SEISMIC VELOCITY AND ATTENUATION CONSTRAINTS ON THE FORMATION OF OCEANIC PLATE AND THE ORIGIN OF THE LOW-VELOCITY ZONE

Seismic attenuation has been long neglected in traditional seismic tomography since attenuation effects on amplitude are hard to separate from other effects such as focusing/defocusing and scattering. However, attenuation is an important property of Earth's materials and can provide us additional knowledge other than elastic velocity about temperature, fluid content, phase changes, and density of solid-state defects in the crust and mantle. Fundamental mode surface wave studies at different periods allow us to constrain regional and vertical variation of the quality factor Q. We use 2-D sensitivity kernels for surface waves based on single-scattering (Born) approximation to account for the effects of scattering on amplitude in regional surface wave studies.

We invert phase and amplitude data of Rayleigh waves for shear wave velocities and shear wave quality factor (Qu) in very young (less than 10 Ma) Pacific plate using teleseismic sources recorded on ocean-bottom seismometers. There is a high seismic velocity lid about 50 km thick underlain by a low velocity zone. Qu shows a similar pattern: Qu is larger than 150 at depths shallower than 50 km and ~120 at depths greater than 60 km. Models that attribute the origin of the low-velocity zone beneath old seafloor solely to temperature and pressure effects predict Qu values an order of magnitude too low beneath young seafloor. An alternative model considering the variation of water content in the upper mantle can explain both the velocities and attenuation pattern. Partial melting in the shallow upper mantle at mid-oceanic ridges during the production of the oceanic crust effectively remove the water in the MORB source region and lead to a “dry” depleted peridotite layer underlain by a “wet” fertile peridotite. The presence of water in the asthenosphere lowers Qu and seismic velocity beneath old sea floor, so that the effects attributed to thermal variations are much smaller and thus attenuation in the hot, young lithosphere is less pronounced. Our seismic observations provide strong constraints on the argument that the structure of oceanic plates is controlled by compositional as well as thermal parameters.