FY 2022

Hydroacoustic monitoring of seafloor spreading and transform faulting in the equatorial Atlantic Ocean

Parnell-Turner, R., D.K. Smith, and R.P. Dziak

J. Geophys. Res., 127(7), e2022JB024008, doi: 10.1029/2022JB024008, View online (open access) (2022)

Seismicity along mid-ocean ridges and oceanic transform faults provides insights into the processes of crustal accretion and strike-slip deformation. In the equatorial Atlantic ocean, the slow-spreading Mid-Atlantic Ridge is offset by some of the longest-offset transform faults on Earth, which remain relatively poorly understood due to its remote location far from land-based teleseismic receivers. A catalog of T-phase events detected by an array of 10 autonomous hydrophones deployed between 2011 and 2015, extending from 20°N to 10°S is presented. The final catalog of 6,843 events has a magnitude of completeness of 3.3, compared to 4.4 for the International Seismic Center teleseismic catalog covering the same region, and allows investigation of the dual processes of crustal accretion and transform fault slip. The seismicity rate observed at asymmetric spreading segments (those hosting detachment faults) is significantly higher than that of symmetric spreading centers, and 74% of known hydrothermal vents along the equatorial Mid-Atlantic Ridge occur on asymmetric spreading segments. Aseismic patches are present on nearly all equatorial Atlantic transform faults, including on the Romanche transform where regional rotation and transpression could explain both bathymetric uplift and reduction in seismic activity. The observed patterns in seismicity provide insight into the thermal and mechanical structure of the ridge axis and associated transform faults, and potentially provide a method for investigating the distribution of hydrothermal vent systems.

Plain Language Summary. New oceanic seafloor is created at mid-ocean ridges where tectonic plates move apart, causing seafloor volcanism and deep-sea hydrothermal venting. Meanwhile, changes in the direction of plate motion occur along strike-slip faults which can be hundreds of kilometers long, called oceanic transforms, and offset the mid-ocean ridge. Neither of these processes are fully understood, but constantly generate numerous small earthquakes which in places like the remote equatorial Atlantic Ocean, often go undetected by far-away instruments on land. Using a network of hydrophones spaced thousands of kilometers apart and moored ∼800 m below the sea surface, over 6,000 earthquakes in the equatorial Atlantic ocean were located from 2011 to 2015, roughly 10 times more than would have been detected by land-based stations alone. Similar to previous studies, we find that transform faults have patches where very few earthquakes occur and hence may be locked or creeping, likely due to spatial variability in the thermal properties of the lithosphere and underlying mantle. More earthquakes occurred in places where the plates spread apart asymmetrically, and hydrothermal vents are also more likely to occur in places where spreading is asymmetric.