Seismicity of Mid-oceanic Ridges and its relation to Properties of the Upper Mantle and Crust

Abstract

EARTHQUAKES occurring along the median rift of the Mid-Atlantic ridge have a significantly steeper body wave magnitude–frequency relation than those occurring along the fracture zones which offset the ridge¹, according to a survey of earthquakes reported by the US Coast and Geodetic Survey for the period 1963 to May 1967. Surface wave magnitudes of the earthquakes used in the body wave analysis have now been measured. The amplitudes of 20 s period Rayleigh waves were measured from copies of the vertical-component long-period records of ten World Wide Standard Stations situated around the Atlantic Ocean. The magnitudes obtained are essentially Gutenberg M_s. No corrections were made for depth of focus but, as all mid-oceanic ridge earthquakes seem to have shallow foci, this does not introduce much error. M_s could be ascribed to sixty-eight of the seventy-five fracture zone earthquakes for which m_b was available, and to sixty-four of the seventy-nine rift zone events. Cumulative numbers of earthquakes for the two groups are plotted against m_b in Fig. 1, and against M_s in Fig. 2. The slope of the rift zone group is steeper in both cases. The significance of this difference has been estimated in each case by drawing separate magnitude–frequency plots for each fracture zone and rift area (where enough earthquakes have been observed for this to be done). Slopes obtained in this manner are given in Table 1. The t distribution at 5 degrees of freedom shows that the difference in the mean slopes for the body wave magnitude is significant at just over 80 per cent, but evidence adduced in the earlier paper from the remainder of the Mid-Atlantic ridge supported the conclusion that the slopes were indeed different. In the case of surface wave magnitudes the four fracture zones give an average gradient b = 0.63 ± 0.14 (standard deviation) and the three rift areas give b = 1.08 ± 0.18 (standard deviation). The t distribution shows these slopes to be different with more than 95 per cent significance.