Migration of the Lowermost Mantle Structure Under the COCOS - fig. 2
Migration of the Lowermost Mantle Structure Under the COCOS - fig. 2
Credit:
Christine Thomas • University of Liverpool, United Kingdom; Edward J. Garnero • Arizona State University; Thorne Lay • University of California, Santa Cruz/IRIS
Description
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.
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 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.
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.
Photographer / Contributor: Christine Thomas • University of Liverpool, United Kingdom; Edward J. Garnero • Arizona State University; Thorne Lay • University of California, Santa Cruz
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