Nationwide frequency-dependent seismic site amplification models for Iceland

TL;DR

This study develops nationwide frequency-dependent seismic site amplification models for Iceland, integrating local geological data and recent large-scale models, revealing significant discrepancies with non-Icelandic models and emphasizing the importance of local site effects.

Contribution

It presents the first comprehensive frequency-dependent amplification maps for Iceland based on local data, and compares their performance with large-scale models, highlighting the necessity of local calibration.

Findings

Local models outperform non-Icelandic models in predicting site amplification.

Discrepancies are mainly due to Iceland's young geology and formation history.

Frequency-dependent site factors are characterized at 1-30 Hz and PGA.

Abstract

Seismic wave amplification due to localized site conditions is an important aspect of regional seismic hazard assessment. Without systematic studies of frequency-dependent site-effects during strong Icelandic earthquakes, various local site proxies of large-scale studies in other seismic regions have been used in Iceland. Recently, earthquake site-effects were rigorously quantified for 34 stations in Southwest Iceland for the first time and correlated to distinct Icelandic geological units of hard rock, rock, lava rock, and sedimentary soil. These units are prevalent throughout Iceland and herein we present 1) nationwide maps of proxies (slope, Vs30, geological units) that may contribute to a better estimation of site effects and associated, 2) frequency-dependent site-amplification maps of Iceland. The frequency-dependent site factors for each geological unit are presented at 1-30 Hz and PGA. Finally, we generate site amplification maps based on recent large-scale models developed in other seismic regions (ESRM20) and various site proxies they are based on (geology- and slope-based inferred Vs30, geomorphological sedimentary thickness). We compare site-proxy maps and amplification maps from both Icelandic and large-scale, non-Icelandic, models. Neither spatial patterns nor amplification levels in either proxy or amplification maps from large-scale non-Icelandic studies resemble those observed from local quantitative strong-motion research as presented in this study. We attribute the discrepancy primarily to the young geology of Iceland and its formation history. Additionally, we compare model performance across frequencies by assessing the bias of model predictions against empirical site amplifications in the South Iceland Seismic Zone, accounting for site-to-site variability of residuals indicating the superior performance of the local amplification model.