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INNER CORE 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Support for Inner Core Super-Rotation from High-Quality Waveform Doublets Xiaodong Song, Yingchun Li, Xinlei Sun • University of Illinois at Urbana-Champaign Jian Zhang, Paul G. Richards, Felix Waldhauser • Lamont-Doherty Earth Observatory of Columbia University Differential inner core rotation was inferred from temporal change of travel times through the inner core (Song and Richards, 1996). However, claims of such a travel-time change have DF BC AB A DF been challenged and reinterpreted as artifacts of systematic t1-0-86a-86b event mislocations, mantle heterogeneity, and other causes t1-1-86b-87c (e.g., Souriau, 1998). Waveform doublets potentially can provide much stronger proof of temporal change in travel times t1-1-86a-87c d-2-97a-99-BC04 through the inner core (Li and Richards, 2003). The high d-3-87a-90 similarity of the whole waveforms in a waveform doublet d-4-98-02b ensures that the two events indeed occur at the same location d-7-95c-02a-BC01 and sample the same Earth structure. The waveform similarity also allows measurements of relative time shifts with high d-7-86d-93 d-8-93c-01-BC01 precision. We have recently found 18 high-quality waveform doublets with time separation of up to 35 years in the South Sandwich Islands (SSI) region, for which the seismic signals that have traversed the inner core as PKP(DF) show a consistent temporal change of travel times at up to 58 stations in and near Alaska, and a dissimilarity of PKP(DF) coda (Zhang et al., 2005). Using waveform doublets avoids artifacts of earthquake mislocations and the contamination of small-scale heterogeneities. Our new results greatly strengthen the original claim of seismological evidence for inner core super-rotation. The figure overlays waveforms of the SSI doublets in order of increasing time separation, most of which were recorded at the GSN station at College Alaska (COL/COLA). We see that the waveforms of the PKP branches that do not traverse the inner core, PKP(BC) and PKP(AB) at College station and Beaver Creek array stations (BC01 and BC04), are highly similar. The high similarity of BC and AB signals in our doublets is due to propagation paths outside the inner core that sample the same heterogeneities. Observed differences in DF give information on changes in the inner core. d-8-87b-95 t2-10-93b-03 d-13-82a-95a d-18-86c-04 d-18-64-82b d-23-61-84 t2-23-70-93b t2-33-70-03 d-35-62-97b -15 -10 -5 0 5 10 15 20 -2 -1 0 1 2 3 4 Travel Times (s) Travel Times (s) (A) PKP waveforms at College, Alaska station and Beaver Creek array stations BC01 and BC04 of 18 SSI doublets. The traces (grey for the earlier event, dark for the later one) are aligned with the PKP(BC) phase. They are sorted with increasing time separation from top to bottom (the year difference and the years of the two events are indicated in the label). (B) Enlarged view of PKP(DF) segments marked by ticks in (A). The traces have been shifted so that the onset of the DF arrival of the earlier event of each doublet is roughly aligned. The arrow marks the measured difference of BC-DF times. Our basic observations are that when signals from these high-quality waveform doublets are aligned on the BC phase, the DF phases for event pairs with time separation of less than 4 years overlap with each other rather well; but the DF phase of the later event arrives consistently earlier than that of the earlier event for doublets separated by more than 4 years, and the DF phase is seen to arrive progressively earlier as the time separation increases. The temporal change of the DF travel times is about 0.09 s per decade with standard error of 0.005 s. We also see that the waveforms of the DF coda become dissimilar when the time separation is larger than 7-10 years. The DF coda is presumably caused by scattering within a complex anisotropic heterogeneous structure. Thus, the observed breakdown of waveform similarity provides new and powerful evidence for motion of the inner core. B Zhang, J., X.D. Song, Y.C. Li, P.G. Richards, X.L. Sun, F. Waldhauser, Inner Core Differential Motion Confirmed by High-Quality Earthquake Waveform Doublets, Science, submitted, 2005. 194
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL INNER CORE The Earth’s Free Oscillations and the Differential Rotation of the Inner Core Gabi Laske, Guy Masters • IGPP, Scripps Inst. of Oceanography Differential rotation of the inner core (IC) was initially inferred by body-wave studies that reported a super-rotation rate of 0 to 3 degrees per year. The wide range of inferred rotation rate is caused by the sensitivity of such studies to local complexities in structure. The analysis of free oscillations is insensitive to local structure and is therefore a better candidate for estimating differential rotation. In principle, the analysis of the time dependence of ʻʼsplitting functionsʼʼ that display local frequency shifts for a mode caused by Earthʼs internal 3D-structure gives us clues whether structure has changed over the last few decades. Unfortunately, the data volume and quality for earthquakes 20 years ago is insufficient to construct splitting functions with high precision. We have therefore chosen a forward approach in which we optimize the hypothesis of an assumed rotation rate against the fit of older data. We construct the ʻʼrecentʼʼ splitting function using data for recent great earthquakes. We isolate the contribution from the inner core and ʻʼcorrectʼʼ the splitting function with an assumed rotation rate and determine the fit to older data. We repeat this procedure for each inner-core sensitive mode and then determine the mean inner core rotation rate. Analyzing 9 modes, our initial results indicated that IC differential rotation has essentially been zero over the last 20 years, though our error bars allowed for a small relative rotation of up to 0.3°/yr. A subsequent study with 4 additional modes and 23 additional earthquakes cut our error bars in half and our current estimate is a small super-rotation of 0.13°/yr. Since inner-core sensitive modes are also sensitive to mantle structure, we need to make sure that our result does not depend on the model we choose to correct our splitting function. It turns out that results using different models are not significantly different. However, we suspect systematic effects for certain modes that consistently give westward rotation rates. A possible process that is currently investigated is the coupling to other modes. a) Recent and past splitting functions for mode 13 S 2. The past splitting function is not accurate enough to reliably constrain inner core rotation b) Innercore rotation rates obtained for 13 inner-core sensitive modes. Grey and symbols mark results when using our own mantle model for corrections, blue symbols mark models from other workers. Our data are most consistent with a small rotation rate of 0.24°/yr or less. G. Laske, and G. Masters, The Earthʼs Free Oscillations and the Differential Rotation of the Inner Core, in: V. Dehant, et al. (eds.) ʻʼEarthʼs Core: Dynamics, Structure, Rotationʼʼ, Geodynamics Series 31, AGU, Washington, D.C, 5-21, 2003. G. Laske, and G. Masters, Limits on differential rotation of the inner core from an analysis of the Earthʼs free oscillations, Nature, 402, 66-68, 1999. 195
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