Angola-Brazil Geotraverse: Ocean-bottom seismoacoustic observations

Abstract

OBS observations were carried out during the period 1980-1986 at more than 100 sites along the Angola-Brazil Geoiraverse (12°S between l2°E and 30°W). Microearthquake activity .. was registered in the region of the Mid-Atlantic Ridge (MAR) within the area where strong earthquakes exist Standard features are similar to other MAR segments: (1) the occurrence of earthquakes at contrasting relief forms; (2) shallow hypocenter depth (2 to 7 km); (3) rapid attenuation of seismic waves under the seafloor; (4) velocity ratio Vp/Vs=l.74. Frequency-temporal variations in the seismic background and in the microseismicity of the Gabon Fault ~ere detected, presumably connected with volcanic processes. T-phase variations at distances exceeding 5000 km suggest the formation of a hydroacoustic signal near the epicenter on the continental slopes (maximum), and on smooth slopes of the continental margin. The intensity of the hydro-acoustic waves in the frequency range 3 to 20 Hz decreases with depth of recording. Residuals of PKP-waves, recorded in the Angola Basin and in Europe, suggest that the properties of the medium under these structures are similar. No lateral variations of velocity were found under a 600 x 600 km area in the Angola Basin.