The influence of shear‐velocity heterogeneity on ScS2/ScS amplitude ratios and estimates of Q in the mantle

Regional waveforms of deep‐focus Tonga‐Fiji earthquakes indicate anomalous traveltime differences (ScS2‐ScS) and amplitude ratios (ScS2/ScS) of the phases ScS and ScS2. The correlation between the ScS2‐ScS delay time and the ScS2/ScS amplitude ratio suggests that shear wave apparent Q in the mantle below the Tonga‐Fiji region is highest when shear wave velocities are lowest. This observation is unexpected if temperature variations were responsible for the seismic anomalies. Using spectral element method waveform simulations for four tomographic models, we demonstrate that focusing and scattering of shear waves by long‐wavelength 3‐D heterogeneity in the mantle may overwhelm the signal from intrinsic attenuation in long‐period ScS2/ScS amplitude ratios. The tomographic models reproduce the trends in recorded ScS2‐ScS difference times and ScS2/ScS amplitude ratios. Although they cannot be ruled out, variations in shear wave attenuation (i.e., the quality factor Q) are not necessary to explain the data.Key PointsThe influence of complex 3‐D wave propagation in the mantle on ScS2/ScS amplitude ratiosScS2‐ScS difference times are delayed and ScS2/ScS amplitude ratios are high on Samoa indicating low wave speeds but no attenuationBody wave amplitudes may be useful for evaluating the accuracy of tomographic models and as complementary data in tomographic inversionsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134191/1/grl54786_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134191/2/grl54786.pd

Observations of core‐mantle boundary Stoneley modes

Core‐mantle boundary (CMB) Stoneley modes represent a unique class of normal modes with extremely strong sensitivity to wave speed and density variations in the D” region. We measure splitting functions of eight CMB Stoneley modes using modal spectra from 93 events with M w > 7.4 between 1976 and 2011. The obtained splitting function maps correlate well with the predicted splitting calculated for S20RTS+Crust5.1 structure and the distribution of S diff and P diff travel time anomalies, suggesting that they are robust. We illustrate how our new CMB Stoneley mode splitting functions can be used to estimate density variations in the Earth's lowermost mantle. Key Points We present CMB Stoneley mode splitting function measurements The CMB Stoneley mode splitting correlates well with diffracted body wave data Our measurements allow to constrain density variations in the lowermost mantlePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99080/1/figS2_plot_prem_freq_Q_stoneley_paperrotated.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99080/2/grl50514.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99080/3/figS1_plot_coef_stoneley_paper_deg2_newrotated.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99080/4/README_suppl_mat_GRL.pd

An analysis of SS precursors using spectral‐element method seismograms

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/89460/1/j.1365-246X.2011.05256.x.pd

Small‐scale Shear Velocity Variance of the D″ Layer beneath the Indian‐Eurasia Collision Zone

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149523/1/acgs13921.pd

Lateral Variations of Shear‐Wave Velocity in the D″ Layer Beneath the Indian‐Eurasian Plate Collision Zone

Seismic tomography has demonstrated that the shear‐wave velocity is relatively high over a 3,000‐km wide region in the lowermost mantle beneath southern and eastern Asia. This seismic anomaly demarcates the current position of slab remnants that may have subducted in the Cretaceous. To further characterize the seismic structure at smaller scales, we measure 929 residual travel time differences (δt) between the phases ScS and S using recordings of eight earthquakes beneath the Indian Ocean at stations from the Chinese Digital Seismic Network. We interpret variations of δt up to 10 s as due to horizontal shear‐velocity variations in D″ beneath northern India, Nepal, and southwestern China. The shear velocity can vary by as much as 7% over distances shorter than 300 km. Our observations provide additional observational evidence that compositional heterogeneity and possibly melt contribute to the seismic structure of the lower mantle characterized by long‐term subduction and mantle downwelling.Plain Language SummarySeismic tomography indicates that the seismic wave speed is relatively high in the lowermost mantle (i.e., the D″ region) beneath regions, such as eastern Asia, influenced by subduction since the Mesozoic era. Our new analysis of the propagation time of shear‐wave reflections off the outer core (i.e., the phase ScS) corroborates the result from seismic tomography that the shear velocity in D″ beneath eastern Asia is high overall. However, we also find that the shear velocity can vary by as much as 7% over distances shorter than 300 km within a region of D″ beneath northern India, Nepal, and southwestern China. This provides new evidence for the thermochemical nature of D″ beneath downwelling regions of the mantle.Key PointsUsing waveforms of S and ScS, we estimate the shear‐wave velocity (Vs) structure in D″ beneath the Indian‐Eurasian plate collision zoneVs is relatively high overall but we resolve horizontal variations by 3–7% over distances shorter than about 300 kmOur models place new constraints on geodynamic scenarios for the generation of thermochemical heterogeneity in downwelling regions of D″Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154654/1/grl60374.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154654/2/grl60374_am.pd

Lithospheric cooling trends and deviations in oceanic PP‐P and SS‐S differential traveltimes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97534/1/jgrb50092.pd

A new catalogue of normal-mode splitting function measurements up to 10 mHz

The splitting of the Earth's free-oscillation spectra places important constraints on the wave speed and density structure of the Earth's mantle and core. We present a new set of 164 self-coupled and 32 cross-coupled splitting functions. They are derived from modal spectra up to 10 mHz for 91 events with Mw ≥ 7.4 from the last 34 yr (1976–2010). Our data include the 2001 June 23 Peru event (Mw = 8.4), the Sumatra events of 2004 (Mw = 9.0) and 2005 (Mw = 8.6), the 2008 Wenchuan, China event (Mw = 7.9) and the 2010 Chile event (Mw = 8.8). The new events provide significant improvement of data coverage particularly in continental areas. Almost half of the splitting functions have never been measured before. In particular, we measured 33 new modes sensitive to mantle compressional wave velocity, 10 new inner-core sensitive modes and 22 new cross-coupled splitting functions. These provide new constraints on the large-scale compressional structure of the mantle and the odd-degree structure of the mantle and inner core and can be used in future inversions of heterogeneous Earth structure. Our new splitting function coefficient data set will be available online

Comparing ray-theoretical and finite-frequency teleseismic traveltimes:Implications for constraining the ratio of S-wave to P-wave velocity variations in the lower mantle

A number of seismological studies have indicated that the ratio R of S-wave and P-wave velocity perturbations increases to 3–4 in the lower mantle with the highest values in the large low-velocity provinces (LLVPs) beneath Africa and the central Pacific. Traveltime constraints on R are based primarily on ray-theoretical modelling of delay times of P waves (ΔTP) and S waves (ΔTS), even for measurements derived from long-period waveforms and core-diffracted waves for which ray theory (RT) is deemed inaccurate. Along with a published set of traveltime delays, we compare predicted values of ΔTP, ΔTS, and the ΔTS/ΔTP ratio for RT and finite-frequency (FF) theory to determine the resolvability of R in the lower mantle. We determine the FF predictions of ΔTP and ΔTS using cross-correlation methods applied to spectral-element method waveforms, analogous to the analysis of recorded waveforms, and by integration using FF sensitivity kernels. Our calculations indicate that RT and FF predict a similar variation of the ΔTS/ΔTP ratio when R increases linearly with depth in the mantle. However, variations of R in relatively thin layers ( 20 s). This is because FF predicts that ΔTP and ΔTS vary smoothly with epicentral distance even when vertical P-wave and S-wave gradients change abruptly. Our waveform simulations also show that the estimate of R for the Pacific LLVP is strongly affected by velocity structure shallower in the mantle. If R increases with depth in the mantle, which appears to be a robust inference, the acceleration of P waves in the lithosphere beneath eastern North America and the high-velocity Farallon anomaly negates the P-wave deceleration in the LLVP. This results in a ΔTP of about 0, whereas ΔTS is positive. Consequently, the recorded high ΔTS/ΔTP for events in the southwest Pacific and stations in North America may be misinterpreted as an anomalously high R for the Pacific LLVP

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0002 layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0003, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations