[S-SS07] Crustal Structure

[SSS07-04] Revealing structures in the shallow crust beneath Botswana, southern Africa: considering dip and anisotropy

*Admore Phindani Mpuang^1,2, Takuo Shibutani² (1.Graduate School of Science, Kyoto University, 2.Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University)

In receiver function analysis, velocity models developed under the assumption of horizontal isotropic interfaces suffer inaccuracies in the presence of dipping and anisotropic structures. In such cases, the receiver functions exhibit back-azimuthal variation of amplitude and delay time. Such features of receiver functions have been observed in records from stations located in the Okavango rift zone and Limpopo belt in Botswana, with evidence for strong P-to-S conversions from high Vs contrasts in the shallow crust. In this study, the presence and nature of seismic structures at shallow crustal depths beneath Botswana is investigated in order to improve the current understanding of the seismic structure beneath the various tectonic regions in southern Africa. As a first step, 3Hz P-to-S receiver functions are computed for teleseismic events from epicentral distances between 30 and 85 degrees recorded at 37 stations located within Botswana, with magnitudes Mw>5.8. After carrying out a quality assessment, receiver functions with good quality are stacked in 30-degree back azimuth bins. These receiver function stacks are then subjected to harmonic decomposition (Bianchi et al., 2010) to constrain the presence, or lack thereof, of dipping and anisotropic structures in the shallow crust. The resulting harmonic stacks suggest that at shallow depths in the Limpopo belt, the metasedimentary sequences lie unconformably over uneven crystalline metamorphic basement surfaces, while plunging anisotropy may be present in the metamorphosed clastic sediments and basalts of the Kheis belt. In the Okavango rift zone, harmonic stacks indicate presence of very shallow strong heterogeneity in the overlying unconsolidated sediments. Applying a genetic algorithm inversion technique is expected to give optimum estimates for Vs, dip and anisotropy magnitude and orientation of the structures.