HR: 0830h
AN: U51B-0011    [PDF]
TI: Attenuation at the base of the mantle: Global mapping using core-reflected and core-diffracted waves
AU: Solander, K
EM: kurtsolander@hotmail.com
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: Lawrence, J F
EM: jfisher@levee.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: * Wysession, M E
EM: michael@wucore.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AB: We use ScS-S and Sdiff data to examine the shear-wave attenuation in the lowermost part of the mantle (the D'' layer). There is significant lateral variation in the shear-wave attenuation, as sampled by long- and intermediate-period wavelengths. Maps of attenuation show some degree of correlation with shear-wave velocity anomalies, but a greater degree of uncorrelation. There is significant coherence to the maps of D'' attenuation at lateral spatial scales of hundreds to thousands of kilometers. The ScS-S study involved more than 40,000 high-quality digital broadband measurements using IRIS GSN and PASSCAL data from 1990-2001. All data had simultaneous ScSH-SH and ScSV-SV traveltime measurements obtained as well, using the attenuation measurements as an important correction. The attenuation values were obtained from differential t* measurements taken using spectral division of the S and ScS signals. At closer epicentral distances (less than 70 deg) the ScS waveform usually displayed a greater deal of attenuation than the S waveforms, as to be expected from the greater path lengths. However, as the raypaths of ScS and S approach each other (beyond 70 deg) the ScS pulse usually displayed less attenuation, indicative of generally high Q values at the base of the mantle. To a first order, shear-wave attenuation agrees with the large degree-2 anomalies observed in seismic tomographic models, with high D'' attenuation beneath the Pacific and lower attenuation around the rim of the Pacific and beneath Asia. This is also evident in maps that show regions of slow velocities and high attenuation (and regions of fast velocities and low attenuation). There is also a significant amount of smaller-scale features that could represent a combination of chemical, thermal, or structural variations. Interestingly, the attenuation observed for ScSH waves differed from ScSV waves, being generally higher but also varying at smaller scales. Only in D'' beneath eastern Asia was the ScSV attenuation significantly higher than for ScSH waves. These results compare interestingly with global maps of D'' attenuation obtained from Sdiff waves. Here, the amplitudes of Sdiff waves are compared to predicted amplitudes from synthetic counterparts. There is an added complication with the diffracted wave amplitude variations, however, in that the amplitudes are actually more sensitive to the vertical velocity gradient across D'' than to the intrinsic attenuation within it. There is a significant trend of larger Sdiff amplitudes with increased diffraction distance around the core, suggesting a negative velocity gradient is more appropriate for an averaged D'' model than a zero gradient. Because of the anomalously high amplitudes of Sdiff waves, it is not possible to support a large amount of attenuation at the base of the mantle, agreeing with the ScS-S results.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
SC: U
MN: 2003 Fall Meeting