Monitoring seismic velocity changes in the Saint Peter and Saint Paul Archipelago-Equatorial Atlantic using ambient noise interferometry

Abstract The Saint Paul Transform System (SPTS) is located in the Mid -Atlantic Ridge (MAR), Equatorial Atlantic and is formed by a multi-fault complex system, where many faults are interacting and small-to-moderate events are routinely located in the area using global seismic networks. This seismicity is challenging to charecterize since no permanent seismic stations near this area are available. As a result, the interplay between seismicity, faulting and fluid in the MAR is poorly constrained. In this paper, we quantify the seismic velocity perturbations due to small-to-moderate magnitude earthquakes in the Saint Peter Saint Paul Archipelago (SPSPA) using one seismological station installed in the region. We calculate noise auto-correlation functions using the data recorded by this station. The auto-correlograms were obtained by two different approaches: classical auto-correlation geometrically normalized (ACGN) and phase autocorrelation (PAC). We found that both approaches give very similar results and we are able to estimate seismic velocity changes due in the propagating medium to small earthquakes (between 3.0 and 4.7 ML). The changes in the medium were investigated through the joint analysis of decorrelation curves and velocity changes obtained by Moving Window Cross Spectral (MWCS) method. Our analysis shows that the seismic velocity changes are possibly associated with a hydromechanical coupling in which the observed velocity increase is driven by static stress changes and fluid-rock interaction is responsible for the observed seismic velocity drop. This behaviour is qualitatively explained by the multi-fault complex system nature of the SPTS.