Capturing Regional Variations of Hard-Rock Attenuation in Europe

Date: 4/24/2019

Time: 05:00 PM

Room: Pine

A proper assessment of seismic reference site conditions has important applications as they represent the basis on which ground motions and amplifications are generally computed. Besides accounting for the average S wave velocity over the upper 30 m (vS30), the parameterization of high‐frequency ground motions beyond source‐corner frequency has received significant attention in last few decades. κ, an empirical parameter introduced by Anderson and Hough (1984), is often used to represent the spectral decay of the acceleration spectrum at high frequencies. Definitions of reference site conditions are mostly based on indirect large-scale crustal velocity inversions while site effects for hard‐rock sites are typically computed using analytical models for the effect of κ0, the site‐specific component of κ at zero epicentral distance. Following Mayor et al. (2018), we propose an alternative procedure for capturing the reference κ0 on regional scales by linking the well‐known high-frequency attenuation parameter κ and the properties of coda waves. Using near‐distance records of more than 10,000 crustal earthquakes at more than 1300 sites, we observe that κ0 from coda waves seems to be independent of the soil type but correlated with the hard-rock κ0, showing significant regional variations across Europe. The values range between 0.004 s for northern Europe and 0.020 s for the southern and south-eastern parts. On the other hand, measuring κ (and correspondingly κ0) on the S wave window (as classically proposed), the results are strongly affected by transmitted (reflected, refracted and scattered) waves included in the analysed window. This effect is more pronounced for soft soil sites. In this way, κ0 of coda waves can serve as a proxy for the regional hard-rock κ0 at the reference sites.

Presenting Author: Marco Pilz

Authors