OBSERVATIONS OF AMBIENT NOISE AND SIGNAL COHERENCY ON THE ISLANDS OF HAWAII FOR TELESEISMIC STUDIES

Ambient seismic noise conditions at several sites on the Island of Haw aii have been studied by analyzing noise power spectral density (PSD) and signal-to-noise ratio (SNR) of teleseisms. Broadband digital wavef orm data used in this study were collected during Project ALOHA. Direc t comparison of noise PSD and SNR of teleseisms simultaneously recorde d at stations on Hawaii and at station KIP on Oahu indicates that the stations on Hawaii are quieter than station KIP on Oahu in the frequen cy band of 0.1 to 1 Hz, suggesting that the Island of Hawaii can provi de better noise conditions than station KIP. Our interpretation of thi s observation is that the area and volume of Hawaii are much larger th an those of Oahu and distances to the coast for the stations on Hawaii are larger than that for station KIP on Oahu. Stronger attenuation an d relatively low coherency for teleseismic signals on Hawaii in freque ncies of 0.4 to 1.2 Hz are attributed to the presence of partial melt in the upper mantle. Teleseismic signal coherency is very low at frequ encies above 2 Hz for a small array with a radius about 2 km. The rela tively high coherency of the tangential component is strongly indicati ve of severe scattering effects caused by the heterogeneity in structu re of the active volcanic area.