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CORE–MANTLE BOUNDARY 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Lateral Variation of the D” Discontinuity Beneath the COCOS Plate Thorne Lay • University of California, Santa Cruz Edward J. Garnero • Arizona State University S. A. Russell • Weston Geophysical Broadband shear-wave signals from 15 deep South American earthquakes reveal small-scale variations in the D” shear-velocity discontinuity beneath the Cocos Plate. The data, from GSN, BDSN and TRInet stations in the western U.S., provide imaging for a preliminary stage of the USArray deployment. This dense distribution of stations is used for double-array stacking procedures, using suites of observations from different sources and receivers that sample common mid-points in spatially localized bins. The data sampling each of 4 bins provide coherent stacks of energy turning at depths above the core-mantle boundary, which is associated with ScS reflections observed in all of the data. When the data are processed using a uniform reference model for stacking, reflections of varying strength are detected at apparent depths that vary by over 100 km. Models that match the amplitudes require lateral variations in the structure above or below the reflector. If all the variations are mapped into the D” layer, satisfactory models are found for a discontinuity at uniform depth of 264 km above the CMB, with laterally varying velocities of 0.9% to 2.6% within D”. This set of models matches all of the data except in the southernmost region, where differential times suggest a somewhat deeper discontinuity (by 50 to 100 km), with some velocity decrease above the D” region. Map indicating the raypath configuration and fine scale bins used for double-array stacking of the recordings in the western U.S. for South American earthquakes. The average velocity anomaly for each bin inferred from ScS travel time anomalies is shown. Double array stacks of energy relative to ScS (imaged at the coremantle boundary) as a function of depth relative to the CMB, for each of the 4 bins (dark solid lines, with bootstrap variances in dotted curves). The number of traces contributing to the stack at each depth is shown in dashed lines. The red lines are stacks of synthetics for a uniform discontinuity depth 264 km above the CMB, with laterally varying velocity in the D” layer. Lay., T., Edward J. Garnero, S. A. Russell, Lateral variation of the D” discontinuity beneath the Cocos plate, Geophys. Res. Lett., 31, L15612, doi:10.1029/ 2004GL020300, 2004. Supported by NSF grants EAR-0125595 and EAR-0135119. 168
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL CORE–MANTLE BOUNDARY Migration of the Lowermost Mantle Structure Under the COCOS Christine Thomas • University of Liverpool, United Kingdom Edward J. Garnero • Arizona State University Thorne Lay • University of California, Santa Cruz A simplified point-scattering migration method is applied to shear-wave observations from GSN, BDSN, and TRInet stations for deep South American earthquakes. The S waves traverse the lowermost mantle beneath the Cocos Plate. Migration allows image development with few a priori assumptions about the structure, in contrast to forward modeling waveforms. In this preliminary application to deep mantle structure, we migrate subsets of data from individual events that sample very localized areas. This allows a detailed mapping of apparent reflector depth across the localized study area to be obtained. It should be recognized that use of a homogeneous background Maps indicating the depth of a reflector at the top of D” giving rise to positive (left) and negative (right) arrivals in the waveforms. The migration procedure used seeks very fine-scale resolution of the reflector depth, under the assumption of a homogeneous background model. The apparent topography of the boundary will tradeoff with volumetric velocity heterogeneity within the D” layer. The positive reflector appears to deepen by about 100 km on average in the southern portion of the study area. The northern portion of the study area has evidence for a negative reflector about 100 km below the positive reflector. model and very localized data sampling can lead to artificial topography in the image, but significant detail may also be revealed. The image of the D” reflector that is recovered indicates the presence of a southward increase in depth, rather abruptly at around 5°N. There is clear evidence for an additional reversed-polarity arrival in the northern region, but the migration used here cannot resolve robustly where this energy comes from. It may be from a deeper velocity decrease or, as appears more likely, from out-of-plane scattering. A smoothed 3D image of the two reflectors imaged in this study. The images are formed from piecing together local reflector images from stacks of small numbers of observations, so some of the undulations may be exaggerated. However, the main features of southward deepening of the shallower reflector at the top of D” and the presence of some form of scatter of negative arrivals in the northern region are resolved. Thomas, C., Edward J. Garnero, and T. Lay, High-resolution imaging of lowermost mantle structure under the Cocos Plate, J. Geophys. Res., 109, B08307, doi:10.1029/2004JB003013, 2004. Supported by NSF grants EAR-0125595 and EAR-0135119. 169
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2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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