Attenuation of Broadband P and S Waves in Tonga: Observations of Frequency Dependent Q

Teleseismic broadband recordings of intermediate and deep focus earthquakes are used to quantify both compression (Qa ) and shear (Qb ) wave attenuation within the Lau backarc basin. A spectral-ratio method is employed to measure differential attenuation (dt*) between the depth phases sS, pP, and sP and the direct S and P phases over the frequency band 0.05 and 0.5 Hz. We use a stacking algorithm to combine the spectra of several phase pairs from a single event, having similar azimuth and range, to obtain more robust dt* measurements; these estimates are then used to compute the average Q above the focal depth. Qb and Qa are measured directly from the sS-S and pP-P phase pairs respectively, however, the interpretation of dt* measured from sP-P requires assumptions about the ratio Qa :Qb. We find an empirical ratio of Qa :Qb 1.93 for this region and use it to compute Qa and Qb from the QsP observations. We observe lateral and depth variations in both Qb and Qa beneath the tectonically active Lau Basin and the geologically older, inactive Lau Ridge and Fiji Plateau. The upper 200 km beneath the Central and Northern Lau Basin show a Qb of 45–57 and a Qa of 102–121, and Q appears to increase rapidly with depth. The upper 600 km beneath the Lau backarc basin has a Qb of 118–138, while over the same depth interval we observe a higher Qb of 139–161 beneath the Lau Ridge and Fiji Plateau. We also find Qa of 235–303 beneath the northern Lau Basin and a higher Qa of 292–316 beneath the Fiji Plateau and the Lau Ridge measured directly from pP-P phase pairs. These geographic trends in the broadband Q measurements correlate with our previous long-period estimates of Qb in this region, however, the broadband measurements themselves are higher by about a factor of two. These observations suggest substantial frequency dependence of Q in the upper mantle, beginning at frequencies less than 1.0 Hz and consistent with the power-law form: Q8va with a between 0.1 and 0.3.