On the Distribution of Earthquake Strong-Motion Amplitudes and Site Effects Across the Icelandic Strong-Motion Arrays

Abstract eng:
In Iceland, the most seismically active region in northern Europe, large earthquakes occur almost exclusively in two major fracture zones, the South Iceland Seismic Zone (SISZ) in the populated and cultivated farmlands of south Iceland and the mostly offshore Tjörnes Fracture Zone (TFZ) in north Iceland. Both regions are known for being densely populated and have been historically subjected to destructive near-fault earthquake strong-motions. Generally speaking, populated urban areas in Iceland consist of structures of relatively similar construction. Although site effects are generally assumed to be negligible and regionally similar, recent data collected on new strong-motion arrays, the ICEARRAY-I in Hveragerði in the SISZ and ICEARRAY-II in Húsavík in the TFZ now provide the opportunity to investigate the geological impact on site responses and study spatial distribution of ground motions. In this study, we apply the Nakamura’s method using earthquake strong-motion data and microseismic measurements to assure the consistency of the results. To further investigations of geostructural influences on site responses, we present the spatial distribution maps for key parameters such as Peak Ground Acceleration (PGA) and Spectral Acceleration (SA) as a function of oscillator period as well as predominant frequency (f0) and the relative amplification (A0). Contrary to the fairly uniform site condition, the data exhibit variability across both arrays; however, despite the scatter of the results, we find a noticeable correlation between near surface geology and site response. Additionally, due to strong velocity reversals with depth due to repeated lava-sediment stratigraphy under most of ICEARRAY-I, we model it as a discrete dynamic structural system instead of using the body wave approximation. The results presented herein support the use of site effect investigations in combination with surface geology to improve seismic microzonation studies and also present a simple, practical and reliable strategy to model soil amplification in the presence of the velocity reversals.

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