NEW_Accomplishments.indd - IRIS

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GLOBAL MANTLE STRUCTURE 2006 IRIS 5-YEAR PROPOSAL Using IRIS Digital Data to Determine the Radial Attenuation Structure of the Lower Mantle Jesse Fisher Lawrence • Washington University, now at University of California, San Diego Michael Wysession • Washington University We employed a niching genetic algorithm to invert ~30,000 differential ScS/S attenuation values for a new spherically-symmetric radial model of shear quality factor (Qµ) with high sensitivity to the lower mantle. The data were obtained from all broadband shear waves available from the IRIS DMC after 1990. The new radial Qµ model, QLM9, possesses greater sensitivity to Qµ at large mantle depths than previous studies. Differential ScS-S attenuation as a function of event-to-station distance provides an excellent mechanism for determining lower mantle quality factor structure regardless of upper mantle structure. This model is robust for both shallow and deep event data sets. On average, lower mantle Qµ increases slightly with depth, which supports models of increasing viscosity with depth. QLM9, when compared to viscosity and temperature (with significant caveats), is in agreement with geodynamic expectations. There are two higher- Qµ regions at ~1000 and ~2500 km depth, which roughly correspond to high-viscosity regions observed by Forte and Mitrovica. There is a lower-Qµ layer at the core-mantle boundary, as expected based upon the presence of the lowermost mantle thermal boundary layer. There is also a relatively low-Qµ region in the mid-lower mantle. However, as three-dimensional tomographic modeling of shear wave attenuation shows, A. The lower mantle radial Q model QLM18 of Lawrence and Wysession (2005). Gray shading shows the range of other low-cost models from the there is tremendous lateral variation in the shear-wave genetic algorithm search. B. Mantle viscosity models: 1. Hager and Richards (1989); 2. Forte and Mitrovica (1996); 3. Forte and Mitrovica (2001); 4. attenuation of the lower mantle, so a radial model of Qµ may be of some general use for geodynamics Steinberger and Calderwood (2001); 5. McNamara et al. (2003). C. Comparison of the PREM and QLM9 models with the laterally averaged radial applications, but it is important to remember that it is model from the 3D mantle model VQLM3D (Lawrence and Wysession, averaging over huge lateral variations. As such, it is 2004). D. Radial profiles for VQLM3D from 4 different geographic regions. interesting how nearly-constant the vertical profile is across the lower mantle. The slight increase in Qµ in the mid-lower-mantle may be suggesting that descending subducted lithosphere doesnʼt spend a lot of time in the mid-lower mantle. Lawrence, J. F., and M. E. Wysession, QLM9: A new radial quality factor (Q) model for the mantle, Earth Planet. Sci. Lett., 2005 (in press). Lawrence, J. F., and M. E. Wysession, 3D Whole-Mantle Velocity and Quality Factor, Eos Trans. AGU, 85(17), Jt. Assem. Suppl., Abstract S43A-05, 2004. NSF Grant #: NSF-EAR-0207751 152
2006 IRIS 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE Whole-Mantle 3D Seismic Attenuation: Evidence for Global Mass Flux Jesse Fisher Lawrence • Washington University, now at University of California, San Diego Michael Wysession • Washington University By combing through available broadband digital data from the IRIS DMC, we constructed a three-dimensional tomographic model of the seismic shear wave attenuation in the mantle. The model was obtained from all available digital broadband records during 1990-2001. The data consist of over 70,000 high-quality differential attenuation measurements of ScS-S, SS-S, ScS-SS, S-S, sScS-sS, sSS-sS phase pairs. The use of differential measurements removes contaminating source effects, which is the same in both phases, as well as near-source and near-receiver structural anomalies. The result is that our data have the best resolution in the lower mantle, which has previously been largely unexamined with 3D modeling. The differential attenuation measurements (taken as t* values) are inverted using the LSQR routine over a long-wavelength 5 x 5 degree grid for a 3D model of shear wave quality factor (Q). These t* star measurements are also used to generate a whole-mantle 3D shear wave velocity model. While the velocity model does not have resolution equal to other models that incorporate surface wave and normal mode data, because it uses the same paths as went into the Q model, it is of interest in interpreting the attenuation anomalies. There is a very strong suggestion of wholemantle flow in the attenuation tomography. High- Q sheet-like anomalies extend from the surface to the core-mantle boundary region (CMBR) at subduction zones, dominating the model. The African and Pacific superplumes involve low-Q anomalies that extend from the CMBR to the surface. For the Pacific superplume, the low-Q anomalies extend vertically from core to crust, but with the African superplume, the low-Q anomalies do not extend directly up into the sub-African upper mantle, but rather branch east and west up towards the Atlantic and Indian Ocean spreading centers. While the interpretation of seismic Q in terms of anelasticity is challenging (as there are other factors such as seismic scattering, water content, grain size, and deviatoric strain that can influence it), if we assume that the Q anomalies are influenced to the first order by temperature variations, then this model strongly supports whole-mantle mass flux between the surface and the base of the mantle. Two global slices through the shear velocity and attenuation models of Lawrence and Wysession (2004), obtained from differential shear wave phases. Lawrence, J. F., and M. E. Wysession, QLM9: A new radial quality factor (Q) model for the mantle, Earth Planet. Sci. Lett., 2005 (in press). Lawrence, J. F., and M. E. Wysession, 3D Whole-Mantle Velocity and Quality Factor, Eos Trans. AGU, 85(17), Jt. Assem. Suppl., Abstract S43A-05, 2004. NSF Grant #: NSF-EAR-0207751 153
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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