Microearthquakes beneath the Median Valley of the Mid‐Atlantic Ridge near 23°N: Hypocenters and focal mechanisms

We report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the Mid‐Atlantic Ridge near 23°N during a 3 week period in early 1982. The network consisted of seven ocean‐bottom hydrophones and three three‐component ocean bottom seismometers. The instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence. The hypocentral parameters of the 26 largest microearthquakes are reported; 18 of these events have epicenters arid focal depths which are resolvable to within ±1 km formal error at the 95% confidence level. Microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor. Composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30° or more; these solutions are similar to the mechanisms of nearby large earthquakes. Microearthquakes also occur beneath the steep eastern inner rift mountains. The rift mountain earthquakes have nominal focal depths of 5–7 km and epicenters as distant as 10–15 km from the center of the median valley, but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure. We interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7–8 km. We infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment.