Mode of the strain release along the Gibbs fracture zone, Mid-Atlantic ridge

Abstract

The Gibbs fracture zone (52°N, 35°W) is one of the major transform faults in the Atlantic. A large earthquake (m_b = 5.8, M_s = 6.9) occurred on October 16, 1974 on this fracture zone. This earthquake was preceded by similar large earthquakes along the fracture zone in 1967 (m_b = 5.5, M_s = 6.5), 1954 (M_s = 6.5), 1941 (M_s = 6.25) a 1923 (M_s = 6.5). Detailed analyses of seismic body waves and surface waves over the period range from several seconds to 200 sec are made for the 1967 and 1974 events. The major conclusions are: (1)|In addition to the obvious disparity in the m_b vs. M_s relation, the excitation of long-period waves by these earthquakes is also anomalously large. The seismic moments determined by using the long-period (40–200 sec) G_1 and R_1 waves are 3.4 · 10^(26) and 4.5 · 10^(26) dyne · cm for the 1967 and 1974 events, respectively. (2)|Wave form analyses of body waves suggest a fault length of 60 km for the 1967 event and 70 km for the 1974 event. The point dislocation particle velocity of about 20 cm/sec obtained for both events is about an order of magnitude smaller than for other earthquakes of comparable magnitude. The large fault length and the slow particle velocity explain the disparity between m_b and M_s, and Ms and seismic moment. (3)|The amount of dislocation (about 170 cm) together with the fault length and the repeat time (13 years on the average) suggest a seismic slip rate of about 2.6 cm/year along the Gibbs fracture zone — a rate which is comparable to that for sea-floor spreading in the northern Atlantic. This agreement suggests that the slip along the fracture zone is primarily seismic at least to a depth of 10 km, the plates moving in jerks once every 13 years on the average, rather than in continuous creep. (4)|Seismic activity along the ridge crest was very high during the 5-year period before the 1974 event, while activity along the fracture zone was very high during the 3-year period prior to the 1967 event. The seismic activity became very low after the 1967 event. This pattern may be interpreted in terms of an intermittent upwelling at the ridge which subsequently triggered, either directly or indirectly, the fracture zone events. The sudden release of stress in these events may lead to an initiation of the next episode of upwelling and eventually renewed seismic activity along the ridge—fracture zone system.