Microearthquake characteristics and crustal velocity structure at 29 degrees N on the Mid-Atlantic Ridge: The architecture of a slow spreading segment

We report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the Mid-Atlantic Ridge segment at 29 degrees N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two-dimensional (2-D) and three-dimensional (3-D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetric high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The microearthquakes located along axis during the 41-day recording period cluster in three separate along-axis regions: (1) the southern segment end near 28 degrees 55'N, (2) the central along-axis topographic high at 29 degrees 11'N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29 degrees 02'N. The greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset. This pattern of off-axis microearthquake activity, and the cross-axis asymmetry in crustal thickness at the segment end, support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for microearthquakes beneath the along-axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center, in contrast, may be the result of volcanic and hydrothermal processes, such as magma movement or thermal stresses generated near cooling plutons. A comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature, whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust. Along axis, well-resolved focal depths determined with a 3-D velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center. These observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state.