Anatomy of a seafloor spreading event captured by in situ seismogeodesy

Key Points:

• On 26 April 2024, a seismic swarm began in segment I1 of the axial valley, starting with small events detected by hydrophones and followed by a Mw 4.9 normal-faulting earthquake; subsequent seismic activity migrated over 8 km southeastward and then reversed northwestward, indicative of rapid dyke propagation.
• The axial valley floor experienced significant coseismic subsidence, reaching 4.2 m over six days, mostly within the first 16 hours, attributed to drainage of an underlying magma reservoir, with rapid subsidence linked to dyke intrusion and slower subsidence reflecting ongoing magma drainage.
• Acoustic ranging data showed large horizontal displacements across the axial valley floor, with extension and contraction patterns consistent with a deflating magmatic sill, dyke opening, and aseismic slip on a normal fault; modeling suggests a sill collapse of 12–18 m, dyke opening of 1.0–3.3 m, and fault slip of 0.6–3.1 m.
• Hydroacoustic monitoring with a network of seafloor hydrophones and bottom-pressure recorders provided precise event locations and deformation measurements, revealing shallow earthquake depths (<5 km) and enabling detailed analysis of tectonomagmatic processes during the spreading event.
• The integrated geophysical observations and elastic dislocation modeling indicate that the 2024 event involved rapid dyke propagation, magma reservoir deflation, and significant aseismic fault slip, representing a major extensional episode equivalent to decades of seafloor spreading at this mid-ocean ridge segment.