduction in seismic impedance (product of density and shear wave velocity) typically towards the surface. Particularly, a significant reduction in seismic impedance can lead to large amplification due to resonance at a particular frequency related to the thickness and average shearwave velocity of a layer. The fundamental resonant frequency (f0) of a single soil layer over an elastic half-space from vertical propagation of a transverse shear wave is [1] where h is thickness and V Save is the soil layer’s average Vs (Haskell 1960). Alternatively, the fundamental period (T0) is defined as T0 = .

[2]

The time-averaged Vs of the upper 30 meters (Vs30 ) has been in use as a simplified site response predictor or metric since the late 1990’s (Borcherdt 1994) and is still prevalent in seismic hazard analysis today. Vs30 is able to capture broadband impedance-based amplification and correlates well with observed earthquake motions, but does not account for amplification at particular frequencies due to resonance in a one-dimensional (1D) soil column or up to three-dimensions (3D) inclusive of basin or topography effects (Di Alessandro et al., 2012; Steidl, 2000; Zhao et al., 2006). Regardless, Vs30 is the

Journal Pre-proof most commonly used metric to characterize and map earthquake site effects. In Canada, Vs30 was adopted as the site effect metric for seismic design guidelines in the National Building Code of Canada in 2005 and recently adopted in the Canadian Highway and Bridge Design Code (CHBDC) in 2015. There are six categories of earthquake site classification based on Vs 30 summarized in Table 1. Table 1. Seismic site classes in 2015 CHBDC. Vs30 (m/s) Vs 30 > 1500 760 < Vs 30 ≤ 1500 360 < Vs 30 < 760 180 < Vs 30 < 360 Vs 30 < 180 N/A

Description Hard rock Rock Firm soil or soft rock Stiff Soil Soft Soil Site-specific evaluation required

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Site Class A B C D E F

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There is a general need to define or map zones of increased seismic hazard due to earthquake shaking effects for various purposes including land-use or emergency-response planning, or seismic risk analyses. To accomplish regional amplification hazard (Vs30 or site class) mapping, a 3D geodatabase of the subsurface elastic properties is required, but the spatial density of the geodatabase is rarely achieved at the desired resolution (e.g., city block). In Canada, seismic microzonation maps (primarily Vs30) have been accomplished for cities with the highest seismic risk: Vancouver, British Columbia (BC) (Taylor et al. 2006), Montreal, Quebec (Rosset et al. 2015), Ottawa, Ontario (Motazedian et al. 2011), Victoria, BC (Monahan et al. 2000), and Québec City (Leboeuf et al. 2013). Nastev et al. (2016) developed maps of Vs30 and fundamental site period for the St. Lawrence Lowlands spanning from Ottawa to Quebec City. Preliminary efforts in seismic amplification hazard mapping have been accomplished for lower seismic risk cities including Toronto, Ontario (Mihaylov 2011) as well as Nanaimo, BC (Molnar et al. 2014). The 3D geodatabase used to produce maps for each of these regions varies significantly. The Ottawa mapping has the most comprehensive geodatabase including 15 boreholes with downhole Vs measurements, 686 seismic refraction or reflection profiles, 25-km of highresolution shear wave reflection ‘landstreamer’ profiling, and ~400 measurements of site period from microtremor horizontal-to-vertical spectral ratio (MHVSR) measurements; similar geodatsets are used in the Québec City mapping (Perret et al. 2013). The mapping for Montreal also has a significant geodatabase consisting of 26,600 boreholes, 3 downhole Vs profiles, 29 locations of multi-channel analysis of surface waves (MASW) Vs profiling, 7.5-km of high-resolution seismic reflection profiling, and 700 MHVSR measurements. The geology of Metro Vancouver is quite variable. A simplified geologic map based on sediments’ age is shown in Figure 1a. The youngest Holocene deposits in the region are modern alluvial, deltaic and bog deposits. The Fraser River (FR) delta, south of Vancouver, is a topographically lowland region comprised of deltaic silts and sands, with thickness up to 300 m (Rogers et al. 1998). Surrounding the delta lowland, are ‘uplands’ of higher elevation hills comprised of overridden (inter) glacial sediments and tills from repeated glaciations. These Pleistocene sediments are mostly composed of ice-compacted till present at or near the surface across most of Vancouver and Burnaby and glaciomarine and glaciofluvial sediments (Luternauer et al. 1994). The Holocene-Pleistocene sediment package overlies Tertiary sedimentary bedrock and pinches out to the north, from a maximum thickness of 800-1000 m in Ladner in Delta to only several meters at the edge of the delta (Britton et al. 1995). A simplified cross section along the FR delta is shown in Figure 1b (Rogers et al. 1998). Tertiary bedrock outcrops are observed in central

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Vancouver (Queen Elizabeth Park) and its northern limit (cliffs in Stanley Park). Furthest north, preTertiary plutonic igneous rocks are exposed at the highest elevations comprising the southern limit of the Coast Mountain range. For the Holocene FR delta sediments, Vs varies from a minimum of 70 m/s at surface to 500 m/s at 300-m depth dependent on sediment facies; Vs is generally lower than 250 m/s in the upper 30 m (Monahan and Levson 2001).. The average velocity of Pleistocene glacial sediments is ~475 m/s ranging between 400-1200 m/s with poorly defined depth dependency and the Tertiary bedrock velocity is more than 1500 m/s ( Hunter and Christian 2001, Monahan and Levson 2001, Hunter et al. 2016).

Figure 1. (a) Simplified geology map of Metro Vancouver based on age of sediments. (b) Geologic cross section along line A-A’ (modified from Rogers et al. 1998).

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Previous studies have accomplished amplification hazard (Vs30) mapping in the Metro Vancouver region but with variation in the data from which Vs30 was determined. An amplification hazard map for Metro Vancouver (Figure 2a) was developed from geologic mapping and borehole stratigraphy in combination with Vs measurements located outside of the FR delta area (Taylor et al. 2006). Hunter et al. (2002) developed a Vs30 map for the FR delta area (Figure 2d) using a geodataset consisting of 115 S-wave refraction profiles, 88 seismic cone penetration testing (SCPT) logs, and 52 downhole Vs profiles (Figure 2c). The amplification hazard map for the District of North Vancouver (Figure 2b) is developed largely from surficial and bedrock mapping supplemented with ambient vibration array (AVA) Vs profiling at 3 locations, 80 standard penetration tests (SPT), and 90 MHVSR site period measurements (Journeay et al. 2015). Overall, it is apparent that the Vs30 site class map for the FR delta in Figure 2d developed from a reasonable spatial density of site-specific Vs measurements (cost in the millions of dollars) is not the same mapping achieved using geological proxies shown in Figure 2a (cost in the thousands of dollars).

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Figure 2. Site class microzonation maps for (a) Metro Vancouver (modified from Taylor et al. 2006), where high amplification zones are assigned at the edges of the Fraser River delta and (b) District of North Vancouver (modified from Wagner et al. 2015). (c) Locations of Vs data and (d) contoured Vs 30 map for the Fraser River delta region (modified from Hunter et al., 2002). Site class maps for (b) North Vancouver and (d) the Fraser delta region occur within Metro Vancouver (dashed and solid line box insets in plot a, respectively).

In this paper, we summarize our progress in 1D amplification hazard (Vs 30 , site class, T0) mapping of Metro Vancouver following our first summer 2018 field campaign. Our development of a comprehensive geodatabase as well as methodologies used in determining the 1D amplification hazard are presented. All available earthquake recordings in the region are compiled to examine earthquake site amplification and to validate our field-based and theoretical methodologies for predicting site amplification. The geodata and analyses presented in this paper are the first steps towards accomplishing regional amplification hazard mapping (Vs30 , site class) for Metro Vancouver. Three additional field campaigns will be conducted as well as 3D numerical wave propagation to predict 3D sedimentary basin amplification (Ghofrani and Molnar 2019). These efforts lie within the framework of developing comprehensive regional seismic microzonation maps for Metro Vancouver including amplified shaking, and liquefaction and landslide potential as part of a multi-year project spanning 2017 to 2023 funded by

Journal Pre-proof the BC provincial government (Emergency Management British Columbia) and a non-profit centre for disaster prevention research and communication (the Institute for Catastrophic Loss Reduction) .

2. Geodatabase development

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Development of an appropriate geodatabase (e.g., balance between density and regional coverage of subsurface measurements) is vital for seismic microzonation mapping. We initiated development of our geodatabase in 2017 by assembling all publicly available geological, geophysical and geotechnical (summarized as “geo”) data for the Metro Vancouver region. The Geological Survey of Canada (GSC), within Natural Resources Canada (NRCan), provides digital mapped products online, e.g., surficial geology (Dunn and Ricketts 1994), and access to previous velocity measurements at ~500 locations across the FR delta (Hunter et al. 1998; 2016). Detailed topographic information (e.g., digital elevation models) and water well information are available at the provincial government level. We then contacted agencies, engineering firms, local geo-consultants, municipalities, and organizations (summarized as “stakeholders”) in the region to share their proprietary geodata. Sharing of the existing or previously collected geodata held by stakeholders in the region has ranged from “easy” (readily provided) to logistically challenging (e.g., requiring client permissions, data sharing agreements). In this way, we capitalize on previously collected geodata primarily from laboratory and invasive testing methods, e.g., borehole stratigraphy, cone penetration testing. We perform non-invasive seismic testing (described in next section) to supplement this proprietary geodata from stakeholders; only the non-invasive seismic testing will be shared publicly at the project’s conclusion.

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Development of our project’s geodatabase involves two main phases. In Phase I, we have extracted basic information about the geodata including: a unique file identifier, digital file location (folder) within the database, year of test or report, geolocation, a unique identifier for the relevant method (borehole, Vs, SPT, CPT, dynamic CPT (DCPT)), maximum depth, water table depth, Atterberg limits, sieve analysis (yes/no), liquefaction analysis (yes/no), and landslide analysis (yes/no) . This allows us to query and display geodata locations based on simple metrics like field method or maximum depth. In Phase II, we are extracting the relevant geodata (measured values) from selected reports for amplification, liquefaction and slope stability analyses and is ongoing. The current geodatabase consists of over 3,800 unique locations of geodata (Figure 3a); we have added 85 locations with Vs measurements in combination with the GSC’s ~500 locations of Vs measurements across the FR delta.

3. Non-invasive seismic testing methods Non-invasive seismic testing is accomplished to supplement previously collected geodata from stakeholders in development of a comprehensive 3D geodatase. This 3D geodatabase of subsurface material properties will allow us to predict site amplification via numerical 1D to 3D wave propagation modelling. Our multi-method non-invasive testing approach includes: MHVSR calculation to obtain each site’s peak frequency or site period and its amplification, and active- and passive-source surface wave array methods to provide each site’s dispersion curve, which are jointly inverted to resolve subsurface layered earth models and thereby Vs depth profiles for Vs 30 estimation.

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Site amplification from MHVSR measurements

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There is ample literature that documents the effectiveness and reliability of non -invasive methods for seismic site characterization compared to invasive methods. Several blind-test experiments have validated Vs profiles and/or Vs 30 estimates from non-invasive methods (e.g., Molnar et al. 2010; Garofalo et al. 2016 a,b). Overall, in simple geologic settings (e.g., soft material at surface increases in Vs with depth), non-invasive methods provide reliable Vs values with a similar degree of error as invasive methods over the depth interval corresponding to frequencies or wavelengths of the non -invasive data. In more complex geologic settings (e.g., stiff ground with low velocity zones or rock sites), the variability in the Vs estimate will increase, which is also true for invasive methods (Garofalo et al. 2016b). Smallscale variations in Vs with depth are best measured by discrete invasive measurements; however, both invasive and non-invasive methods provide robust average estimates (e.g., Vs30) for the same site. Noninvasive methods provide additional benefits for seismic site characterization. For example, the combination of both active- and passive-source surface wave array testing is recommended in practice (e.g., Foti et al. 2018) to ensure dispersion estimates are measured over a wide frequency (wavelength) bandwidth, i.e., the site’s ‘full’ dispersion curve and thereby its ‘entire’ velocity depth profile. In addition, the use of MHVSR, either on its own or in conjunction with surface wave dispersion techniques, resolves subsurface information (Vs estimates) to greater depths (e.g., Molnar et al. 2018; Pratt 2018). Hence our non-invasive approach is an efficient and effective methodology for regional coverage and to obtain informative site effect metrics for amplification hazard mapping. In our 2018 field campaign, we collected over 400 MHVSR measurements and performed multi-method non-invasive testing for Vs profiling at 44 sites (Figure 3b and 3c, respectively).

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Empirical evidence has demonstrated that peak MHVSR amplification occurs at, or close to, the site’s f0 as determined from earthquake recordings provided that there is a sufficiently strong impedance contrast, e.g., Bard (1999). Cassidy and Molnar (2009) showed that MHVSRs at earthquake recording sites on firm-to-soft sediments across British Columbia agree with earthquake horizontal -to-vertical spectral ratios (EHVSRs) and earthquake standard spectral ratio (SSR; reference rock site) up to and including the first peak (f 0). MHVSRs typically do not replicate EHVSR amplification at higher frequencies (resonance modes). This agreement of MHVSR peak amplification with f 0 and its amplification determined from earthquake recordings is observed at soft and stiffer sites in Alberta (Farrugia et al. 2018) and Ontario (Braganza et al. 2016). Generally, there is a sufficiently strong impedance contrast across Canada due to multiple glaciations (e.g., softer post-glacial sediments over glacial tills or competent unweathered (bulldozed) rock). Hence, MHVSRs provide a reliable measure of f 0 and its amplification for most sites across Canada (Cassidy and Molnar 2009) but does not replicate the entire earthquake site amplification spectrum. The exception is sites with a very strong impedance contrast, e.g., soft glaciomarine silts and clays overlie competent unweathered rock , particularly in the St. Lawrence Lowlands. In these cases, the MHVSR peak amplification is outrageously high (> 40) yet outrageously high amplification is also observed from weak earthquake shaking (EHVSR and SSR amplifications > 10 and exceeds 80 in one case, Adams 2007; EHVSR amplification of 50, Cassidy and Molnar 2009). In this case, only the MHVSR f 0 is a reliable measure; the outrageously high amplification is indicative of the very strong seismic impedance contrast. With our confidence in MHVSRs to provide f 0 and its amplification in Canada, we collected over 400 MHVSR measurements across Metro Vancouver during our first field campaign in 2018. These MHVSR measurements are supplemented with previous MHVSR measurements (Onur et al. 2004; Molnar et al.

Journal Pre-proof 2013) collected by the first author and personnel of the Earthquake Engineering Research Facility at the University of British Columbia between 2004 and 2012. Our current MHVSR database for Metro Vancouver consists of over 1100 measurements at a ~600 m grid resolution (Figure 3b) to detect regional subsurface variability.

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Microtremor measurements were performed during daytime hours with appropriate recording durations (i.e., longer durations at deeper sites), ensuring soil-sensor coupling, and protection against weather conditions (e.g., wind reduction). In few cases, repeated measurements were performed at a single site to ensure reliability and repeatability. The sampling rate of recordings ranged between 100 and 200 Hz, which is high enough to ensure that low frequencies are properly sampled. The recordings were performed for a minimum of 15 minutes throughout the region, except on the FR delta, where a minimum 30 minutes duration was performed. At select locations on the FR delta, a Guralp 40T broadband seismometer (flat instrument response to 0.03 Hz) was used for validation of low frequency MHVSR peaks. At these FR delta locations, co-located MHVSR measurements of up to 2-hour duration were performed using both a Tromino® and the Guralp broadband seismometer. Figure 4 shows that MHVSRs calculated from recordings by the two types of seismometers are consistent. This provides confidence we are able to reliably measure f 0 as low as 0.15 Hz from long-duration recordings using either seismometer type.

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Figure 3. (a) Simplified geology map showing locations of discrete geodata (e.g., borehole, CPT, SPT). Coloured crosses identify geodata locations with Vs measurements. (b) Map of MHVSR measurement locations. (c) Map showing locations of our tested AVA and MASW array sites, as well as both free-field strong-motion stations and three instrumented borehole arrays . Select strong-motions stations referred to in this study are labelled.

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Figure 4. Comparison of average MHVSRs (solid lines) measured at the same location using two different seismometers; dashed lines show +/-one standard deviation.

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MHVSRs are calculated using the proprietary Grilla software as a first-pass level of quality control. The microtremor recordings are converted to ASCII and uploaded to a Geopsy database (ver. 2.9.1; Wathelet 2008) and calculated at this secondary stage using Geopsy’s HVSR toolbox which enables overlapping of time windows, an anti-triggering algorithm to remove transient windows, additional spectral smoothing options, and is generally more user-friendly. Time windows are cut from the recording with a defined length of 30 to 120 s duration depending on the expected site period as well as measurement duration. Windows with large transient signals are removed either using an anti-triggering algorithm or by user selection. A 5% cosine taper is applied to each time window before fast Fourier transformation to the frequency domain. The amplitude spectra are smoothed using a Konno-Ohmachi filter with a coefficient of 40 (Konno & Ohmachi, 1998). For each selected time window, the ratio of the quadratic mean of the horizontal component spectra to the vertical component spectra is calculated then averaged over all selected time windows. Geopsy automatically picks the maximum peak of the MHVSR using a statistical process that utilizes the SESAME criteria (Bard 2004). The primary function of these criteria is to establish the clarity of a peak by comparing its peak amplification (A0) to the amplification at other frequency bands and assessing whether the standard deviation of both f 0 and A0 are acceptably low. We employ a slightly more conservative approach to that recommended by the SESAME guidelines, in which a peak’s amplification is required to be greater than 2. Each individual MHVSR is visually inspected and suitability of the automatically picked peak frequency verified as the likely fundamental resonance frequency. In the case of multiple peaks, all peaks that met SESAME criteria are recorded. In the case of unclear low frequency peaks, broad peaks, or multiple peaks, peak selection is challenging, and knowledge of local geology is integral to accurate interpretation. As a final quality control measure, OpenHVSR (Bignardi et al. 2018) is used to validate the Geopsy-calculated MHVSRs as it allows batch processing of multiple MHVSRs and has a built-in automatic peak-picking algorithm. Of the 1100 MHVSR measurements, 730 demonstrate at least one clear peak that is interpretable as f 0 according to SESAME criteria; many MHVSRs do not provide a peak frequency, primarily from the pre-

Journal Pre-proof 2018 field campaigns due to recordings of very short duration or poor soil-sensor coupling, and are removed from our MHVSR site period database. Where two or more clear peaks are defined, f 0 is recorded as the lowest peak frequency regardless of whether this peak has the global maximum amplitude.

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Figure 5a shows the MHVSR locations shaded according to their lowest peak frequency (i.e., f 0). f 0 values are below 0.5 Hz in the deep FR delta and generally incre ase to the north as the depth to Tertiary sedimentary bedrock decreases. In Vancouver and the North Shore, where Tertiary or Pre -Tertiary bedrock may be a few meters to tens of meters below surface, f0 above 5 Hz is observed. Preliminary assessment of f 0 measurements is generally consistent with our understanding of regional geology; trends in soil thickness and stiffness included in the simplified geology map are largely consistent with trends observed in f 0 of the MHVSR dataset. Figure 5b shows the median values of f 0 values grouped by the 4 geologic units; median values of f 0 increase with age from 0.31 Hz for soft Holocene sediments to 4.92 Hz for pre-Tertiary bedrock unit. Even though this crude grouping does not distinguish between various subsurface conditions within each geologic unit, it clearly demonstrates the good correlation between the stiffness of geologic materials and fundamental peak frequencies in the region.

Figure 5. (a) Simplified geology map showing 730 MHVSR measurement locations shaded according to site fundamental frequency (f 0 ). (b) Boxplots of f0 values for each geologic unit with increasing age from left to right.

To further investigate different MHVSRs in the region, we define 5 representative MHVSR categories that cover the range of observed amplification spectra (Table 2). Category I is related to flat MHVSR spectra (no f peak), categories II and V are characterized by a single MHVSR peak of low or high amplification, respectively, and categories III and IV correspond to double and single peak MHVSR spectra observed on the FR delta. A representative ‘template’ MHVSR is selected for each category and cross-correlation performed to identify and group similar MHVSRs into one of the five categories. If the maximum normalized correlation coefficient is greater than 0.75, the measurement was grouped in the respective category. Of the 730 MHVSR measurements, only 265 satisfied this criterion and were

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successfully categorized (Table 2). Further, the measurements were qualitatively inspected to ensure correct categorization. Once the measurements were grouped, the MHVSR amplitudes were normalized by f peak and averaged to highlight their functional response. Normalization by f peak has been used by some authors to emphasize similarities in the shape of MHVSR curves (e.g., Braganza et al. 2016, Farrugia et al. 2018). The average and standard deviation MHVSRs amplification spectra of each category are shown in Figure 6.

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Figure 6. Representative averaged MHVSR for each category normalized by peak frequency; error bars denote one standard deviation. Table 2. Statistics of different representative MHVSR categories.

I II III IV V

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Number of measurements

Flat Single, low amplitude peak Double peak deltaic response Single peak deltaic response Single high amplitude peak

64 98 16 50 37

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fpeak, mean ± standard deviation (Hz) N/A 1.86 ± 0.95 0.73 ± 0.14 0.23 ± 0.02 4.52 ± 5.85

Min fpeak (Hz)

Max fpeak (Hz)

N/A 0.66 0.56 0.22 0.56

N/A 4.59 1.10 0.31 23.36

Dispersion curves from active- and passive-source array methods

During our 2018 field campaign, typically both active- and passive-source surface wave array methods for Vs profiling were performed at 44 array sites in Metro Vancouver. Twenty of the 44 array sites were selected to characterize the subsurface conditions near strong-motion instrument stations (within 430 m; Figure 3c). Both active-source multichannel analysis of surface waves (MASW) and passive-source ambient vibration array (AVA) measurements were performed at each site to provide dispersion

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MASW measurements were conducted using 24 4.5-Hz vertical-component geophones deployed in a linear array. The seismic source is a 5 kg hammer vertically impacted with a steel plate coupled to the ground. Shorter-length arrays of 11.5 and 23 m (geophone spacings of 0.5 and 1 m, respectively) included source shots at 5-m offset distance from each end of the linear array. Each shot was recorded with a 2 seconds time window and a sampling rate of 500 Hz. The longest length array of 69 m (3-m geophone spacing) also included source shots at mid-array. Passive-source AVA measurements were performed using 7 Tromino® seismometers in a circular array geometry with a central sensor. The array aperture was typically varied four times with radial distances of 5, 10, 15 and 30 m, depending on available space. Ambient vibrations were recorded simultaneously by all array sensors with 15-minute duration for smaller array radii and up to 30-minute duration for larger array radii. Time synchronization between recordings was established using Global Positioning System (GPS) time stamps. If required, adjustment in the timing of the array recordings was accomplished visually when obvious at small radial spacings. Further, a cross-correlation analysis is performed to adjust timing for larger array spacings and/or when dispersion estimates indicate timing errors. Accurate positioning was accomplished by use of measuring tapes.

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The vertical-component recordings of the AVA and MASW array recordings were imported into a Geopsy database for dispersion analysis. It is typically assumed the vertical-component recordings are dominated by Rayleigh waves; hence, we seek to obtain the fundamental -mode Rayleigh wave dispersion curve for inversion. Dispersion estimates were produced for each AVA using the Modified Spatial Auto Correlation (MSPAC) method (Bettig et al. 2001). This method converts the azimuthal average of the spatial autocorrelation function derived from cross-correlation of sensor-pair recordings within a particular distance (ring) and all time windows into phase velocity estimates at each selected frequency. Example MSPAC-derived dispersion estimates for two array sites are shown in Figure 7. MSPAC-derived dispersion estimates were stacked together from all arrays from which we pick fundamental-mode Rayleigh-wave dispersion estimates between the minimum resolution and maximum aliasing limits of all arrays. For MASW dispersion analysis, the 24 vertical-component array recordings were cut into 1-s windows around the active-source waveform and amplitudes were normalized. Frequency-wavenumber (f-k) dispersion analysis was performed for the various different shot offsets and receiver spacings and consistent dispersion results were stacked from which the fundamental-mode dispersion curve is picked (Figure 7). The three-component AVA recordings were used to generate time-averaged MHVSRs for each sensor of the smallest and largest aperture array recordings. Verification in the consistency of the MHVSR curve from small- to large-sized arrays confirms uniformity of subsurface ground conditions and validates the lateral homogeneity assumption in our forward and inverse modelling algorithms. The time -averaged MHVSRs of individual sensors were averaged to generate a spatially-averaged MHVSR, representative of the site MHVSR curve (Figure 7), for joint inversion with the site dispersion curve. Figure 7 presents AVA and MASW dispersion estimates, as well as site-representative MHVSR curves, for two sites in Metro Vancouver. Richmond (RI) array site 091 and Vancouver (VA) array site 051 are located 325 and 249 m from strong-motion stations RMD01 and VNC23, respectively (Figure 3c). VA051 is located on higher-elevation glaciated uplands in Vancouver and RI091 is located 5.4 km southward,

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across the Fraser River’s northern arm, on the FR delta lowland in Richmond. These two selected array sites demonstrate ‘typical’ dispersion and MHVSR results for subsurface conditions on glacial upland areas and the FR delta. On the FR delta, very low phase velocities (Holocene deltaic sediments) occur at higher frequencies (near surface) and transition to moderate phase velocities (Pleistocene glacial sediments) at lower frequencies (with depth); the degree and depth of this impe dance contrast (Holocene-Pleistocene boundary) is also measured by the MHVSR peak at 1.0 Hz. The MHVSR provides the degree and depth of a secondary impedance contrast (Pleistocene-Tertiary boundary) at a very low frequency (deep depth) of 0.25 Hz. For VA051, on Pleistocene glacial uplands, the dispersion estimates span the same frequency range but are slightly higher overall (200 to 1000 m/s). The degree and depth of this single impedance contrast (Pleistocene-Tertiary boundary) is also measured by the single MHVSR peak at ~1.5 Hz.

Figure 7. Picked dispersion estimates shown in blue from passive-source AVA testing in (a) and (b) and from activesource MASW testing in (c) and (d) for sites in Richmond (RI091) and Vancouver (VA051) . Average and +/- one standard deviation MHVSR curves calculated from all sensor recordings in the smallest and largest arrays are shown in (e) and (f).

Figure 8 shows the combined AVA and MASW dispersion curves for 20 array sites co-located with strong motion stations (locations shown in Figure 3c). The dispersion curves are coloured or grouped by site class; we calculate a preliminary estimate of Vs 30 based on the measured phase velocity equivalent to a 40-m wavelength Rayleigh wave, Vr40, using the relationship of Martin and Diehl (2004): Vs30 = (1.045)Vr40 . [3] Dispersion curves of Richmond sites on the FR delta are relatively consistent with each other and exhibit lower phase velocities than Vancouver sites. This is expected as the softer FR delta sediments do not vary significantly in Richmond. Our tested Richmond sites correspond to class D, but are notably close to the D/E class boundary. In comparison, dispersion curves of Vancouver sites are clearly distinct from the dispersion data in Richmond with higher phase velocities. Most dispersion data for Vancouver

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corresponds to site class C; two tested sites correspond to the class C/D boundary (VA072 and VA051) and one site is class D (VA060). The higher phase velocity (class C) Vancouver dispersion data is also relatively consistent with a mid-frequency plateau or hump (e.g., VA017 between 5-20 Hz). We currently include this phase velocity plateau or hump as the fundamental-mode indicative of a significant velocity reversal within the glaciated upland Pleistocene sediment package which is geologically feasible since the sequence includes several glaciations (advances and retreats) and likely reversals in sediment stiffness. Alternatively, we cannot yet rule out potential mode transition or contamination, i.e., effective mode dispersion estimates.

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Figure 8. Combined passive-source AVA and active-source MASW dispersion curves from 20 array sites co-located with strong-motion stations in Metro Vancouver. Shading corresponds to Vs 30 -based site classifications; Vs 30 is calculated from the measured Vr 40 dispersion data (black line shows phase velocities corresponding to a 40-m wavelength Rayleigh wave).

Shear-wave velocity profiling

Inversions are performed using Dinver software within the Geopsy software package. A modified version of the global-search neighborhood algorithm (Wathelet 2008) is used to determine theoretical layered earth models whose forward model Rayleigh-wave dispersion and ellipticity solutions minimize misfit with the site’s experimental dispersion and MHVSR curves, respectively. The elastic-media model parameters that define each layer are thickness (H), compressional wave velocity (Vp), Vs, and density (ρ) as well as Poisson’s ratio (. The theoretical dispersion and ellipticity functions are most sensitive to Vs and H. Inversion of Vp is therefore linked to Vs via Poisson’s ratio (0.2-0.5 for all layers) and fixed density values of 1950, 2300, and 2500 kg/m3 are used (Onur et al. 2004) for upper and middle layers and the elastic half-space, respectively. The model parameter of each layer is a uniform value, and velocities are set to increase with depth for sites whose dispersion curves show a gradual increase of phase velocity towards lower frequencies. For sites whose dispersion curves show a phase velocity plateau or hump in intermediate frequency ranges (e.g., VA017), Vs reversals are allowed (i.e., deeper

Journal Pre-proof layers can have lower Vs than shallower layers). The range in Vs for upper ‘soil’ layers is set to vary between 50-1500 m/s and 150-1500 m/s for Richmond and Vancouver sites, respectively. The elastichalf space is constrained to 1000 m depth for Richmond sites and 100 m for Vancouver sites with a maximum Vs of 3500 m/s. The Vs and H model parameter ranges are set intentionally wide for the data to resolve the optimal model. A minimum of 500,000 model solutions are searched in each inversion.

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The model parameterization (i.e., number of layers) is an unknown. To address inversion nonuniqueness and potential bias of a selected model parameterization, we use an inversion approach adapted from Farrugia et al. (2017). We initiate inversions with a single layer over an elastic half-space and progressively add additional ‘soil’ layers (more parameters) until the functional form (e.g., curvature or number of peaks) of the empirical dispersion and/or MHVSR data is adequately fit by the computed forward models. The final layers in the retrieved models range from 3 to 5 layers for Richmond sites and 2 to 3 layers for Vancouver sites, excluding the elastic half -space. In addition, we start with only inverting the measured dispersion curve to determine near-surface velocities then progress to joint inversion with MHVSR peak(s) to determine depths and velocities of impedance contrasts. We assume the resolvable depth of our dispersion data is half the maximum wavelength calculated from the phase velocity of the lowest frequency dispersion datum. Joint inversion with lower frequency MHVSR peaks enables resolution of the Vs profile to greater depth. The joint misfit function is calculated based on equal weighting assigned to dispersion data and MHVSR peak(s). We assume measured MHVSRs are dominated by surface waves and invert MHVSRs as Rayleigh wave ellipticity functions (e.g., Arai and Tokimatsu 2005, Rosenblad and Goetz 2010, Teague et al. 2018). Section 4 further examines MHVSRs in comparison to earthquake recordings and one-dimensional shear wave propagation theory. The frequency range of the target MHVSRs used in the inversions only includes peaks, which result from strong impedance contrasts.

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Figure 9 presents the Vs profiles of the global minimum misfit model from inversions of the dispersion and MHVSR datasets for sites RI091 and VA051. The assumed resolvable depth of the inverted dispersion data for the Vancouver site is deeper than the plotted depth limits in Figure 9. Vs profiles obtained from 4- and 5-layer models for the Richmond site and 2- and 3-layers for the Vancouver site show less variability amongst each other such that additional layered model s were not pursued. In general, Figure 9 shows that Vs variability between different minimum misfit models increases with depth. Beyond the maximum resolved depth of the dispersion data, the Vs profiles are determined solely by MHVSR peaks which are trading-off between Vs and thickness; a higher Vs in the overlying layer will result in a deeper impedance contrast for the same peak frequency . We now refer to other known subsurface information to validate observed excursions (impedance contrasts) in the Vs profiles. For RI091, our inverted Vs profiles determine a significant Vs increase (≥ 500 m/s) at depths between 40 and 63 m, which is consistent with geological evidence that the depth of stiffer Pleistocene sediments here is ~50 m. The depth and degree of a secondary significant increase in Vs associated with the 0.25 Hz MHVSR peak is not yet well-constrained but is quite deep (> 500 m); this is interpreted as the Pleistocene-Tertiary boundary. For VA051, our inverted Vs profiles determine a significant Vs i ncrease at 57 to 78 m depth. The closest available borehole log is located ~3 km east with a 115 m depth of the Pleistocene-Tertiary boundary (Armstrong 1984).

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Figure 9. Shear wave velocity profiles obtained from inversions using different model parameterizations for array sites (a) RI091 and (b) VA051. The theoretical and experimental phase velocity dispersion estimates are shown in (c) and (d), and theoretical ellipticity function with experimental MHVSR peaks are shown in (e) and (f) for sites RI091 and VA051, respectively. Joint refers to joint inversion results and DC refers to dispersion inversion only.

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4. Multi-method empirical site amplification at surface

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Our significant efforts to acquire and assemble a regional geodatabase are for the purpose of developing soil column models to predict 1D earthquake site amplification across the Metro Vancouver region. Empirical earthquake site amplification enables testing validity of the 1D wave propagation theory (assumptions) as well as the 1D model’s parameters. A major challenge in this project is the scarcity and suitability of empirical earthquake recordings in Vancouver. However, these empirical earthquakes recordings currently provide the only opportunity to validate empirical and theoretical site amplification in Metro Vancouver. Following the 2001 M 6.8 Nisqually earthquake, NRCAN designed the Internet Accelerometer (IA; Rosenberger et al. 2006) which records continuously and sends digital data via internet connectivity. Over 100 IAs have been installed in southwestern British Columbia with over 40 in Metro Vancouver, and primarily comprise free-field stations of the current BCSIMS strong-motion network (Kaya and Ventura 2015). Table 3 reports 6 earthquakes since 2002 ‘recorded’ at Metro Vancouver IA stations. The signal-to-noise quality of IA earthquake recordings are much more variable, very few are of high quality. It is worth noting that larger magnitude (≥ 6) earthquakes which produce more low frequency energy typically occur in the Nootka fault zone, west of Vancouver Island, several 100’s of kilometers west of

Journal Pre-proof Metro Vancouver. The recorded PGA in Vancouver from two such events in 2004 is very weak, ~0.3% g (Molnar et al. 2006). In contrast, the most recent and widely felt earthquake in the region, the 2015 M 4.7 Victoria earthquake, occurred 75 km southwest of Vancouver at ~60 km depth in the subducting Juan de Fuca plate (inslab event); Jackson et al. (2017) analyzed source and site effects from both weakand strong-motion recordings of this event. This event generated significant higher frequency energy typical of inslab earthquakes and resulted in a higher maximum PGA in Vancouver of ~4% g. The 2015 earthquake is the first event recorded by three instrumented borehole arrays which are further analyzed in section 4.1. Table 3. Earthquakes recorded by Internet Accelerometers (IA) since 2002. Latitude

Depth (km)

Year

M

Distance from Vancouver (km)

Vancouver Island

-123.30

48.62

60

2015

4.7

75

Offshore Vancouver Island

-127.26

49.64

10

2014

6.6

303

Offshore Vancouver Island

-127.26

49.34

22

Sidney

-123.55

48.54

Offshore Vancouver Island

-127.23

49.51

Offshore Vancouver Island

-127.30

49.51

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Longitude

6.3

301

2006

3.9

88

12

2004

5.8

299

2004

6.4

304

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2011

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Earthquake location

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We rigorously review and assemble all strong-motion earthquake recordings available in Metro Vancouver to consistently reanalyze empirical site amplification via Fourier horizontal -to-vertical spectral ratios (HVSR). Calculation of HVSRs are performed using the ObsPy toolbox (Beyreuther et al. 2010). Time windows of 40 to 180 s are extracted from the continuous IA recordings and includes the largest amplitude arrivals whether S-wave or typically surface waves. Time windows are baseline corrected, 5% tapered and padded according to Converse and Brady (1992) prior to applying a 4th order Butterworth filter with a pass band of 0.05-20 Hz. In addition, pre-event noise windows are cut. Fourier spectra are calculated via the fast Fourier transform, smoothed using the Konno and Ohmachi (1998) filter with a b-value of 40 and the geometric mean of the two horizontal component spectra is calculated. The geomean horizontal spectrum is divided by the vertical spectrum to produce the HVSR. The signal and noise Fourier spectra are corrected by dividing by the square root of its respective time window duration according to Perron et al. (2018). The signal-to-noise ratio (SNR) is calculated as the ratio of the corrected Fourier event spectrum with the corrected Fourier noise spectrum. The earthquake HVSR (EHVSR) is considered valid here when the SNR is ≥ 2. Figure 10 shows EHVSRs and SNRs calculated from 4 earthquake recordings at an IA strong-motion station on the FR delta. Only the valid portions of the EHVSRs are shown in Figure 10a after applying our SNR ≥ 2 criterion shown in Figure 10b. The 2006 earthquake signal is embedded within or saturated by the IA instrument noise (SNR < 2), so the 2006 EHVSR for this station is not calculated. The difference in frequency content of the earthquake signals is apparent in Figure 10. The 2011 and 2014 larger magnitude offshore earthquakes generate lower-frequency surface-wave-dominant waveforms and provide valid EHVSRs at frequencies < 2 Hz for this site. In contrast, the higher-frequency 2015 inslab event’s waveform is dominated by S-wave arrivals and provides a reliable EHVSR at frequencies > 0.7 Hz. In this way, we generate the ‘full’ bandwidth of empirical earthquake site amplification (EHVSR) fo r this station from multiple EHVSRs.

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Figure 10. (a) Valid EHVSRs for station RMD01 determined from (b) SNRs calculated from 4 available earthquake recordings. Line colour and style correspond to earthquake recording identified by year.

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We are able to compare two forms of empirical site amplification, EHVSRs and MHVSRs, at the 20 strong-motion stations (Figure 3c) we co-located our passive-source AVA testing with. In addition, MHVSRs are recalculated from three-component broad-band seismometer recordings of 30-minute duration performed by the first author at select strong-motion stations in 2004 (Onur et al. 2004). The microtremor recordings in 2004 were performed immediately outside of the building that houses the IA; AVA recordings in 2018 are performed on the ground surface w ithin ~430 m of the strong-motion station. As in, perfect replication of EHVSRs by MHVSRs is not expected; however, we seek to confirm if the overall EHVSR (site amplification spectrum) agrees with MHVSRs for each station and thereby validate the use of MHVSRs as a proxy for earthquake site amplification, noting all our earthquake data is weak-motion or corresponds to linear site response. As a minimum, it is expected that the f 0 , and notably its amplification (A 0), will agree between EHVSRs and MHVSRs as has been shown at other British Columbia strong-motion stations (Molnar and Cassidy 2006; Molnar et al. 2013; 2017).

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Figure 11 shows comparison of earthquake and microtremor HVSRs at 4 strong-motion stations located on the FR delta (Figure 3c). The f 0 is consistently observed in all EHVSRs and MHVSRs as expected. The MHVSR amplification of the f 0 typically agrees with the EHVSR and provides confidence that the MHVSR is a suitable EHVSR-proxy for both f 0 and A0. The notable exception is the large A0 at RMD15 from the 2014 earthquake. The MHVSRs also exhibit lower-amplification secondary peaks that do (RMD09, RMD15) and do not (RMD02, VNC09) correspond to secondary EHVSR peaks. At higher frequencies, the MHVSRs do not agree with EHVSR site amplification also as expected and typically observed (e.g., Molnar et al. 2017). Kawase et al. (2018) developed a modification factor to back-calculate EHVSRs from MHVSRs for 100 K-NET and KiK-net stations in Japan. Deriving a similar MHVSR-to-EHVSR modification factor for Vancouver is a future goal, if our earthquake database quality allows, to ensure MHVSRs best approximate earthquake site amplification for our inversions and hazard mapping purposes.

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Figure 11. EHVSRs and MHVSRs calculated from earthquake and microtremor recordings, respectively, at 4 selected strong-motion stations. MHVSRs are calculated from AVA testing in 2018 (labelled MHVSR Array) and broadband seismometer recordings in 2004 (labelled MHVSR BR).

Validating theoretical 1D site amplification

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Three vertical borehole arrays are installed at terminus ends of the Port Mann Bridge at three depths between surface and 57 m maximum depth in each borehole (Table 4). The bottom sensors in the 3 boreholes are located in stiff glacial till. Boreholes 1 and 2 (900 m apart) are located beneath the north bridge approach and Borehole 3 is located beneath the south bridge approach across the Fraser River, locations are shown in Figure 3c. The 2015 inslab earthquake was recorded by these three vertical borehole arrays. Jackson et al. (2017) evaluated earthquake site amplification by calculating EHVSRs and upper-to-lower spectral ratios at each instrumented depth, as well as determined interval Vs depth profiles of each borehole via waveform cross-correlation.

Journal Pre-proof Table 4. Summary of three borehole arrays (Jackson et al. 2017). Borehol e

Sens or

Depth (m)

1

1

-4

VS! (m/s )

Soi l Type Concrete pad

144 2

9

Dense sand grading i nto very dense s a ndy gra vel 267

2

3

41

Ti l l

1

2

Brown s and

2

36

227

333 1

16

Ti l l

Gra y s a nd 283

2

33

Gra y s i lt grading i nto coarse sand, over s i lty gra y clay

57

Ti l l

obta i ned from cros s -correl a ti on a na l ys i s

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Orga ni cs, over sand with thin gra vel l a yer, over silty to sandy gra y clay

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We calculate the surface EHVSR (H1/V1) and the surface-to-base ‘standard’ spectral ratio (H1/H3) (shown in Figure 12) using the same waveform processing procedure described in Section 4 with a time window of 40 seconds. The EHVSR demonstrates a lower f 0 because the HVSR calculation includes amplification from greater depth than the ‘standard’ H1/H3 spectral ratio which only considers amplification between sensors 1 and 3 (upper ~45 m maximum). At each borehole, two MHVSR measurements were performed in 2018 within a few meters of the borehole, referred to as MHVSR1 and MHVSR2 in Figure 12. The MHVSRs and EHVSRs generally agree, continuing to support that MHVSRs are an excellent proxy for EHVSRs in British Columbia. To predict theoretical site amplification, we use each borehole’s Vs profile determined by Jackson et al. (2017) as reported in Table 4. Our theoretical 1D site amplification should therefore predict the ‘standard’ H1/H3 amplification. Following Onur et al. (2004), we assign the unit weight of the Holocene deltaic sediments as 19.5 kN/m3 and apply a small-strain damping ratio (Dmin) of 4%. DEEPSOIL v.6.1 software (Hashash et al. 2016) is used to compute the 1D linear transfer function using a rigid halfspace. Our theoretical 1D site amplification best agrees with the observed ‘standard’ H1/H3 ratio at borehole 1 up to the 5th mode. At boreholes 2 and 3, the fundamental peak frequency of the 1D linear transfer function is slightly higher frequency than the H1/H3 ratio with less agreement in the higher modes. The amplitude of the theoretical amplification generally overestimates the observed H1/H3 up to the 3rd peak frequency in the three boreholes; this can be attributed to the chosen Dmin value. Thompson et al. (2012) proposed choosing a Dmin value that minimizes that misfit between the median observed linear amplification function from at least 10 recordings and the theoretical amplification function; however, we chose not to select an optimal Dmin value based on one recording only. The theoretical 1D site amplification agrees with observed site amplification at borehole 1, the most inland

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site, and this agreement in site amplification begins to degrade at boreholes 2 and 3 approaching the banks of the Fraser River (rapid change in topography).

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Figure 12. Comparison of EHVSR (H1/V1) and ‘standard’ spectral ratio (H1/H3) of the 2015 inslab earthquake recordings compared to MHVSRs and theoretical 1D transfer functions for (a) Borehole 1, (b) Borehole 2, and (c) Borehole 3.

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5. Challenges and Future Work We have presented our approach and methodologies to generate comprehensive seismic amplification hazard maps (Vs30 , site class, site period) for the Metro Vancouver region. However, there are several challenges and limitations that need to be addressed. To improve estimation of earthquake site effects in the region, more details (data) about the subsurface characteristics are required. The development of our geodatabase has been very labour intensive but is crucial to our understanding of the 3D subsurface geology and material properties. To date, the compiled geodatabase provides the best spatial coverage and highest density of information in softer lowland areas including the FR delta. As in, we know the most about material properties of the youngest sediments in the region. In contrast, material properties, especially with depth, of Pleistocene and older glaciated and inter-glacial sediments are rarely tested and therefore poorly represented in our geodatabase currently. Our active - and passivesource surface wave array testing is the most cost-efficient approach to increase quantity of subsurface velocity profiling in these areas of stiffer ground where invasive methods cannot penetrate or would be more expensive. Outcomes from our first non-invasive testing campaign have been consistent with previous measurements, e.g., Vs of FR delta sediments, double peak site amplification with very low

Journal Pre-proof ~0.2-0.3 Hz f 0 on the FR delta. We have also gained new knowledge including variation in f 0 and quantification of ~5 categories (groupings) of site amplification across the region as well as complexity in Vs with depth (potential low velocity zones) in glaciated upland areas. An additional challenge of in situ testing on stiffer upland areas is significant or rapidly-changing surface topography. We first attempt to minimize 2D or 3D effects in our measurements by testing on terraces or flat-areas within hilly or mountainous areas. At the processing stage, a higher level of effort is required to remove or ‘map out’ potential 2D or 3D effects. For example, we identify non -1D effects by rapid changes in f 0 from MHVSRs across the site and analyze sub-arrays within arrays to document changes in retrieved 1D velocity profiles, if found. Overall extra care and caution in both data collection and processing is required.

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We have determined subsurface layered earth models from joint inversions based solely on measured dispersion and MHVSR datasets. At this stage in the microzonation mapping, we are essentially treating the dispersion and MHVSR curves as independent geophysics-based datasets to determine subsurface ground conditions at each array site. Our current inversions are essentially ‘blind’; no other subsurface information (e.g., geologic layering) is used to constrain the inversion. For the final microzonation mapping, inversions are likely to be re-performed inclusive of regional or site-specific subsurface information, e.g., velocities constrained by invasive method or laboratory measurements, or model parameterization informed by the regional 3D geologic model. Love wave dispersion analysis will be performed for a select number of sites to evaluate additional benefit to site characterization. A regional grid of MHVSR measurements provides a suitable dataset for inversion in future to improve 2D-to-3D subsurface models. We continue to compile measurements of Vs and geologic stratigraphy (thickness) to provide ‘tie-points’ or known constraints for future inversion of the regional MHVSR dataset.

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The regional earthquake ground motion database is another major challenge. Neither the quality nor the quantity of available strong-motion network recordings allow us to fully explore complex amplification phenomena in Metro Vancouver. We are currently investigating the consistency between 1D theoretical site response, using our joint inversion results, and empirical amplification (EHVSRs and standard spectral ratios (SSR) referenced to hard ground or rock). The vertical borehole array recordings of the 2015 earthquake presented a rare opportunity to evaluate and validate empirical and theoretical site amplification measures. We plan to install an array of broadband seismometers, primarily for ambient noise tomography and improvement of the current 3D velocity model of the upper few kilometers used in our 3D basin modelling, which will also provide the nature of the ambient vibration wavefield and hopefully high-quality seismic recordings across the region. Three additional field campaigns will occur until and including 2021. The planned 600-m grid of MHVSR measurements will provide base-level spatial coverage across Metro Vancouver, which has been achieved across Vancouver and Burnaby. Spatial density of these MHVSR measurements will be increased to confirm and improve mapped boundaries of site amplification changes and/or assignment of site classification. We will continue to perform surface wave array testing to provide more than 5 velocity profiles for each of the region’s geologic units. This will increase the quantity of velocity measurements to update the regional velocity model (Monahan and Levson 2001), i.e., the average velocity and its standard deviation for each geologic unit. During the summer 2019 field campaign, we will perform a large-scale array (radius in hundreds of meters) to demonstrate maximum resolvable depth on the FR delta. In latter field campaigns, other (more expensive) in situ testing methods will be

Journal Pre-proof performed as required to confirm accurate measurement of subsurface properties and/or to fill in spatial gaps. We are in the process of assembling the highest quantity and largest spatial coverage 3D geodatabase for the Metro Vancouver region to date. We will continue to validate our theoretical site effect predictions using our geodatabase in comparison to observed earthquake site amplification. The final geodatabase will provide comprehensive and accurate mapping of site predictor variables (e.g., Vs30, Z1.0, f 0) as well as constrain a 3D geologic and velocity model for Metro Vancouver.

6. Acknowledgements

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Project funding provided by Emergency Management British Columbia through the Institute of Catastrophic Loss Reduction. Non-invasive measurements were collected and processed by Univ. of Western Ontario (UWO) personnel: Jamal Assaf (PhD Candidate, Civil Engineering), Sujan Raj Adhikari (PhD student, Earth Sciences), Chris Boucher (MSc student, Earth Sciences), Alex Bilson Darko (Field Technician), Meredith Fyfe (MSc Student, Earth Sciences & Env. Sustainability), Sameer Ladak (MSc student, Earth Sciences) and Aamna Sirohey (MSc student, Earth Sciences) with field support from Ali Fallah Yeznabad (PhD Candidate, Civil Engineering) and Alireza Javanbakht Samani (PhD student, Earth Sciences). Geotechnical database managed by S. R. Adhikari with support of UWO students Andrew Beney, Jacob Edgett, and Tyler Beattie. Additional financial support provided by NSERC USRA program for A. Sirohey, UWO pre-thesis graduate funding for M. Fyfe and UWO work study funding for A. Beney.

7. References

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Arai H, Tokimatsu K (2005). S-Wave Velocity Profiling by Joint Inversion of Microtremor Dispersion Curve and Horizontal-to-Vertical (H/V) Spectrum. Bulletin of the Seismological Society of America 95:1766–1778. doi: 10.1785/0120040243

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Armstrong JE (1984). Environmental and Engineering Applications of the Surficial Geology of the Fraser Lowland, British Columbia Bard, PY (1999). Microtremor measurements: a tool for site effect estimation. The effects of surface geology on seismic motion, 3, 1251-1279. Bard PY (2004). Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations–measurements, processing and interpretations. SESAME European research project WP12 – Deliverable D2312. Bettig B, Bard PY, Scherbaum F, Riepl J, Cotton F, Cornou C, Hatzfeld D (2001). Analysis of dense array noise measurements using the modified spatial auto-correlation method (SPAC): application to the Grenoble area. Bollettion di Geofisica Teorica ed Applicata 42:281–304 Beyreuther M, Barsch R, Krischer L, Megies T, Behr Y, Wassermann J (2010). ObsPy: A Python Toolbox for Seismology. Seismological Research Letters 81:530–533. doi: 10.1785/gssrl.81.3.530 Bignardi S, Fiussello S, Yezzi AJ (2018). Free and Improved Computer Codes For HVSR Processing and

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Castellaro S, Mulargia F (2009). The Effect of Velocity Inversions on H/V. Pure and Applied Geophysics 166:567–592. doi: 10.1007/s00024-009-0474-5 Converse AM, Brady AG (1992). BAP basic strong-motion accelerogram processing software version 1.0. US Geological Survey, Open File Report 92-296A 174 pp. doi: 10.3133/ofr92296A

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Dunn D, Ricketts B (1994). Surficial Geology of Fraser Lowlands Digitized From GSC Maps 1484A, 1485A, 1486A, and 1487A. Geologic Survey of Canada, Open File 2894. doi: 10.4095/194084 Farrugia JJ, Molnar S, Atkinson GM (2017). Noninvasive Techniques for Site Characterization of Alberta Seismic Stations Based on Shear‐Wave Velocity. Bulletin of the Seismological Society of America 107:2885–2902. doi: 10.1785/0120170086 Farrugia JJ, Atkinson GM, Molnar S (2018). Validation of 1D Earthquake Site Characterization Methods with Observed Earthquake Site Amplification in Alberta, Canada. Bulletin of the Seismological Society of America 108:291–308. doi: 10.1785/0120170148 Foti S, Hollender F, Garofalo F, et al (2018). Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project. Bulletin of Earthquake Engineering 16:2367–2420. doi: 10.1007/s10518-017-0206-7 Garofalo F, Foti S, Hollender F, et al (2016a). InterPACIFIC project: Comparison of invasive and non invasive methods for seismic site characterization. Part I: Intra-comparison of surface wave methods. Soil Dynamics and Earthquake Engineering 82:222–240. doi: 10.1016/j.soildyn.2015.12.010

Journal Pre-proof Garofalo F, Foti S, Hollender F, et al (2016b). InterPACIFIC project: Comparison of invasive and non invasive methods for seismic site characterization. Part II: Inter-comparison between surface-wave and borehole methods. Soil Dynamics and Earthquake Engineering 82:241–254. doi: 10.1016/J.SOILDYN.2015.12.009 Ghofrani H, Molnar S (2019). 3D sedimentary basin effects in the Metro Vancouver area and its seismic hazard implications: updates and validations of the Georgia Basin velocity model. In: 12th CANADIAN CONFERENCE ON EARTHQUAKE ENGINEERING. Quebec City, Canada Hashash YMA, Musgrove MI, Harmon JA, Groholski DR, Phillips CA, Park D (2016). “DEEPSOIL 6.1, User Manual”

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Canadian Digital Elevation Model. https://open.canada.ca/data/en/dataset/7f245e4d-76c2-4caa-951a45d1d2051333

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Sheri Molnar: Conceptualization, Methodology, Resources, Writing & Editing, Supervision, Project administration, Funding acquisition. Jamal Assaf: Conceptualization, Methodology, Formal analysis, Investigation, Software, Validation, Writing & Editing, Visualization, Aamna Sirohey: Methodology, Formal analysis, Investigation, Software, Data Curation, Writing, Visualization, Sujan Raj Adhikari: Investigation, Data Curation, Resources, Visualization

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights

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Comprehensive seismic microzonation mapping is underway for Metro Vancouver Multiple non-invasive seismic methods are employed Vs profiles are developed near strong motion stations Site amplification is evaluated using microtremor and earthquake recordings Few well-recorded earthquakes present a challenge in validating methodologies

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