Source mechanisms of earthquakes near mid‐ocean ridges from body waveform Inversion: Implications for the early evolution of oceanic lithosphere

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Abstract
To investigate the early tectonic evolution of oceanic lithosphere, we present a synthesis of the characteristics of near‐ridge earthquakes, i.e., earthquakes which occur off the axis of major mid‐ocean spreading centers but in lithosphere less than 35 m.y. old. Near‐ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems, with the exception of the central Indian Ocean and the eastern part of the Cocos plate, which have experienced a high rate of earthquake activity during that time. For 32 near‐ridge earthquakes we determined the double‐couple orientation, seismic moment, centroid depth, and source time function, using a formal inversion technique based on matching synthetic and observed P and SH waveforms. All types of faulting styles are observed. Seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m.y., and occurs almost entirely in the uppermost mantle, from just below the Moho to depths at which the temperature estimated from standard thermal models exceeds 800°C. Thrust faulting is not observed at depths greater than 10 km below the seafloor. The deepest near‐ridge earthquakes are generally characterized by normal‐faulting focal mechanisms, with the T axis oblique or perpendicular to the local spreading direction; most of these earthquakes occur in the central Indian Ocean. The transition from mechanisms characteristic of ridge axes to those characteristic of the near‐ridge environment occurs within the first few million years. For lithosphere less than 35 m.y. old, horizontal compressive stress associated with the cooling and subsidence of the lithosphere (“ridge push”) does not appear to be the dominant source of stress released in earthquakes; this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere. This bound, together with the observed depth of faulting, implies that the strain rate associated with seismic deformation is locally at least 10−15 s−1. We propose that thermoelastic stresses associated with plate cooling play an important role in near‐ridge earthquakes. This source of stress can account for the mechanisms and depths of many near‐ridge earthquakes and for the concentration of activity in very young lithosphere. The comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the Moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis. The higher level of near‐ridge seismicity in the Indian Ocean may be the result of modification or enhancement of the more typical near‐ridge stress state by such local processes as the continental collision between India and Asia or active secondary convection beneath the young lithosphere of the Indian plate.