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INNER CORE 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Waveform Search for the Innermost Inner Core Vernon F. Cormier, Anastasia Stroujkova • University of Connecticut Waveforms of the PKIKP seismic phase in the distance range 150° to 180° are analyzed for evidence of an inner-most inner core of the type proposed by Ishii and Dziewonski having an abrupt change in elastic anisotropy near radius 300 km. Seismograms synthesized in models having a discontinuity at 300 km radius in the inner core exhibit focused diffractions around the innermost sphere at antipodal range that are inconsistent with observed PKIKP waveforms. Successful models have either a transition in elastic properties spread over a depth interval greater than 100 km or an innermost sphere that exceeds 450 km radius. Evidence of a sharp discontinuity in the lower to mid-inner core is sparse in existing global seismic data. Some examples, however, can be found of PKIKP complexity near 161°, consistent with a triplication created by a 475 km radius discontinuity. An abrupt change in either viscoelastic or scattering attenuation at this radius is also observed in PKIKP waveforms, suggesting the existence of an innermost sphere with low, regionally uniform, seismic attenuation. In contrast to the relatively uniform inner-most inner core, a 0 to 100 km thick region at the top of the inner core exhibits strong lateral variations in attenuation and velocity structure, suggesting lateral variations in the processes of solidification, flow and re-crystallization at the inner core/outer core boundary. Analogous to the evidence for an abrupt fabric change in the uppermost inner core, the seismic evidence for an innermost inner core may represent another fabric change near 700 km depth from the inner core/outer core boundary. This last and deepest change may simply signify the end stage of solidification, flow and re-crystallization, resulting in the highest ordering and largest grain sizes of intrinsically anisotropic crystals. Waveform profiles stacked in 0.5 distance intervals for different orientations of PKIKP rays with respect to the rotational axis ( polar cosξ < 0.3, intermediate 0.3 < cosξ < 0.75 (middle panel), and equatorial cosξ > 0.75 ). Seismograms synthesized in inner core models having a +2% P-velocity discontinuity at radii 300 km (left), 400 km (middle), and 500 km (right). Cormier, V.F., and A. Stroujkova, Waveform search for the innermost inner core, Earth Planet. Sci. Lett., submitted. 182
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL INNER CORE Near-Podal Observations of PKPPKP Waves and Implications for Central Inner Core Structure Hrvoje Tkalcic • Lawrence Livermore National Laboratory The amplitude and radial dependence of hypothesized inner core anisotropy are not well known. Apart from previously observed complexity in geometry, some recent results suggest changes in anisotropic properties in the regions of the inner core close to the planetary center. With the spatial distribution of large earthquakes and current configuration of seismographic stations worldwide, it is difficult to achieve sampling of the deep interior of the inner core, except for paths nearly parallel to the equatorial plane. Figure 1 demonstrates this property. Even if a large earthquake occurs at extreme latitudes, say at -50°s, although possible to find antipodally positioned locations on the globe, such geometries could not produce angles between the PKP leg and the Earthʼs rotation axis (ksi) smaller than 35 degrees. This makes interpretation of anisotropic properties near the planetʼs center, at a minimum, very challenging. PKPPKP and even more exotic seismic waves are used to examine inner core structure. Breger et al. (2000) reported that the travel times of PKPPKP waves recorded at NORSAR network with polar geometries sampling the top 400 km of the inner core were not anomalously advanced. The near-podal PKPPKPdf waves show much promise for studying inner core structure. Figure 2 illustrates Figure 1. Theoretical space of minimum angle between PKPdf leg in the inner core and the Earth’s rotation axis, that can be achieved for antipodal (epicentral distance larger than 170 degrees) PKP paths. Figure 2. Vertical components of the Kyrgyz Seismic Telemetry Network records bandpassed between 0.2 and 1.0 Hz for an earthquake near the Afganistan-Tajikistan border. PKPPKPdf waves arriving at a steep incidence angle are visible at all records. an example of one near-podal PKPPKPdf observation, recorded by the Kyrgyz Seismic Telemetry Network for an Afganistan- Tajikistan border earthquake. Preliminary results from analyzing near-podal PKPPKPdf travel times suggest that the central part of the inner core is not fast in the direction parallel to Earthʼs axis, which is in contradiction with presently hypothesized anisotropy models. Painstakingly collected data points from near-podal PKPPKPdf travel times will thus have very important implications for our understanding of the anisotropy and other properties of the deep inner core. Breger, L., B. Romanowicz and S. Rousset, New constraints on the structure of the inner core from PʼPʼ, Geophys. Res. Lett., 27, 2781-2784, 2000. 183
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2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
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