The search for hard-rock kappa (κ) in NGA-East: A semi-automated method for large,challenging datasets in stable continental regions

This article describes the work undertaken within the Next Generation Attenuation (NGA)-East project with the aim of estimating κ₀ (the site-specific component of the high-frequency decay parameter, κ) for rock sites in Central and Eastern North America (CENA), using the project’s shallow crustal dataset. We introduce a methodology to address the numerous challenges in CENA: a large dataset in a low-seismicity stable continental region, with poor magnitude and distance coverage, undesirable recording sensor characteristics (low sampling rates leading to poor high-frequency resolution), high uncertainty in the regional stress drop, and lack of site-specific velocity characterization. We use two band-limited κ estimation approaches, the acceleration and displacement spectrum (AS and DS), applied above and below the source corner frequency (fc), respectively. For band-limited approaches, the key requirement is an estimate of fc, which—apart from the event magnitude readily available in the flatfile—also heavily depends on the highly uncertain stress drop. By considering lower and upper bounds on regional stress drop, we propose a new method to quickly and automatically screen such very large datasets to identify all possible recordings for which band-limited κ approaches can be used. Combining them produces better-quantify estimates of κ and its epistemic uncertainties for this challenging dataset. The mean κ₀ values combining the two methods are 13 ± 23 ms for horizontal ground motion.