The Search For Hard-Rock Kappa (Kappa) 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 kappa(0) (the site-specific component of the high-frequency decay parameter, kappa) 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 kappa 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 kappa approaches can be used. Combining them produces better-quantify estimates of kappa and its epistemic uncertainties for this challenging dataset. The mean kappa(0) values combining the two methods are 13 +/- 23 ms for horizontal ground motion.