Attenuation of Broadband P and S Waves in Tonga: Observations of Frequency Dependent Q
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—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 employed 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 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 <delta>t* measurements; these estimates are then used to compute the average Q above the focal depth. Q <beta> and Q <alpha> are measured directly from the sS-S and pP-P phase pairs respectively, however, the interpretation 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 for this region and use it to compute Q <alpha> and Q <beta> from the Q sP observations. We observe lateral and depth variations in both Q β and Q >alpha< 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 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 basin has a Q <beta> of 118–138, while over the same depth interval we observe a higher Q <beta> of 139–161 beneath the Lau Ridge and Fiji Plateau. We also find Q <alpha> of 235–303 beneath the northern Lau Basin and a higher Q <alpha> 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 Q <beta> 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 Q∝ωα with <alpha> between -0.1 and -0.3.
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