ISSN:
1420-9136
Keywords:
Key words: Attenuation, Frequency dependence, subduction zone.
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
,
Physics
Notes:
Abstract —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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s000240050199
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