Identification of crustal and upper mantle heterogeneity by modelling of controlled-source seismic data

Abstract

High-frequency controlled-source seismic sections with dense spatial sampling show the existence of heterogeneity at different depth levels of the continental crust and upper mantle. Our sources of information are the Peaceful Nuclear Explosion (PNE) seismic data sets recorded to large offsets in the former Soviet Union supplemented by recordings from the North American Early Rise deep seismic experiment and normal-incidence reflection seismic sections collected in northwest Europe. Heterogeneity in the crust and upper mantle can be uniquely identified in reversed high-frequency (2–10 Hz) PNE seismic sections collected with dense spatial sampling (nominal receiver spacing of 10–15 km) out to 4000 km offset. We document pronounced seismic scattering from three heterogeneous zones: The lower crust from ∼20 km to ∼40 km depth, an ∼80 km thick low-velocity zone below ∼100 km depth, and the ∼320–460 km depth interval around the top of the mantle transition zone. We calculate the full seismic wavefield in heterogeneous crust–mantle models with a two-dimensional finite-difference algorithm. We represent the heterogeneous layers by random fluctuations of the elastic parameters and Q-values. The spatial (horizontal and vertical) correlation lengths and the standard deviation of the scattering media are constrained by comparison of observed and calculated seismic sections. The lower crustal heterogeneity causes a coda to the upper mantle arrivals at all recorded frequencies. This coda is a prominent feature for whispering-gallery phases (teleseismic Pn), which travel as multiply reflected refractions below the Moho to more than 3000 km offset from the PNE sources. The heterogeneous mantle low-velocity zone causes a scattered coda trailing the first arrivals in the ∼800–1400 km offset range. The best fit to the observations along profile Kraton in Siberia is obtained by an 80 km thick heterogeneous low-velocity zone below 100 km depth, represented by fluctuations described by a von Karman distribution function with a Hurst number of 0.5 and spatial correlation lengths of 5–10 km (horizontally) and 3–5 km (vertically). Scattered arrivals, which trail the reflection from the ‘410’ discontinuity and a reflection from a shallower depth of ∼320 km, constrain heterogeneity around the top of the transition zone. This heterogeneity is modelled by fluctuations in the 320–460 km depth interval with correlation lengths on the order of 20–40 km by 5–10 km. Scattering from the two shallower heterogeneous zones improves the description of the phases from around the transition zone.