Abstract
Although rotational seismology has progressed in recent decades, the links between rotational ground motion and site soil conditions are poorly documented. New experiments were performed on Kefalonia Island (Greece) following two large earthquakes (MW = 6.0, MW = 5.9) in early 2014 on two well-characterized sites (soft soil, VS30 ~ 250 m/s; rock, VS30 ~ 830 m/s, VS30 being harmonic average shear-wave velocity between 0 and 30 m depth). These earthquakes led to large six-component (three translations and three rotations) datasets of hundreds of well-recorded events. The relationship between peak translational acceleration versus peak rotational velocity is found sensitive to the site conditions mainly for the rotation around the vertical axis (torsion; dominated by Love waves): the stiffer the soil, the lower the torsion, for a given level of translational acceleration. For rotation around the horizontal axes (rocking; dominated by Rayleigh waves), this acceleration/rotation relationship exhibits much weaker differences between soft and rock sites. Using only the rotation sensor, an estimate of the Love-to-Rayleigh energy ratios could be carried out and provided the same results as previous studies that have analyzed the Love- and Rayleigh-wave energy proportions using data from translational arrays deployed at the same two sites. The coupling of translational and rotational measurements appears to be useful, not only for direct applications of engineering seismology, but also to investigate the composition of the wavefield, while avoiding deployment of dense arrays. The availability of new, low-noise rotation sensors that are easy to deploy in the field is of great interest and should extend the use of rotation sensors and expand their possible applications.Graphical abstract.
Highlights
At present, the definition of seismic hazard involves almost exclusively translational ground-motion analysis along the three dimensions of space
We introduce two new 6-DOF datasets that were recorded on well-characterized sites with different subsoil properties, during the post-seismic sequence of the two large earthquakes (MW = 6.0 and MW = 5.9) that occurred in early 2014 on the island of Kefalonia, in the Ionian Sea (Greece) (Hollender et al 2015)
The post-seismic survey that was conducted on Kefalonia Island after the two large earthquakes (MW = 6.0, MW = 5.9) that occurred on January 26, 2014, and on February 3, 2014, allowed to gather new comprehensive datasets combining translational and rotational measurements (6-DOF) on rock and soil sites with well-characterized site conditions
Summary
The definition of seismic hazard involves almost exclusively translational ground-motion analysis along the three dimensions of space. We introduce two new 6-DOF datasets that were recorded on well-characterized sites with different subsoil properties, during the post-seismic sequence of the two large earthquakes (MW = 6.0 and MW = 5.9) that occurred in early 2014 on the island of Kefalonia, in the Ionian Sea (Greece) (Hollender et al 2015) This survey was conducted within the framework of the SINAPS@ program (Berge-Thierry et al 2016). Note that the rotation sensor was “virtually” placed here in the center of a dense array that had been installed previously in the framework of the NERA program (funded by the European Union), which allowed complete analysis of the ground-motion coherency and the wavefield composition (Imtiaz 2015) This array was no longer in place for the SINAPS@ post-seismic survey, but the results of the rotation measurement could be analyzed using the results of the Imtiaz (2015) study.
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