Shear velocity and shear attenuation models inverted from the world-wide and pure-path average data of mantle Rayleigh waves (0S25 to 0S80) and fundamental spheroidal modes (0S2 to 0S24)

Abstract

Summary. Global and regional structures of the Earth's mantle are studied. Original phase velocities and Q−1 of Rayleigh waves are converted to equivalent free oscillation periods and Q−1. Global average upper mantle shear velocity models are derived from the inversions of the world-wide average eigenperiods corrected for attenuative dispersion. Resolution analysis shows that the thickness and velocity of the lithosphere cannot be determined from our data set. Inverted models have a well-developed low-velocity zone in the depth range between 70 and 280 km. Global average Q−1μ models for the whole mantle are derived from the world-wide average Q−1 of mantle Rayleigh waves and fundamental spheroidal modes. The resolution analysis indicates that Q−1μ at depths shallower than about 100 km cannot be determined by our data set and that the depth resolution for Q−1μ of the lower mantle is poor. All inverted Q−1μ models are characterized by a well-developed high-Q−1μ zone in the depth range between about 100 and 300 km in the upper mantle. Qμ has a minimum value of about 90 at depths around 200 km, begins to increase at a depth of about 300 km, rapidly increases in the depth range from 300 to 400 km, and gradually increases at depths between 400 and 800 km. Qμ of the lower mantle is about 330. The boundary between the high-Q−1μ upper mantle and low-Q−1μ lower mantle is sharper than has been suggested in the past. Inverted Q−1μ models predict QScS= 226 to 250. The Earth's surface is divided into mountainous, oceanic, ridge, shield, and trench and marginal sea regions, and a regionalized shear velocity model of the upper mantle is derived. The inversions of the pure-path eigenperiods without correction for attenuative dispersion show that strong lateral variations in shear velocity exist at depths shallower than 250 km in the upper mantle. A low-velocity zone for shear waves exists in the upper mantle beneath all the tectonic regions except perhaps the shield region. If the Sn velocity of the starting model is equal to or higher than 4.7 km s−1, the shield data require a moderate low-velocity zone. Data from the mountainous region require a much lower shear velocity than the shield region at depths shallower than 250 km. The oceanic, ridge, and trench and marginal sea models have a well-developed low-velocity zone. The shear velocity model for the trench and marginal sea region is characterized by a velocity increase at depths between 250 and 400 km. This increase in velocity is interpreted as a consequence of the subducted lithospheric slabs beneath many of our trench and marginal sea regions. Regional differences in Rayleigh wave Q−1 are found for modes from 0S25 to 0S45. Since the resolving power of this data set is limited, we could not locate the depths at which regional variations of Q−1μ exist. Since the oceanic Q−1 for modes from 0S45 to 0S78 appear to be stable, the oceanic Q−1μ model is derived. This model has a well-developed high-Q−1μ zone. Qμ has a minimum value of about 110 at depths between 200 and 300 km, and rapidly increases to a high value of about 250 at a depth of about 400 km.