doi:10.3850/978-981-08-7619-7_P063

ABSTRACT

This study analyzes data recorded at a unique dense array in the Parkway Valley, Wainuiomata, New Zealand, a small alluvial valley surrounded by greywacke outcrops. Power spectral densities and lagged coherencies are estimated from the data during two windows: A faster-propagating-wave window at the onset of the excitation, rich in lower and higher frequency components, and a later, slower-propagating-wave window with high energy at lower frequencies. Dramatic amplitude variability of the motions is observed at the valley stations depending on their location and the time window analyzed. During the second, dominant surface-wave window, the peak amplification of the data at the lower frequencies can be as high as four times the peak amplification during the onset of the excitation. On the other hand, the lagged coherency of the motions appears to be higher during the onset of the excitation as compared to the lagged coherency during the dominant surface-wave window. Higher correlations in the data are observed for a station pair in the middle of the valley and along the direction of propagation of the waves, and lower correlations between the data at stations in the middle of the valley and data at stations close to the valley edges. The analysis provides insight in the pattern of the spatial variability of the seismic ground motions at sites with irregular subsurface topography.

Keywords: Seismic motions, Site conditions, Spatial variability, Wave propagation, Power spectral density, Coherency.

© 2011 RPS