An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Abstract

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.

Introduction to the special issue of the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) international guidelines for applying noninvasive geophysical techniques to characterize seismic site conditions

“…Summarily, Hayashi et al (2022) provide a synopsis of the fundamental principles for determining the Rayleigh-wave phase velocity from ambient noise sources; general recommendations are provided for field data acquisition and limitations and uncertainties of the MAMs are discussed. An earlier review effort by Asten and Hayashi (2018) was the genesis for the COSMOS 2018 blind trial (Asten et al 2022, in this issue), whose findings are included in Hayashi et al (2022). Louie et al (2022, in this issue) and Pancha and Apperley (2022, in this issue) present a pair of papers focusing on the refraction microtremor (ReMi) technique.…”

“…To this end, eleven papers were curated by the COSMOS guest editors. These articles are assigned to three main themes: the first topic articulates the best practice for applications of various site characterization methods (Louie et al 2022;Pancha and Apperley 2022;Hayashi et al 2022;Hunter et al 2022;Molnar et al 2022;Stephenson et al 2022); the second is agnostic to the aforementioned techniques and focuses on processing and analyzing data (Toro 2022;Vantassel and Cox 2022), including one paper on the role of analysts (Asten et al 2022); and the third involves reviews of select topics that are fundamental for consideration in all techniques (Gosselin et al 2022;Parolai et al 2022). All papers are aligned on issues relating to uncertainty, which are paramount to the practice of site characterization as performed at the time of publication.…”

“…Analysts were encouraged to apply their choice of processing and analysis techniques (beam-forming, cross-correlation, seismic interferometry, or SPAC) using customized and/or commercial software, which allows comparisons of the effectiveness of differing wave field distributions and techniques. To quantitatively compare V S profiles from multiple analysts, Asten et al (2022) develop the M quality index, which is defined using estimates of the time-averaged V S of the upper 10 (V S10 ), 30 (V S30 ), 100 (V S100 ), and 300 (V S300 ) meters from the surface. Asten et al (2022) conclude that-subject to a sufficient azimuthal distribution of the passively acquired seismic noise sources-the use of sparse arrays is adequate for accurate estimates of V S10 , V S30 , V S100 , or V S300 ; no techniques/software packages are observed to outperform others for any portion of the trial; and that analyst skill and experience is a stronger factor than that of technique and software choices.…”

Flexible multimethod approach for seismic site characterization

“…Y19 evaluated the inter-and intra-method variability of V S30 values derived from multiple combinations of surface wave methods (e.g., MASW, SASW, MAM) for 31 seismograph sites in California and presented results based on substantially more sites than have been reported (Boore and Asten 2008;Cornou et al 2009;Garofalo et al 2016a, b;Asten et al 2019Asten et al , 2022. They estimated V S30 values from independent surface-wave dispersion and compared the values estimated from the same (intra-method) and alternative methods (inter-method), as well as to V S30 values estimated from combinations of multiple methods.…”

“…When accounting for ergodic (representative) site effects commonly used in ground motion models, site parameters such as V S30 and the site dominant frequency (f d ) have become sought-after measurements. S-wave slowness, the inverse of the S-wave velocity, is a lesser-known measure that has been found useful for highlighting detailed layers of the near surface (Brown et al 2002;Boore 2004;Boore and Asten 2008;Asten et al 2022;Stephenson et al 2021;Mital et al 2021).…”