S-wave velocity structure at the Galápagos Archipelago (Ecuador) using ambient seismic noise

The Galápagos Archipelago originates from a plume-like structure that rises from the mantle about 250 km south of the islands. The Isabela Island, located on the western part of the Archipelago, contains several of the most active volcanoes in Galápagos, among them Alcedo, Cerro Azul, and Sierra Negra, whose last eruptions occurred in 1953, 2008, and 2018, respectively.

Several studies from different disciplines have been performed to image the subsurface structures at the volcanoes on Isabela. They report a melt-rich sill located at 2 km depth, a crystal-mush zone below Sierra Negra located at depths approximately between 8 to 15 km, and a magma intrusion for depths between the sill and the crystal mush before the 2010 eruption of Sierra Negra. However, the resolution of these studies is limited along many areas and depths because of multiple reasons, like non-ideal station distribution, limitations on the selected methodologies, or sparse earthquake locations.

Using seismic data recorded by two temporal seismic networks deployed in the Archipelago, we used the ambient seismic noise to obtain a 3D S-wave velocity model; we used this information to improve the understanding of the structure of the subsurface in the area. One of the networks -XE array- was composed of 18 stations deployed between July 2009 and June 2011; the second network -YH array, composed of 10 stations, was deployed between August 1999 and March 2003. Provided the distribution of the seismic stations, a higher resolution was obtained on Isabella Island. Therefore, we focused our analysis on the regional-scale feeding systems of the volcanoes in Isabela, in particular, Alcedo, Sierra Negra, and Cerro Azul volcanoes.

Through an iterative linear-least-squares inversion methodology, we obtained Rayleigh phase-velocity maps for periods in the range 2.5-25 s. Subsequently, we inverted the obtained tomographic maps for retrieving the S-wave velocity distribution as a function of depth. Our results indicate two main discontinuities, located at 3 and 11 km depth, agreeing with the expected depth for the discontinuity between old and new oceanic crust. The first layer presents an average S-wave velocity of 2.4 km/s, while the second and third layers - 3.0 km/s and 3.4 km/s, respectively. Our results show two relevant low-velocity zones in the subsurface: one is located between Sierra Negra and Alcedo volcanoes centered at 20 km depth, the second one is below Sierra Negra at 8 km depth, which we interpret as magma accumulation zones. In addition, our results show a high-velocity zone at 3 km depth, coincident with the previously reported melt-rich sill.

This work not only validates the results obtained by previous works but provides information with higher resolution for certain depths of the subsurface of hazardous volcanoes on Galápagos.