Microearthquake Characteristics of a Mid‐Ocean Ridge along‐axis high

Abstract

We report results from a 3‐week microearthquake survey of the segment of the Mid‐Atlantic Ridge axis near 26°N. The segment is centered on an along‐axis median valley bathymetric high that includes the site of the TAG hydrothermal field. The seismic network, consisting of seven ocean bottom hydrophones and two ocean bottom seismometers, spanned the median valley inner floor and eastern valley wall. Hypocenters were determined for 189 earthquakes, with good resolution of focal depth obtained for 105 events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor, with earthquakes occurring at shallower depths (less than 4 km) beneath the along‐axis high. No events were detected in the immediate vicinity of the hydrothermal field. The along‐axis high is the site of a midcrustal low‐velocity zone, significant attenuation of P wave energy, and high b values; the low‐velocity volume extends about 10 km south of the high to the vicinity of volcano within the axial neovolcanic zone. Fault plane solutions indicate high‐angle (or very low angle) normal faulting beneath the along‐axis high and the base of the adjacent western wall, reverse faulting beneath the axial volcano, and a more conventional normal‐faulting geometry for earthquakes beneath the eastern wall. The distribution of seismicity and the diversity of faulting styles suggest a spatially variable tectonic state for the ridge segment at 26°N. These variations are likely a signature of along‐axis differences in thermal structure and state of stress. We suggest that the low‐velocity volume beneath the along‐axis high is the site of a relatively recent crustal injection of magma. Continued cooling of the now largely solid but still hot intrusion, and associated thermal stress and fracturing in the immediately surrounding crust, can account generally for the distribution of areas of most intense earthquake activity, the diversity of observed faulting mechanisms, and the presence of the high‐temperature vent field. These results are supportive of the spreading cell model for segmentation of magmatism and thermal structure along a slowly spreading ridge.