Constraints on Shear Wave Velocity Heterogeneity and Anisotropy in D” from Finite-Frequency Differential Traveltime Residual Analysis
Date Issued
2014
Date
2014
Author(s)
Liao, Tzu-Yun
Abstract
The D’’ region which lies in the lowermost ~250 km of the mantle has long been postulated as a major thermo-chemical boundary layer in the earth’s dynamic evolution, where the upwelling plumes most likely originate and the downwelling cold slabs terminate. Numerous seismological investigations have found D” exist both strong velocity heterogeneity and anisotropy.
In this study, we collect broadband waveforms from earthquakes with epicentral distances of 40-145o and magnitudes greater than 5.8 during 1997-2012. A cluster analysis with waveform cross correlation is employed to measure differential traveltime residuals of S(Sdiff)-SKS, ScS-S, ScS2-ScS, ScS3-ScS relative to predictions of the spherically-symmetric earth model ak135. Our results show the phase pairs passing through D” beneath the Circum-Pacific Rim generally have large negative residuals while those through D” beneath the Central Pacific and Africa have positive residuals. The residual variations are in agreement with large-scale degree 2 pattern of mantle convective circulation in the lowermost mantle as revealed from global tomographic images, in which the circum-Pacific ring of high velocities associated with the remnants of ancient subducted slabs surround the two antipodal large low-velocity regimes originating from the upwelling hot superplumes anchored at the core-mantle boundary beneath the central Pacific and African. Besides, the residuals from the paths through the Northeastern Pacific plate and Siberia give both positive and negative values within small sampling areas in D”, suggesting that these regions may exists complex, small-scale structural heterogeneity.
The split times between the vertically (SV) and transversely (SH) polarized arrivals of S(Sdiff) phases after correcting for upper mantle anisotropy are further analyzed to investigate D” seismic anisotropy. We observe both positive and negative SV-SH split times in seismically slow regions; however, the proposed viable scenario in which partial melts produced at the base of the hot mantle plumes in D” have been aligned horizontally by the basal boundary flow can only explain advanced SH arrivals relative to SV. Positive SV-SH times (i.e., SH traveling faster) found in high-velocity regions are consistent with intrinsic LPO of anisotropic minerals in horizontally-lying slabs and apparent splitting resulting from finite-frequency sensitivity. Numerical waveform modeling has indicated that because of difference of the finite-frequency sensitivities, the SHdiff phase arrivals would advance and delay with respect to SVdiff while propagating through high- and low-velocity D” regions, respectively, similar to our observed apparent splitting between them. Both finite-frequency effects and elastic anisotropy may equally contribute to the observed splitting results.
Subjects
下地幔
D’’
剪力波速度側向非均質性
剪力波分離
彈性非均向性
Type
thesis
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