Polarization and Frequency-Wavenumber Spectrum Analysis for the Strong-Motion Array Data in Ashigara Valley, Japan.

Abstract

In order to understand the propagation characteristics of earthquake strong motion in a sedimentary basin, combined analyses were made using running spectra, polarization and frequency-wavenumber spectra for array data obtained in Ashigara Valley, Japan. The data from three moderately large events (M= 5.5-6.5) which occurred in Sagami Bay, south of the Ashigara Valley, were analyzed. The long-period (5-10 s) Love waves were predominant during the two shallow events ( <10 km), while short-period waves less than 2 s dominated in the event of intermediate depth (20 km). The predominant waves propagated from the azimuths of about 10°-20° west of the sources. Not only the sedimentary basin at the Ashigara Valley, but also the underground structure beneath Izu Peninsula may affect the propagation path from source to station. A significant variation in the Love wave propagation direction was found during the earthquake of Feb. 20, 1990, the shallow largest event. Three distinct arrivals of Love waves were identified in the event. Two wave groups were identified as Love waves propagated from the azimuth of 15° west of the source. The other one group propagated from the azimuth of 30° west of the source. Its propagation path turned towards a more easterly direction. The arrival was identified as Love waves from both the polarization and propagation direction. This identification was only possible by combining the results of polarization and frequency-wavenumber spectral analyses. The significance of the effects of three-dimensional heterogeneity in the sedimentary basin was also suggested.