Deep Sea Spreading Event Captured by Seafloor Seismogeodesy

Scientists have captured an unprecedented, detailed view of a seafloor spreading event, observing rapid seismic activity and significant seafloor deformation in real-time. The event, which unfolded on April 26, 2024, along the middle of segment I1 of the Juan de Fuca Ridge, offered researchers a unique window into the dynamic processes occurring at tectonic plate boundaries. The observations, made using advanced seafloor monitoring equipment, documented a rapid sequence of earthquakes, dike propagation, and dramatic subsidence, providing a comprehensive dataset of a geological phenomenon previously only theorized in such detail.

The event began with a notable swarm of extensional seismicity at 19:56 UTC, starting with small tremors detected by hydrophones. This was followed by a magnitude Mw 4.9 normal-faulting earthquake at 20:09, with its epicenter located beneath the axial valley. This initial seismic activity, which included a subsequent Mw 5.1 earthquake and 16 smaller tremors, migrated approximately 8 kilometers southeast along the axial valley until 20:35. Minutes later, a second series of earthquakes, including seven with magnitudes Mw ≥ 5 and eight in the 4.4 to 4.8 range, migrated in the opposite direction, moving over 9 kilometers northwest from the segment's center. These migrating seismic patterns are characteristic of dike propagation events at extensional plate boundaries, with migration rates observed in this case reaching 2–3 meters per second, an order of magnitude faster than previously documented sequences.

Seafloor Deformation and Magma Dynamics

Simultaneously, seafloor pressure recorders (BPRs) in the axial valley registered a series of discrete subsidence events. These events involved sudden pressure increases, correlating with vertical ground motion of 1.7 to 3.3 centimeters coinciding with the initial large normal-faulting earthquakes. Analysis of rupture size and focal mechanisms suggested these earthquakes occurred at shallow depths, no more than about 5 kilometers below the seafloor, to produce the observed subsidence. This contrasts sharply with standard catalog estimates, likely due to limited teleseismic data coverage in the remote area.

The subsidence accelerated after a Mw 5.1 event at 21:03, occurring in a stepwise manner and averaging about 5 centimeters per minute as a putative dike propagated northwest. By the time the dike propagation ceased around 21:40, the cumulative seafloor subsidence reached 1.2 meters. The rate then slowed, continuing at a lesser pace for several days, with a total subsidence of 4.2 meters recorded over six days. This rate of subsidence was nearly double that observed during a 2015 eruption at Axial Seamount, a closely monitored volcano on the Juan de Fuca Ridge. Researchers interpret this vertical displacement as evidence of magma drainage from an underlying reservoir, potentially an axial melt lens, into the newly formed dike.

Near-bottom water temperature data also provided clues, with rising temperatures suggesting lava may have reached the seafloor as early as 22:00 on April 26. Slower subsequent seafloor subsidence is believed to reflect magma drainage through open fractures connecting the reservoir to the surface. Furthermore, an acoustic ranging array documented substantial horizontal displacements across the axial valley. By averaging measurements before significant temperature fluctuations, researchers observed baselines on one side of the valley lengthening by up to 1.30 meters, while on the opposite side, baselines shortened by 0.53 to 0.86 meters. These horizontal displacements are an order of magnitude larger than those recorded during a 1998 dike intrusion at Axial Seamount.

The combined seismic and geodetic data allowed scientists to model the complex interplay of deformation sources. Analysis of millions of model combinations indicated a deflating magma sill at least 3,500 meters deep, a dike extending to shallow depths, and significant slip on a normal fault. This integrated approach provided a detailed picture of how magma chamber deflation, dike intrusion, and fault slip collectively contribute to the dramatic reshaping of the seafloor during spreading events, offering invaluable insights into the fundamental processes driving plate tectonics.