Radial profiles of seismic attenuation in the upper mantle based on physical models

Abstract

Thermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ∼1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. © 2008 The Authors Journal compilation © 2008 RAS

Mapping the Distribution of Fluids in the Crust and Lithospheric Mantle Utilizing Geophysical Methods

“…Attenuation is defined by the quality factor Q ¼ 2pE/DE, or its inverse Q -1 , where DE represents the loss of seismic energy per unit cycle and E denotes the elastic energy stored in the system. Estimated values of Q, for the crust and upper mantle, range between the lowest values of~30-40 for Q S (S-wave attenuation) in the asthenosphere to values of several thousands in lowattenuation regions (e.g., Cammarano and Romanowicz 2008). 5.…”

Lithospheric structure in the Baikal–central Mongolia region from integrated geophysical‐petrological inversion of surface‐wave data and topographic elevation

“…Much of the attenuation observed in seismic waves is caused by the thermally activated viscoelastic relaxation of the mineral aggregates within the Earth. The importance of attenuation effects in inversions of seismic data for the thermal and compositional structure of the upper mantle is well known [e.g., Afonso et al , 2010; Cammarano and Romanowicz , 2008; Goes et al , 2000; Sobolev et al , 1996]. Our understanding of viscoelastic relaxation at seismic frequencies (10–10 −4 Hz) in upper mantle mineral aggregates or analogues, and the relation between attenuation and grain size, have improved considerably in recent years thanks to new laboratory studies [e.g., Jackson and Faul , 2010; McCarthy et al , 2011].…”

Inferring the thermochemical structure of the upper mantle from seismic data

“…A short review of uncertainties in anelastic properties can be found in Cammarano & Romanowicz (2008). A more extended review, also illustrating the principles of the physical mechanisms responsible for attenuation, is given by Jackson (2008).…”

“…Alternatively, an unrealistically strong negative thermal gradient with depth would need to be invoked. The purely thermal explanation is not only unphysical, as discussed in Cammarano & Romanowicz (2007), but it would imply intrinsic values of Q S (shear quality factor) that are too high compared to the values compatible with global observations of seismic attenuation (see Cammarano & Romanowicz 2008). We also speculated that a purely compositional explanation would be consistent with an upper mantle that is not chemically equilibrated, but contains 3‐D compositional heterogeneity at a scale beyond the resolution of the long‐period data used in that study.…”