Wavefield characteristics and coherency of seismic ground motion from a rocksite at argostoli Greece
Résumé
The effects of spatial variation of earthquake ground motion (SVEGM) have been of great interest for a long time in the design and analysis of large and extended structures. In current engineering practices, the SVEGM is characterized by coherency functions, representing the degree of correlation between two ground motions, and usually estimated from the analysis of dense seismic arrays as an exponential decay with increasing frequency and interstation distance. However, most of these arrays are located at soil sites, resulting in very few coherency models for rock sites. In reality, many horizontally extended structures (e.g. bridges) may be supported at different site conditions-soil or rock or a combination of both. In order to extrapolate these models for different sites, it is imperative to characterize the loss of coherency based on some physical parameters, which requires identification of the different seismic phases contributing to the recorded signal. The current studies do not focus on such wavefield characterization, probably considering implicitly the ground motions on rocky sites to be minimally affected by the complex wave propagation. However, spatial incoherency has also been observed at rock sites owing to geological complexities such as weathering and shallow fracturation. In this context, this study analyzes the seismic events recorded from a dense array located on a rock site at Argostoli, Cephalonia Island, Greece. The array was installed within the framework of the ANR-PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/). The objective of the current study is to explore to what extent the non-direct, diffracted surface waves influence the seismic wavefield at a rock site and to investigate the loss of coherency of ground motions. The array consists of 21 velocimeters encompassing a central station in four concentric circles with diameters 20, 60, 180 and 360 m. The analyzed seismic dataset includes 40 events with magnitudes ranging from 2 to 5 and epicentral distance up to 200 km. MUSIQUE algorithm has been used to analyze the seismic wavefield by extracting the backazimuth and slowness of the dominant incoming waves and identifying the Love and Rayleigh waves. Lagged coherency has been estimated for all the available station pairs (max. 210 pairs per event) and the results have been averaged at different separation distance intervals (10-20, 20-30, 30-40, 80-90 m) for the entire dataset. The results were also compared with those from a similar array located on an adjacent small, shallow sedimentary valley. The analysis suggests that about 20% energy of the wavefield could be characterized as diffracted Love and Rayleigh waves, primarily arriving from the northeast and north-south directions, respectively. The spatial coherency estimations at the rock site are, generally, observed to be larger than those from the sedimentary array, especially at frequencies below 5 Hz. The directionality of coherency estimates observed from the soil array is absent in case of the rock array data. Comparison with the widely-quoted parametric models reveals that there is little correlation between the decay of coherency observed at the rock site and the models. The results clearly indicate the site dependent nature of the spatial incoherency as well as the locally generated wavefield at the rock and soil sites. The findings suggests that more rigorous studies based on rocky sites could definitely be instrumental in better understanding the causes of spatial variation of ground motion.
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