Abstract

We have observed first-order mantle reverberations, specifically SH-polarized ScSn and sScSn phases reflected at near-normal incidence from upper mantle discontinuities, as discrete phases on long-period digital seismograms of the HGLP and SRO networks. Such arrivals correspond to an n or n + 1 member dynamic ray family denoted by {ScSn, Sd±S}, where n is the multiple number of the parent phase, “d+” signifies a reflection from the top of an internal discontinuity at depth d, and “d” signifies a reflection from the bottom. The travel times and attenuation of these phases place important constraints on the nature of the transition zone. We have employed the phase equalization and stacking algorithm of Jordan and Sipkin (1977) to obtain the differential attenuation operators of {sScSn, S650+S} − sScSn and {ScSn, S650S} − ScSn phase pairs from deep-focus Tonga events recorded by the HGLP station KIP on the island of Oahu. The apparent Q's of the zeroth-order and first-order reverberations over the frequency band of 10 to 30 mHz have been inverted for the average quality factors of the upper mantle (QUM) and lower mantle (QLM) and the reflection coefficient of the 650 km discontinuity (R650). At 20 mHz, the results are: QUM = 82 ± 18, QLM = 231 ± 60, and |R650| = 0.080 ± 0.004 (at normal incidence). The QLM estimate is significantly less than recent normal mode solutions, suggesting that lower mantle attenuation structure is frequency-dependent at very low frequencies. We have measured the differential travel times between the first-order reverberations {sScSn, S400+S} and {sScSn, S650+S}, which yields a direct estimate of the vertical shear-wave travel time between the discontinuities along the Tonga-to-KIP path. The observed two-way time, 92.2 ± 1.5 sec, is less than predicted by some recent oceanic upper mantle models, indicating either a smaller separation of the discontinuities and/or greater shear velocity in the transition zone. Measurements of differential travel times between first-order reverberations and multiple ScS phases suggest that the former is more likely and are consistent with depths of 400 and 650 km for the transition zone discontinuities. Observations of first-order mantle reverberations can potentially provide unique constraints on lateral heterogeneity of the earth's mantle, especially within the transition zone.

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