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Between and Within-Site Variabilities: How Large Are They? How Far Can We Reduce Them?

Description: 

An important component of the variability of seismic motion is related to the effect of surface layers. This site variability has two components: the between-site variability and the within-site variability.

The between-site variability is the spatial variability in the average site response (over multiple events) across many sites in a data set. The between-site variability is mostly epistemic means that its value is equal to zero if the amplification is known at all sites (“non-ergodic” case with many recordings at the considered sites). For sites where no or few records exist, we will never be able to perfectly reproduce the full complexity of real amplifications. The reduction of the between-site variability is then depending on the method used to derive the amplification (1D modelling, empirical prediction based on proxies like vs30, machine learning). We will show a recent compilation of observed between-site variabilities in Japan and Europe and compare the “corrected” between-site variabilities obtained by various site-amplification prediction methods. In particular, we will show the limits of classical 1D models to reproduce site-amplifications and reduce the between site variability at high frequencies (>3Hz). We will also discuss the added value of new methods (new proxies or Machine Learning).

In contrast to the between-site variability, the within-site variability is defined as the randomness in site response from event to event at a single site, and it is considered to be aleatory. Quantifications of this within-event variability show that the within-event variability is much lower than the between-event variability. We will show that it is difficult to identify the geological/geotechnical factors controlling this variability, and we will finally discuss and illustrate new working hypothesis (e.g. time-dependencies of kappa) that may explain part of it.

Session: Understanding and Modeling the Uncertainties in Earthquake Ground Motions

Type: Oral

Date: 4/20/2023

Presentation Time: 10:00 AM (local time)

Presenting Author: Chuanbin Zhu

Student Presenter: No

Invited Presentation: 

Authors