A global search for ultra-low velocity structure at the CMB using SPdKS
Abstract
The phase SPdKS has been used to globally probe the CMB for ultra-low velocity boundary layer structure. In this study, we use a global set of broadband data from the IRIS/GSN, USNSN, and CNSN networks, and PASSCAL experiments to examine SPdKS anomalies. 51 earthquakes Mw >= 5.8 are used, which were chosen based on simple and impulsive waveforms, and good signal to noise ratio. Excellent data coverage is obtained in the SW Pacific and circum-Pacific regions and good coverage is obtained underneath the central Atlantic and southern Africa region. Many other regions are also sampled, but sparsely. The wave shape and timing of anomalous SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening compared to SKS. The cleanest SKS data at distances 90-102 deg are stacked to construct an empirical source wave shape for each event. Reflectivity seismograms are then convolved with trapezoidal source-time functions that best reproduce the SKS stack pulse. Anomalous SPdKS data are modeled with 3 classes of models: (1) mantle-side ultra-low velocity zones (UVLZ), (2) core-side rigidity zones (CRZ), and (3) core mantle transition zones (CMTZ). For ULVZ structures, P and S velocity reductions with dVs:dVp ratios of 1:1 and 3:1 are explored, where 3:1 is appropriate for the partial melt scenario. A systematic model space grid search is conducted, whereby synthetics for a wide range of ULVZ, CRZ, and CMTZ models are cross-correlated with data to quantify goodness of fit for all models. Given the tradeoffs present between ULVZ, CRZ, and CMTZ models, acceptable model space maps are constructed for the highest quality data. The most striking anomalies are found in localized patches with lateral scale lengths on the order of 100 km, such as in the SW Pacific. These results are compared to aspherical P and S-wave velocity structure as well as to the distribution of strong lateral gradients, which have been shown to strongly correlate with hot spots. Our detailed analyses of SPdKS anomalies support the notion of strong variations at the CMB at scales an order of magnitude shorter than resolved in current tomographic inversions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.S12D..06T
- Keywords:
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- 7200 SEISMOLOGY;
- 7207 Core and mantle