High-frequency full-wavefield assessment of ultra-low velocity zone scattering
Abstract
The ultra-low velocity zones (ULVZs) are small-scale heterogeneities detected on the Earth's core-mantle boundary (CMB) with a strong velocity reduction. To constrain the location, geometry and properties of a ULVZ, we require a profound understanding of its scattering effects on the high-frequency body wave phases. We conduct a full-wavefield study on the SKS-SPdKS phases at a 1 Hz frequency, considering three different types of ULVZs: 1D, 2D and 3D. We use the spectral-element code AxiSEM3D to conduct the forward modelling. A 1D ULVZ is a slow global layer above the CMB. The top of the ULVZ forms a new discontinuity, separating the original SKS energy into three parts: SuPKPuS, SuPKSuS+SuSKPuS and SuSKSuS, where "u" denotes refraction at the top. Such phase separation becomes observable only at a high frequency. The SuSKSuS phase may be accompanied by two SPdKS-like phases, diffracted respectively along the top of the ULVZ and the CMB, so are SuPKPuS and SuPKSuS+SuSKPuS. To investigate the effects of path reciprocity, we also truncate the ULVZ on either the source or the receiver side. Next, we study the scattering effects of a 2D ULVZ, i.e., a local structure axisymmetric round the event. A 2D ULVZ has two horizontal discontinuities: the event-side and receiver-side edges. These two edges have two significant effects. First, they limit the distance range for the observation of the SKS separation. Second, they create additional KS phases as if they were two point-sources on the CMB. These additional phases have a move-out different from both SKS and SPdKS and are sensitive to the ULVZ location. Besides, we evaluate the shape and boundary sharpness of a ULVZ. Finally, we examine the SKS-SPdKS waves scattered by a 3D ULVZ. At a frequency as high as 1 Hz, the computation can be prohibitively expensive even using AxiSEM3D (which is faster than a conventional 3D method). To achieve such a high frequency, we introduce AxiSEM3D for wavefield injection, which enables the decomposition into the incident and the scattered wavefields while capturing full multiple scattering effects. Utilising their different axes of symmetry, AxiSEM3D can solve these two wavefields separately and efficiently. Our results reveal strong off-plane effects due to the localisation of the ULVZ, indicating the insufficiency of the 2D in-plane modelling.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI41C0010N
- Keywords:
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- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3621 Mantle processes;
- MINERALOGY AND PETROLOGY;
- 8124 Earth's interior: composition and state;
- TECTONOPHYSICS;
- 8125 Evolution of the Earth;
- TECTONOPHYSICS