Abstract

Ocean-bottom seismographs were deployed twice in the FAMOUS area in 1973. Few earthquakes were observed on the first occasion, but 515 were recorded on the second with magnitudes ranging from 0 to 2.5. These occurred both in the median valley and in the fracture zone adjacent to the instruments. The latter events have been interpreted as activity along a single Riedel fracture cutting obliquely across the trend of the fracture zone. Only a crude velocity structure could be determined on the basis of the earthquake observations. A superficial surface layer with a compressional velocity of 2.7 km/s overlies a main layer with a P velocity of 5.0 km/s and a S velocity of 2.4 km/s. The low-velocity surface layer and the high ratio of P to S velocities for the main layer (2.08) reflect the highly fissured, water-saturated nature of the crust. The earthquake foci are thought to lie in the low-velocity surface layer. Finally, in spite of numerous small earthquakes, an acoustical analysis of the hydrophone data indicates that distant shipping traffic is the dominant source of ambient noise on this part of the Mid-Atlantic Ridge in the frequency band 5 to 32 Hz.

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