Références: Testud et al, 2001, J. Appl. Meteor., 40, 1118-1140

STATION GEOPHYSIQUE DE LAMTO

Development of seismic velocity model of West Africa using the IMS data and IDC product

Ouattara Y. (1), Maggi A. (2)

(1) Université Félix Houphouët Boigny, 22 B.P. 582 Abidjan 22 (Côte d'Ivoire), (2) Ecole et Observatoire des Sciences de la Terre, 5 rue Rene Descartes F-67084 Strasbourg Cedex (France)Abstract:

To predict and mitigate the effects of future earthquakes in the West Africa, more information is needed regarding local earthquake sources and how seismic waves travel through the region. A velocity model describes shear-wave properties in the Earth, which are useful to characterize seismic hazard. The velocity model is also important for earthquake location and discrimination between natural and artificial events. Elastic deformation of ground motion does not only propagate body waves. Much of the energy generated by seismic source propagates as surface waves. The aim of surface wave inversion is to provide 3D distribution of the S wave velocity structure. In West Africa, the density of seismic stations is extremely weak, and there are few seismological investigations. We have measured group velocity dispersion of surface waves on over 400 paths crossing West Africa. We have produced preliminary group velocity anomaly maps that seem consistent with regional geological structures.

CTBT Science and Technology 2015 Conference

1- Introduction West Africa is composed of many different geological structures (shield, basin, mobile zone belt…). Detailed information on the crust and upper mantle structure would make it easier to obtain a clear picture of the tectonic evolution of the region. This study aims to determine the seismic structure of West Africa using surface wave tomography. We use this method because of the low number of local and regional seismic events and also the uneven distribution of seismic stations in the region. We measure the dispersion characteristics of surface waves as a first step towards estimating velocity structure.

2 - Data and methodology

3 - Preliminary results

4 – Conclusion At this stage of our study, we can say that our preliminary results (group velocity anomaly map) seem to match the geological structure of the region, although in some areas the azimuthal coverage is not complete. We observe a differentiation of velocity anomalies between shields, basins and mobile belts. After the step of regionalization, we will perform synthetic tests and then attempt an inversion in depth to have a velocity model of West Africa.

The records were corrected for instrument effects. Group velocities were then isolated by applying the Multiple Filter Technique (MFT) .

We analyze seismic events from 1995 to 2015 with magnitude ≥ 5.We use vertical component seismograms to estimate the Rayleigh wave group velocities on the fundamental modes for the period range 4 - 250 seconds.

From the dispersion curves of the group velocities we perform a first inversion process (regionalization) to generate group velocity maps at different periods (20s, 50s, 100s and 150s) .

Station Latitude Longitude AltitudeASCN -7.932700° -14.360100° 173 mCMLA 37.763700 ° -25.524300 ° 429 mDBIC 6.670160° -4.856560° 125 m

KOWA 14.496700° -4.014000° 321 mMBO 14.392024° -16.955467° 3 m

MSKU -1.655700° 13.611600° 312 mSACV 14.970200° -23.608500° 387 mSHEL -15.958800 -5.745700 537 mTAM 22.791490° 5.528380° 1410 m

Group velocity dispersion and spectral amplitude Dispersion curves

Path coverage Coverage at 50.0 s

Group velocity anomaly at T=50.0 s

West Africa geology

T1.2-P6

Study area showing earthquakes