Lower Mantle Seismic structures investigated with unusual seismic phases

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Understanding structures in the Earth lower mantle provides information about mineralogy and dynamics within the mantle. Especially the lowermost mantle has been shown to contain a number of different structures on several length scales including reflectors at different depth and large regions of reduced velocity (LLSVPs). While it is possible to test many regions in the Earth, a better coverage is often limited by i) the available source-receiver combinations and ii) distance ranges needed to study a particular seismic wave and hence a particular region in the Earth's mantle. In this study, we aim to use unusual seismic arrivals in the wavefield, understand and validate their origin, and eventually use them to study the deep mantle in the Northern and Western Pacific and Atlantic in order to enhance our coverage of the lower mantle structures in the target areas. This proposal is therefore partly exploratory, since we want to test new seismic arrivals that we have observed in previous datasets but we do not understand the origin of these arrivals yet, although first indications suggest that they originate in the lower mantle. The two target regions are places near the edges of the LLSVPs and understanding structures in these areas will provide further knowledge on mantle dynamics and mineralogy as well as connections between lower mantle reflectors and LLSVPs. We will also use standard seismic waves to map reflectors in the lower mantel and extract their properties also for validation of the origin of the unusual seismic waves. An important step is the comparison of the results in the two target regions, i.e. the Northern and Western Pacific with the Atlantic, since this will help to understand i) whether the imaged structures are of global origin or more local phenomena and ii) whether there are differences between the African and the Pacific LLSVP. Only through an integration of all wave information, including amplitudes, polarities, waveforms, arrival times and frequency dependence and subsequent modelling results can we achieve to gain detailed knowledge on seismic structures. Previous studies often use only one type of method or one type of seismic wave to decipher the origin of LLSVPs but here we aim for a much more complete view of the structure by using combinations of standard and unusual seismic phase observations.