Multi-method site characterization to verify the hard rock (Site Class A) assumption at 25 seismograph stations across Eastern Canada

Abstract

Site characterization is a crucial component in assessing seismic hazard, typically involving in situ shear-wave velocity ( VS) depth profiling, and measurement of site amplification including site period. Noninvasive methods are ideal for soil sites and become challenging in terms of field logistics and interpretation in more complex geologic settings including rock sites. Multiple noninvasive active- and passive-seismic techniques are applied at 25 seismograph stations across Eastern Canada. It is typically assumed that these stations are installed on hard rock. We investigate which site characterization methods are suitable at rock sites as well as confirm the hard rock assumption by providing VS profiles. Active-source compression-wave refraction and surface wave array techniques consistently provide velocity measurements at rock sites; passive-source array testing is less consistent but it is our most suitable method in constraining the rock VS. Bayesian inversion of Rayleigh wave dispersion curves provides quantitative uncertainty in the rock VS. We succeed in estimating rock VS at 16 stations, with constrained rock VS estimates at 7 stations that are consistent with previous estimates for Precambrian and Paleozoic rock types. The National Building Code of Canada uses solely the time-averaged shear-wave velocity of the upper 30 m ( VS30) to classify rock sites. We determine a mean VS30 of ∼ 1600 m/s for 16 Eastern Canada stations; the hard rock assumption is correct (>1500 m/s) but not as hard as often assumed (∼2000 m/s). Mean variability in VS30 is ∼400 m/s and can lead to softer rock classifications, in particular, for Paleozoic rock types with lower average rock VS near the hard/soft rock boundary. Microtremor and earthquake horizontal-to-vertical spectral ratios are obtained and provide site period classifications as an alternative to VS30.

Seismic microzonation mapping of Greater Vancouver based on various site classification metrics

“…The geodatabase is developed from two main data sources: 1) from previously collected open and private geodata sources, with the private geodata shared by 24 municipalities, organizations, or consultants, and 2) by Frontiers in Earth Science frontiersin.org performing in situ field-based multi-method non-invasive seismic testing throughout the region annually between 2018 and 2021. The non-invasive seismic field methods include single-station microtremor measurements (Molnar et al, 2022) performed at a 600-to 1,000-m grid spacing across the study area (Figure 1B) to obtain amplification spectra and site peak frequencies (f 0HV and higher frequency peaks f 1HV and rarely f 2HV ) (Sirohey, 2022), combined active-source multi-channel of surface waves (MASW) and passive-source microtremor array method (MAM) array testing to obtain fundamental-mode Rayleigh wave dispersion estimates for joint inversion with MHVSR peak frequency(ies) (f 0HV and f 1HV ) to obtain Vs depth profiles (Ladak, 2020;Assaf et al, 2022;Boucher, 2022) and compression-wave velocity and Vs refraction surveys at selected sloping ground sites (Boucher, 2022).…”

A review of the microtremor horizontal-to-vertical spectral ratio (MHVSR) method

“…Documentation of metadata becomes cumbersome as the quantity of microtremor measurements increases into hundreds and thousands of sites. Large quantity microtremor campaigns performed for seismic microzonation mapping, site characterization of a seismic network’s stations, or post-earthquake reconnaissance purposes (e.g., Puglia et al 2011 ; Albarello 2011 ; Molnar et al 2020 ; Ladak et al 2021 ) may include multiple equipment, practitioners, and/or span multiple years. In such cases, the importance of minimal standardized yet robust metadata of the microtremor measurements and their processing and interpretation increases.…”

“…Theoretically, rock sites are expected to have HVSR amplification of unity (or 1.414 according to equations in Sect. 2.3), but broadband or high-frequency amplification is often observed (e.g., Ladak et al 2021 ). A relatively flat MHVSR can also result from the lack of a strong impedance contrast, even at a deep sediment site, e.g., coarse-grained soils over volcanic ash or heavily weathered (non-glaciated) rock common in Chile (Bonnefoy-Claudet et al 2009 ; Leyton et al 2013 ; Molnar et al 2015 ) or from complex site effects (e.g., Uebayashi 2003 ; Di Giulio et al 2010 ; Le Roux et al 2012 ).…”

Cartografía de la respuesta sísmica local: una contribución a la gestión del riesgo en la zona metropolitana del Valle de Toluca

“…Mientras que los enfoques más recientes proponen relaciones cuantitativas, generalmente empíricas, entre la amplificación de las ondas sísmicas en el sitio y algún parámetro físico de los materiales del suelo. Entre estas últimas relaciones se encuentran las basadas en el parámetro de relación de vacío (Hayes et al, 2017) o en módulo de corte, y las que hacen depender el efecto de sitio del valor de la velocidad de propagación de ondas de corte (Vs) en la estratigrafía somera (Cabas et al, 2017;Ladak et al, 2021). También existen métodos experimentales cuyo objetivo es determinar la función de transferencia, o en su defecto la frecuencia fundamental, de los depósitos superficiales a partir de registros sísmicos o de ruido sísmico.…”