Seismic Attenuation at the Equatorial Mid-Atlantic ridge Constrained by Surface wave analysis from the PI-LAB experiment

Abstract:

We present surface wave Q_μ values for the lithosphere and the asthenosphere beneath 0-80 My old seafloor near the equatorial Mid-Atlantic Ridge. We use Rayleigh waves recorded by the Passive Imaging of the Lithosphere and Asthenosphere Boundary (PI-LAB) experiment which consisted of 39 broadband ocean bottom seismometers. We use a total of 17 local, M > 4.2 earthquakes to obtain the attenuation coefficient (γ) from the amplitude of Rayleigh waves for periods between 15 and 40 s. The estimated average attenuation coefficient value is around 5×10^-4 km^-1 at period 16 s, gradually decreasing to 2×10^-4 km^-1 at higher periods (35-40 s). We invert the attenuation coefficients as a function of period for (Q_μ ) using sensitivity kernels from our best fit V_SV model for the region. The Q_μ value is around ~200 at depth shallower than ~50 km and decreases to a value of ~95-100 value at depth greater than 60 km. The Q_μ model structure is similar to our 1D shear wave velocity models for the study region which include a 50 km thick high velocity lithosphere layer above a slower asthenosphere. Our observations in the lithosphere (Q_μ ~ 200) are lower than the global average from PREM (Q_μ ~ 600) but slightly higher than QRFSI12 model (Q_μ ~ 140), while our observations in the asthenosphere (Q_μ ~ 95-100) are slightly higher, but within error of the global average attenuation from PREM (Q_μ~ 60-90). The lithospheric discrepancy could be explained by the fact that our study area represents younger lithosphere than the global average.