Body wave attenuation in the Tonga Subduction Zone

Abstract

Attenuation P and S waves of in the Tonga subduction zone is estimated from locally recorded data using a spectral ratio method. The data consist of three‐component, digital seismograms of 77 earthquakes recorded at two stations in Fiji and one in Tonga. The stations in the Fiji Islands lie above the deep earthquakes in the subduction zone while the Tonga station is close to the top of the subducted slab. Variations in the high‐frequency spectral decay rate define four attenuation provinces. Paths from earthquakes deeper than 350 km to stations on Viti Levu (Fiji) and the Lau ridge traverse mantle with normal attenuation (Qp=570, Qs=200–400). The subducting Tonga slab has very low attenuation (Qp>2000, Qs≥1800) relative to the surrounding mantle. The wedge of mantle above the slab at depths shallower than 350 km and east of the Lau ridge is extremely attenuating for P waves (Qp=240). Several paths from main thrust zone events to the station in Tonga show intermediate attenuation (Qp=300–500) in the forearc region. The observed spectra do not require Q to be frequency dependent in the 1.0 Hz to 7.6 Hz band in this region.