Sediment Structure at the Equatorial Mid-Atlantic Ridge Constrained by Seafloor Admittance using data from the PI-LAB experiment

Well-constrained marine sediment characteristics (sediment thickness and shear wave velocity) are important not only for the study of climate over geologic times scales but also for correcting and accounting for its presence in seismic data used to investigate deeper structures. We use data from the PI-LAB (Passive Imaging of the Lithosphere Asthenosphere Boundary) experiment, which consisted of 39 broadband ocean bottom seismometers deployed at the Equatorial Mid-Atlantic Ridge near the Chain fracture zone covering 0-80 Myr old seafloor. We successfully compute reliable admittance between the pressure to the vertical displacement at the seafloor as a function of frequency for microseism-generated Rayleigh waves at 18 stations. We examine frequencies between 0.1 to 0.2 Hz. We find a general trend of increasing sediment thickness with the seafloor ages, as expected. We find sediment thickness varies almost uniformly across both sides of the ridge and it indicates that both sides might have experienced by the same sedimentation process. The best-fitting sediment thickness on young seafloor (< 10 Myr) is also consistent with the global model which is based on sediment cores in this region [Straume et al., 2019] and the previous sedimentary thickness from P-to-s (Ps) delay times of conversions from the base of the sediment layer [Agius et al., 2018]. The best-fitting sediment thickness on moderate aged lithosphere (17-30 Myr) is 25-30 m thinner than the global model but about 5-25 m thicker than the sedimentary from Ps delay times. We observed similar characteristics of sediment thickness with the global model for the oldest seafloor in our region (~50-80 Myr). Overlap of the 95% confidence regions between admittance and Ps estimates for thickness and shear velocity is found at 15 stations where we have both Ps and admittance estimates. It suggests that both methods yield accurate estimates for sediment thickness. In addition, combing the two methods can also increase confidence, and here we find we can resolve shear velocities within 0.200 km/s.