Felix Halpaap | Seismology

Earthquakes track subduction fluids from slab source to mantle wedge sink

Felix Halpaap¹, Stéphane Rondenay¹, Alexander Perrin², Saskia Goes², Lars Ottemöller¹, Florian Millet^1,3, Håkon Austrheim⁴, Robert Shaw² and Thomas Eeken²

¹⁾ Department of Earth Science, University of Bergen, Bergen, Norway

²⁾ Department of Earth Science and Engineering, Imperial College London, London, UK

³⁾ Laboratoire de Géologie, Claude Bernard University Lyon, Lyon, France

⁴⁾ Physics of Geological Processes (PGP), The Njord Centre, Department
of Geosciences, University of Oslo, Oslo, Norway

Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. The relationship between these fluids and earthquakes has been debated. Here we combine new seismic observations and new thermal-petrologic models from the Western Hellenic subduction zone in Greece to investigate the relationship between fluids and earthquakes. From seismic images and relocated hypocentres of deep earthquakes (>35 km), we observe that earthquakes occur in three domains of the subduction zone: in the slab, on the plate interface, and in the mantle wedge. These observations are supported by a systematic difference in focal mechanisms and secondary arrival patterns (caused by the interactions of the local seismic waves with subduction discontinuities) between the domains. By comparing across other subduction zones where mantle wedge earthquakes occur (Greece, Lesser Antilles, E Honshu, New Zealand), we find that earthquakes in these cold slabs effectively track the flow of fluids from their source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface. In some locations, this seal breaks and fluids escape from the slab, through a vent on the interface, into a dry mantle wedge. Along their migration path, the fluids cause earthquakes in the slab, on the plate interface, and in the overlying mantle wedge, while the forearc slab updip of the vents consistently lacks earthquakes due to being fluid-starved. In eleven years (2006-2017) of detailed observations for Greece, the interface vents persistently hosted only small (Mw ≤ 3.7) earthquakes. Their combined release of seismic moment points to a small (<0.4 %) fraction of coseismic compared to aseismic slip between the plates. However, the vents coincide with the locations of three major earthquakes (Mw 5.7, 6.8, ~7.0) that occurred at unusually large depths on the plate interface (>55 km), suggesting that the vents are relevant for the nucleation of damaging earthquakes.