Microearthquake characteristics and crustal V-P/V-S structure at the Mid-Atlantic Ridge, 35 degrees N

We report on the results of a microearthquake experiment conducted at the a long-axis bathymetric high of the slow spreading ridge segment near 35 degr eesN on the Mid-Atlantic Ridge (MAR). Of a total of 255 microearthquakes re corded during the 43-day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransf orm 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 s teep 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 segme nt center earthquakes are consistent with normal faulting on faults approxi mately 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 contri bution 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 reli ef is also anomalously low for this segment's center, and on the basis of t his and other MAR microearthquake experiments, there appears to be a correl ation between cross-axis relief and maximum depth of seismicity. From the c orrelation 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.