Microearthquakes beneath Median Valley of Mid‐Atlantic Ridge near 23°N: Tomography and tectonics

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Abstract
Data from a microearthquake experiment in the median valley of the Mid‐Atlantic Ridge near 23°N in 1982 are used to measure earthquake source parameters, to determine the laterally heterogeneous seismic velocity structure across the inner floor, and to develop a kinematic tectonic model for this portion of the median valley. Fifty‐three microearthquakes occurred over a 10‐day period beneath the median valley inner floor and eastern rift mountains. Twenty of 23 well‐located inner floor epicenters define a line of activity, about 17 km long, having a strike of N25°E and located near an along‐axis depression some 300–400 m deeper than surrounding regions. Earthquakes with well‐resolved hypocenters generally have focal depths of 4–8 km beneath the seafloor of both the inner floor and the rift mountains; the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure. Composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45°. Normal faulting mechanisms, although poorly constrained, are also indicated for the rift mountain microearthquakes. Seismic moments, approximate fault dimensions, and average stress drops for the largest events recorded are 1019–1020 dyn cm, 200–400 m, and 1–70 bars, respectively. A twodimensional tomographic inversion of P wave travel time residuals from microearthquakes and local shots indicates a well‐resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor. P wave velocities at 1–5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions. The most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column. By this view, the seismic velocity structure at the ridge axis evolves, probably by the sealing of cracks and pores, within the first few hundred thousand years of crustal accretion. Consideration of the detailed Sea Beam bathymetry in this region of the inner floor, the characteristics of large earthquakes that the region has experienced during the past 25 years, and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years.