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

Abstract

Teleseismic broadband recordings of intermediate and deep focus earthquakes are used to quantify both compression (Q α) and shear (Q β) wave attenuation within the Lau backarc basin. A spectral-ratio method is employed to measure differential attenuation (δt*) 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 δt* measurements; these estimates are then used to compute the average Q above the focal depth. Q β and Qα are measured directly from the sS-S and pP-P phase pairs respectively, however, the interpretation of δt* measured from sP-P requires assumptions about the ratio Q α/Q β. We find an empirical ratio of Q α/Q β = 1.93 for this region and use it to compute Qα and Q β from the Q sP observations.We observe lateral and depth variations in both Q β and Q α beneath the technically 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 β of 45–57 and a Q α of 102–121, and Q appears to increase rapidly with depth. The upper 600 km beneath the Lau backarc basin has a Q β of 118–138, while over the same depth interval we observe a higher Q β of 139–161 beneath the Lau Ridge and Fiji Plateau. We also find Q α of 235–303 beneath the northern Lau Basin and a higher Q α 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 β 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 α between ‒0.1 and −0.3.Key wordsAttenuationFrequency dependencesubduction zone