Determination of Q as a Function of Depth and Tectonic Province.
Abstract
The goal of this study is to determine the relation between upper mantle shear velocity at a particular depth and Q sub u at the depth. Such a relation would provide a means of finding t* biases for any test site by using well developed techniques for remotely sensing shear velocity structure. We have developed upper mantle Q models, to 700 km depth, beneath two distinctly different tectonic regions, the East Pacific Rise and the North American craton. These two regions are end members in terms of upper mantle velocity structure with the East Pacific Rise having the lowest shear velocities and the Canadian shield the highest. The NTS test site overlies an upper mantle similar to the East Pacific Rise. The Soviet and French Saharan test sites overlie mantle with intermediate shear structures. The upper mantle Q structures were derived by modeling the amplitude decay of multiple bounce shear waves as a function of distance. For the East Pacific Rise, S, SS, SSS, and SSSS waves were included for distances from 30 to 80 deg. For the shield data only phases up to SSS were used and the distance range was limited to 60 deg due to the geographic extent of the craton. The data have predominant periods from 20 to 30 sec. The PREM lower mantle Q model was assumed for below 700 km depth and mechanisms for the earthquakes were taken from the literature. The amplitudes of the multiple bounce phases which propagated wholly within the upper mantle were modeled relative to the S wave amplitude with a WKBJ synthetic code which incorporated realistic Q structures. (jhd)
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 14, 1989
- Accession Number
- ADA213805
Entities
People
- Stephen Grand
- Xiao-yang Ding
Organizations
- University of Illinois Urbana–Champaign