Detailed site effect estimation in the presence of strong velocity reversals within a small-aperture strong-motion array in Iceland

• Published in: Soil Dynamics and Earthquake Engineering
• Main Authors: Rahpeyma, Sahar, Halldorsson, Benedikt, Olivera, Christian, Green, Russell A., Jonsson, Sigurjon
• Other Authors: Crustal Deformation and InSAR Group, Earth Science and Engineering Program, Physical Science and Engineering (PSE) Division, Earthquake Engineering Research Centre & Faculty of Civil and Environmental Engineering, School of Engineering and Natural Sciences, University of Iceland, Selfoss, Iceland, ECS Mid-Atlantic, LLC, 6710 Oxon Hill Road, Suite 101, National Harbor, MS, United States, Department of Civil & Environmental Engineering, Virginia Tech, Blacksburg, VA, United States
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier BV 2016
• Subjects: Array; Iceland
• Online Access: http://hdl.handle.net/10754/621537; https://doi.org/10.1016/j.soildyn.2016.07.001

The rock site characterization for earthquake engineering applications in Iceland is common due to the easily exposed older bedrock and more recent volcanic lava rock. The corresponding site amplification is generally assumed to be low but has not been comprehensively quantified, especially for volcanic rock. The earthquake strong-motion of the Mw6.3 Ölfus earthquake on 29 May 2008 and 1705 of its aftershocks recorded on the first small-aperture strong-motion array (ICEARRAY I) in Iceland showed consistent and significant variations in ground motion amplitudes over short distances (<2 km) in an urban area located mostly on lava rock. This study analyses the aftershock recordings to quantify the local site effects using the Horizontal to Vertical Spectral Ratio (HVSR) and Standard Spectral Ratio (SSR) methods. Additionally, microseismic data has been collected at array stations and analyzed using the HVSR method. The results between the methods are consistent and show that while the amplification levels remain relatively low, the predominant frequency varies systematically between stations and is found to correlate with the geological units. In particular, for stations on lava rock the underlying geologic structure is characterized by repeated lava-soil stratigraphy characterized by reversals in the shear wave velocity with depth. As a result, standard modeling of HVSR using vertically incident body waves does not apply. Instead, modeling the soil structure as a two-degree-of-freedom dynamic system is found to capture the observed predominant frequencies of site amplification. The results have important implications for earthquake resistant design of structures on rock sites characterized by velocity reversals. © 2016 Elsevier Ltd Icelandic Centre for Research[141261-051] Icelandic Catastrophe Insurance[S112-2013] University of Iceland Research Fund Leifur Eiriksson Foundation and Virginia Tech