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

Knowledge about local seismic site conditions provides critical information to account for site effects that are commonly observed in strong motion recordings. Certainly, other wave propagation effects can influence these observations, which are attributable to variations in material properties of the paths traveled by the waves, as well as the characteristics of the seismic source. However, local geologic conditions, particularly, when under shear-wave excitation, are known to have a strong influence on the behavior of ground shaking in the frequency range that is expected to directly affect the built environment. Thus, shear waves traveling in the shallow subsurface—defined here as tens to hundreds of meters beneath the ground surface—are the main foci for application and research in the earthquake engineering community. To assess the potential for important site effects, a number of approaches collectively known as site response analyses (SRA) are constantly developed. They are also continuously tested and refined with the aim to reduce the uncertainties associated with each technique. Although SRA can be carried out empirically, a set of popular procedures within the suite of SRA methods relies on numerical techniques (one dimensional [1D] transfer functions) and is further differentiated by earthquake engineers as ground response analysis (GRA). Fundamentally, GRAs require input from measurements through in situ seismic recordings that are generally known as the field data acquisition component of site characterization. Following such acquisitions are the associated data processing and analysis phases that produce the shear-wave velocity (VS) profile as the main output, as well as its derivative, the time-averaged VS of the upper 30 m from the surface (VS30), which is the main site index term in ground motion modeling (Boore et al. 1993; Borcherdt 1994). To advance knowledge about site effects phenomena, special SRA-focused sessions have become common occurrences at internationally held earthquake conferences and scientific journals have frequently devoted special issues (or sections) to document the state of the knowledge (Field et al. 2000; Panzera et al. 2017; Kaklamanos et al. 2021). Recently, Kaklamanos et al. (2021) introduced a collection of papers compiled as a special section entitled Advancements in Site Response Estimation, which originated from a similarly named special session planned for the 2020 Annual Meeting of the Seismological Society of America (which was canceled due to the COVID-19 pandemic). Through open submissions, the guest editors organized articles into five interrelated sections about various aspects of site response (Kaklamanos et al. 2021), including five papers addressing uncertainties as contributed through the SRA framework, as well as one general section on site characterization. Of the six papers included in this section, only two were primarily focused on VS measurements and both focused on the use of surface wave methods to generate in situ VS models (Hobiger et al. 2021; Stephenson et al. 2021). The study locations of each paper were unrelated, but both papers shared the general approach of comparing surface-wave-based analytical estimates of the site dominant frequencies (fd) to that of earthquake horizontal-to-vertical spectral ratios (eHVSR). These independent studies found strong agreement between their modeled and observed fd. In a more recent effort, S. Matsushima and others (http://www.esg6.jp/blind.html; last accessed 4 April 2022) conducted blind tests that were mainly focused on SRA through participation by international analysts as part of the 2021 6th International Symposium of the Effects of Surface Geology on Seismic Motion. During the past two decades, advancements in the field of site characterization have also benefited from activities that were similarly conducted for SRA. This period coincided with a time when applying cost-effective noninvasive surface-wave approaches gained tremendous popularity worldwide. Particularly important were related crossover efforts that attempted to assess uncertainties propagated from methodologies that apply surface-based site characterization to GRAs. To this end, a number of blind trials on-site characterization methods were conducted and most of these activities were directly followed with developments of guidelines for best practices by organizers of the trials (Cornou et al. 2007; Boore and Asten 2008; Garofalo et al. 2016; Foti et al. 2018; Asten et al. 2022, this issue). Unassociated guidelines, technical reports, and textbooks about the application of surface wave methods were also independently published by authors and many were participants of the aforementioned trials (SESAME 2004; Yong et al. 2013; Martin et al. 2014; Dal Moro 2014; Foti et al. 2015; Martin et al. 2017). Despite these accomplishments, the findings illuminated solutions, which also inherently beget more questions, and thus the continuation of these activities is expected for the foreseeable future (Askan et al. 2022).