Effects of trench-zone scattering on receiver functions over a subduction zone: A 3-D finite-difference modelling study

Abstract

As Morozov [Morozov, I. B. (2004). Crustal scattering and some artefacts in receiver function images. Bull. Seismol. Soc. Am., 94 (4), 1492–1499.] suggested, for a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the strong horizontal velocity heterogeneity beneath the trench zone itself produces subhorizontally-propagating waves that should be observed as coherent dipping events in receiver functions (RF). Due to similar RF delay times and moveouts, these events could be difficult to distinguish from backscattered P- and S-wave modes. To further verify this suggestion, we performed a full-waveform, 3-D visco-elastic finite-difference modelling of teleseismic wave propagation within a simplified model of a subduction zone. The synthetics show strong scattering from the area beneath the trench, dominated by the mantle and crustal P-waves propagating at 6.2–8.1 km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered converted modes, and also have similar amplitudes. Although their amplitude decay characters are different, the uncertainty in the knowledge of the velocity and density structure of the subduction zone could make distinguishing between these modes difficult. However, under minimal assumptions, recent observations of receiver function amplitudes decreasing away from the trench support the interpretation of (sub-) trench-zone scattering. Although still limited in its representation of crustal heterogeneity, 3-D modelling suggests that scattering from near-Moho crustal structures plays a key role in the formation of teleseismic wavefields. Recognition of scattered noise in teleseismic records could help to constrain major crustal structures, particularly those with strong horizontal velocity contrasts at near-Moho depths, such as crustal sutures, subduction fault zones, and mountain roots. Matching of the observed arrivals with wavefield synthetics could help constrain the locations and parameters of such structures and also help substantiate the interpretations.