Mapping complexities on the top of the core-mantle boundary using entropy analysis of SKS and SPdKS waveforms
Abstract
Seismological studies reveal patches of strong shear (S)- and compressional (P)- wave velocity anomalies on the top of the core-mantle boundary (CMB) known as ultra-low velocity zones (ULVZs). However, their relation to large-scale structures and mantle dynamics is still uncertain due to an incomplete mapping of ULVZs at the CMB. In the presence of ULVZs, SKS (an S wave passing through the core as P and converted to S on the receiver side) and SPdKS (SKS wave that diffracts as P along the mantle side of the CMB) phases split to additional phases due to conversions at the ULVZ boundaries including both the top-most interface and lateral edges. Identification of these additional phases is challenging, particularly when phases overlap and wavefield interferes constructively/destructively. Here, we analyze sample entropy at various time lags in the time window containing the SKS and SPdKS waveforms, which provide an extensive sampling of the CMB at longer epicentral distances. Sample entropy quantifies waveform complexities based on the degree of similarity and randomness of data points within a given threshold. We perform feasibility of this approach through various 1-D and 2-D ULVZ synthetic waveforms with noise that are convolved with S-wave source time function computed from global seismic stations. We find that sample entropy at higher threshold performs better, and stronger ULVZs produce higher complexity as a function of time lags and epicentral distances. To evaluate the source and station noise bias, we compare sample entropy of observed waveforms from two co-located events that have different source time functions. We find similar sample entropy of waveforms relative to PREM synthetic waveforms, which are convolved with source time functions and station dependent observed noise. The strength of complexities is further quantified using linear discriminant statistics. Finally, we present geographical distribution of complexity at the CMB using a global collection of broadband data from 1990 and onwards.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI41C0013P
- 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