Hendrik Jan van Heijst

New seismic model of the upper mantle beneath Africa

We present a seismic model of the upper 400 km of the mantle beneath Africa and surrounding regions. This model is constructed by inverse modeling of fundamental mode Rayleigh wave phase velocities (40–200 s) for about 8000 propagation paths. Among the most pronounced anomalies are high shear velocity structures (as much as 6% higher than in the Preliminary Reference Earth model) beneath the West African, Congo, and Kalahari cratons that extend to about 250 km depth. These structures have near-vertical margins across which the shear velocity changes by as much as 6% over 500 km distance. Anomalous low shear velocities (3%–4% lower than in the Preliminary Reference Earth Model) structures are observed beneath the East African, Red Sea, and Gulf of Aden rifts, and beneath the northwestern Indian Ocean. These structures extend to a depth of at least 250 km. Our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere, nor does our seismic model indicate that high-temperature, low-density material beneath the lithosphere is responsible for the uplift of southern Africa.

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.

Splitting function measurements for Earth's longest period normal modes using recent large earthquakes

Recent megathrust earthquakes, such as the 23 June 2001 Peru event, the Sumatra events of 2004 and 2005 and the 27 February 2010 Chile event, have given us the opportunity to measure splitting of the longest period normal modes. We use wave spectra to make robust measurements for modes 0S2, 0S3, 0S4, 2S1 and 1S2. Singlet frequencies of these modes have been measured previously using gravimeters, but here we use seismic records to observe splitting functions for 0S2 and 2S1 for the first time. Cross-coupling with nearby modes is included to account for ellipticity and rotation of the Earth and results in significantly improved splitting function measurements for 0S3, 0S4 and 1S2 compared with previous studies. The new splitting function measurements can easily be implemented in future tomographic modelling of aspherical velocity and, particularly, density structure.