RECORDING OF OCEAN-BOTTOM SEISMIC NOISE AND OF A STRONG EARTHQUAKE INTHE HIMALAYAS BY BROAD-BAND DIGITAL OBS INSTALLED ON THE MID-ATLANTICRIDGE

A broadband digital-analog ocean-bottom seismograph (OBS) with frequen cy pass band 0.003-30 Hz was installed in October 1991 on the Mid-Atla ntic Ridge in the area of the Azores, where it operated for 3 days in programmed mode. The general structure of long-period ocean bottom sei smic noise previously observed in several sea-floor experiments was co nfirmed, i.e. storm microseisms lie in the range 0.15-0.30 Hz and mini mum noise is observed in the range 0.04-0.12 Hz (the band most appropr iate for registering remote earthquakes). For still lower frequencies the level of noise increases. The level of background ocean-bottom sei smic oscillations is time dependent. Strong 'storms' of microseisms we re observed. In addition, a strong (M = 7.2) earthquake occurred in th e Himalayas on 19 October 1991 and was recorded in its entirety during one of the 2 h operational periods. The dispersive Love wave train wa s the most interesting one on the record, and dispersion curves of gro up velocity for two harmonics of the wave were derived. The dispersion curves are characteristic of a medium which is intermediate between t ransitional (from continental to oceanic) and continental models.