Scientists have obtained the first in situ record of a mid-ocean ridge rifting event as it unfolded, capturing seismic activity, seafloor movement and lava output in real time, according to a study published in Nature.
The event took place on 26 April 2024 along the Southeast Indian Ridge (SEIR), near 37° S, at a segment known as I1. It began just two months after researchers had installed a network of instruments across the ridge axis and the nearby Amsterdam transform fault, part of a project called OHA-GEODAMS (Observatory with Hydro-Acoustics and Geodesy near Amsterdam Island).
Mid-ocean ridges form a 65,000-kilometre network of tectonic boundaries where new oceanic crust is created as plates drift apart at a pace of centimetres per year. Because monitoring these submarine systems continuously is difficult, little has been known about how they behave on timescales of months or years, despite shaping most of the ocean floor over hundreds of thousands of years.
A rift caught in the act
According to the researchers, the event started with a swarm of extensional earthquakes migrating rapidly along the axial valley. At 19:56 UTC on 26 April, five small quakes were detected by hydrophones, followed 13 minutes later by a magnitude 4.9 normal-faulting earthquake beneath the axial valley. Seismic activity, including a magnitude 5.1 quake and 16 smaller events, then migrated more than 8 kilometres toward the southeastern end of the segment within about half an hour.
Minutes later, a second burst of quakes — seven of magnitude 5 or greater, eight in the 4.4–4.8 range, and four smaller ones — propagated more than 9 kilometres in the opposite direction, toward the northwestern end of the segment. The researchers note that such back-and-forth migration patterns are a known signature of dyke propagation along spreading plate boundaries.
The observatory's bottom-pressure recorder and acoustic-ranging beacons showed that the valley floor subsided by 4 metres and the valley widened by more than a metre horizontally. The team interprets this as the deflation of a sill-like magma reservoir feeding dykes that propagated along the ridge axis.
Over the following roughly 16 days, the dykes fed an eruption of about 160 million cubic metres of lava onto the seafloor, the study reports. The process also triggered both seismic and aseismic slip on the normal faults bordering the valley, and ultimately set off seismic activity on the adjacent transform faults.
The findings suggest that large-scale aseismic slip driven by magmatic processes may be the main way that normal faults at mid-ocean ridges accumulate displacement — a mechanism the authors say could help explain why these faults release far less seismic energy than would be expected given their rapid deformation rates.
The observatory itself combined five autonomous hydrophones covering the wider Saint Paul–Amsterdam volcanic plateau with 15 seafloor acoustic-ranging beacons spanning 11 kilometres across the ridge and transform fault, according to the study, marking the first time such methods have all been deployed together on an active mid-ocean ridge segment.