A large ultra-low velocity zone at the potential base of the Galapagos plume

The seismic stations of the Transportable Array are currently deployed across Alaska (2011-2020), which offers new geometries to map the core-mantle boundary beneath the Eastern Pacific using earthquakes from Chile. Here we show evidence of strong Sdiff postcursory phases for a 2016 Chilean earthquake. These phases can be explained by a broad ultra-low velocity zone (ULVZ, ~600 km width, ~20 km high, and ~-25% shear velocity anomaly). We obtain this model by analysing beamforming stacks and forward modeling full 3D-synthetics for potential ULVZ models (using CSEM, Capdeville et al. 2003). The event-station geometry to Alaska only highlights the ULVZ from one direction, which makes the location of the ULVZ uncertain. Additional evidence comes form an earthquake on the Pacific-Antartic ridge to the Eastern USA. Its waveforms show (weaker) Sdiff postcursors allowing us to locate the ULVZ to the WNW of the Galapagos hotspot.

This ULVZ is similar in extent to ones found beneath Hawaii (Cottaar and Romanowicz 2012), Samoa (Thorne et al. 2012), and Iceland (Yuan and Romanowicz, 2017), and these types of features have been previously dubbed 'mega-ULVZs'. They might constitute a different family of ULVZs from the scattered small piles found elsewhere. The four mega-ULVZs mapped up till now, lie at the base of hotspots where basalts show the strongest ³He /⁴He ratios; three of these have also been confirmed to correlate with anomalous tungsten isotopes (Mundl et al. 2017). Both these signatures strongly suggest entrainment from a primordial material, the source of which could be the mapped mega- ULVZs.