INFERRED ATTENUATION FROM SITE EFFECT-CORRECTED T-PHASES RECORDED ON THE ISLAND OF HAWAII

Coda site amplification factors are used to eliminate the site effect from records of three circum-Pacific T phases recorded by the Hawaiian Volcano Observatory (HVO) network on the island of Hawaii. Observed T -phase amplitudes across the island generally decrease with increasing distance from the conversion point where acoustic waves in the SOFAR channel become seismic waves propagating through the crust. However, t he decay of the observed T-phase signal across the island is not regul ar in regions of dense station coverage, in particular, the Kilauea ca ldera region. We divide the maximum observed T-phase amplitudes at a g iven station by the coda site amplification factor obtained for the sa me station and frequency band (3.0 Hz); the distribution of these ampl itudes reveals a smooth pattern over the entire island. The distance o ver which the site effect-corrected T-phase amplitude decreases by one -half, combined with the apparent velocity of propagation of T phases across the island, allows for an approximate determination of near-sur face Q over much of the island of Hawaii. We found a region of low Q i n the Kilauea summit area (Q approximate to 30) and east rift zone (Q approximate to 60) with considerably higher Q in the Kaoiki and northe rn portions of the island (Q approximate to 150 to 200). The low e val ues obtained in the Kilauea summit region and east rift zone are signi ficantly lower than estimates of coda e in the same region, suggesting T phases may be sampling the earth's near-surface properties.