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Tamp1.5: Estimating the Effect of Site-specific Kappa on High-frequency Ground Motion Utilizing Elastic Response Spectral Shapes

Description: 

Kappa is a parameter that can significantly affect high-frequency ground-motion estimation but is often elusive in its quantification. Commonly, the zero-distance kappa is assumed to be associated with damping effects; however, kappa encompasses more than just damping. If kappa is interpreted as only due to damping, the result can be large scale factors (2-3) being applied to high-frequency ground motions for site-specific applications. The site-specific kappa includes the effects of deviations between the simplified model and the observations. As a result, kappa includes effects of errors in the source, path, and site amplification models at high frequencies and not just damping. Along with the uncertainty of what contributes to a site-specific kappa, accurate estimation of kappa can be highly subjective, often depending on the usable frequency band of the availability site-specific data. We present an alternative methodology for estimating the effects of site-specific deviations in kappa (delta kappa) that are related to damping. The short-period spectral shape is characterized by the shortest period at which the amplitude of the 5%-damped response spectrum (PSA) reaches 1.5 times peak ground acceleration, called Tamp1.5. A ground-motion model (GMM) is developed for Tamp1.5. The Tamp1.5 site term is treated as a proxy for the site-specific delta kappa due to damping only. The relation between the Tamp1.5 site term and the T=0.05 sec site terms from a PSA GMM is developed. The site-specific delta kappa is estimated using the relation between delta kappa and the change in the T=0.05 sec PSA. With these two steps, the Tamp1.5 site term is converted to a delta kappa site term. The Fourier amplitude spectrum (FAS) from a GMM is then scaled by exp(-pi * delta_kappa * freq) to estimate the site-specific FAS including kappa effects. This methodology provides a more objective approach for estimating the site-specific delta kappa due to damping, facilitating more accurate ground-motion modeling and improving site-specific seismic hazard assessments for the short-period ground motion.

Session: Challenges and Opportunities in Constraining Ground-motion Models from Physics-based Ground-motion Simulations [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Micaela

Student Presenter: Yes

Invited Presentation: 

Poster Number: 42

Authors