Microearthquake characteristics and crustal VP/VS structure at the Mid-Atlantic Ridge, 35°N

We report on the results of a microearthquake experiment conducted at the along-axis bathymetric high of the slow spreading ridge segment near 35 degreesN on the Mid-Atlantic Ridge (MAR). Of a total of 255 microearthquakes recorded during the 43-day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a similar to 12-km-long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3-4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged V-P/V-S ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2-km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which microearthquake experiments have been carried out. Cross-axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR microearthquake experiments, there appears to be a correlation between cross-axis relief and maximum depth of seismicity. From the correlation of cross-axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross-axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.