Attenuation of broadband P and S waves in tonga: Observations of frequencydependent Q

Teleseismic broadband recordings of intermediate and deep focus earthquakes are used to quantify both compression (Q(alpha)) and shear (Q(beta)) wave attenuation within the Lau backarc basin. A spectral-ratio method is employ ed to measure differential attenuation (delta t*) between the depth phases sS, pP, and sP and the direct S and P phases over the frequency band 0.05 a nd 0.5 Hz. We use a stacking algorithm to combine the spectra of several ph ase pairs from a single event, having similar azimuth and range, to obtain more robust delta t* measurements; these estimates are then used to compute the average Q above the focal depth. Q(beta) and Q(alpha) are measured dir ectly from the sS-S and pP-P phase pairs respectively, however, the interpr etation of delta t* measured from sP-P requires assumptions about the ratio Q(alpha)/Q(beta). We find an empirical ratio of Q(alpha)/Q(beta) = 1.93 fo r this region and use it to compute Q(alpha) and Q(beta) from the Q(sP) obs ervations. We observe lateral and depth variations in both Q(beta) and Q(alpha) beneat h the tectonically active Lau Basin and the geologically older, inactive La u Ridge and Fiji Plateau. The upper 200 km beneath the Central and Northern Lau Basin show a Q(beta) of 45-57 and a Q(alpha) of 102-121, and Q appears to increase rapidly with depth. The upper 600 km beneath the Lau backarc b asin has a Q(beta) of 118-138, while over the same depth interval we observ e a higher Q(beta) of 139-161 beneath the Lau Ridge and Fiji Plateau. We al so find Q(alpha) of 235-303 beneath the northern Lau Basin and a higher Q(a lpha) of 292-316 beneath the Fiji Plateau and the Lau Ridge measured direct ly from pP-P phase pairs. These geographic trends in the broadband Q measur ements correlate with our previous long-period estimates of Q(beta) in this region, however, the broadband measurements themselves are higher by about a factor of two. These observations suggest substantial frequency dependen ce of Q in the upper mantle, beginning at frequencies less than 1.0 Hz and consistent with the power-law form: Q proportional to omega(alpha) with alp ha between -0.1 and -0.3.