Out-of-plane signals from deep mantle subducted slabs at teleseismic distances: insights from three-dimensional global modelling and three-component array analysis

Global seismology mainly uses seismic waves propagating in the sagittal plane along the great circle path (GCP). However, heterogeneities in the mantle laterally deviate the path of seismic signals, which arrive out-of-plane (OOP) at arrays of sensors at teleseismic distances. Detection and back-projection of these signals have, in the past, provided independent evidence for the location of distant subducted slabs in the deep mantle, complementing global tomographic imaging. To infer physical properties of these subducted slabs, 3-D waveform modelling of OOP waves for a finite-thickness slab is needed but still missing. In this study, we conduct a series of synthetic tests using a spectral element solver. We test the detectability of OOP signals and, by progressively adding complexities, we evaluate to which extent these signals can be used to infer physical properties of the modeled slab. We carry out three-component array analysis and investigate focal mechanism dependency. Our results show that the transverse component might be the best candidate for such studies, also for P-to-P OOP signals. Vertical and radial component recordings are usually dominated by P-SV energy arriving from the earthquakes along the GCP, which masks possible OOP signals. Contrary, the transverse component filters out any P-SV energy arriving directly from the source and, owing to its intrinsic directionality, allows for higher resolution measurement of P-to-P OOP signals. This is especially the case prior to the arrival of the S-wavefield. We pick a series of OOP arrivals which are back-projected using a multiphase trial-and-error approach, that is considered successful only when different OOP seismic phases converge to the modelled (true) structure. We retrieve the location of the slab, its bottom and top edges and its thickness in the lower mantle. These inferences are tested against varying topography, orientation and size of the modelled slab. The insights gained with modelling are confirmed with real data examples, supporting higher resolution mapping of 3-D mantle structure based on OOP seismology.