HR: 0830h
AN: S51C-0060    [PDF]
TI: Upper mantle attenuation structure beneath East Africa from relative t* measurements
AU: Nyblade, A A
EM: andy@geosc.psu.edu
AF: Penn State University, Department of Geosciences, 447 Deike Bldg., DeUniversity Park, PA 16802
AU: * Venkataraman, A
EM: anupamav@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, 397, Panama Mall, Stanford, CA 94305-2215
AU: Ritsema, J
EM: jeroen@gps.caltech.edu
AF: California Institute of Technology, 252-21, Seismological Laboratory, Pasadena, CA 91125
AB: We are studying the attenuation structure of the upper mantle beneath east Africa using body waves recorded by a PASSCAL array that was deployed in Tanzania between 1994-1995. The array runs along two profiles (EW and NE-SW) that traverse the Tanzania craton and the surrounding rifted mobile belts. Studying this region may provide important clues on the initiation of Cenozoic rifting in east Africa and hence, give us insights into the tectonic and geodynamic processes that occur during the initial stages of rift development. The same dataset has been used to obtain tomographic velocity models, so we can compare the velocity structure to the attenuation structure. We determined the P wave spectral amplitude ratios of data from the same earthquake recorded at different stations (thereby eliminating the source effect), and computed relative t* (path integrated attenuation) from the slopes of these ratios. We then used a least-squares inversion to determine the t* at each station relative to the average t* of the array. We applied the same procedure to determine relative t* for S waves from the transverse components of the data (i.e., SH only). To simplify the problem, we selected events that have azimuths along the two profiles, and we have initially focused on events along the EW profile. On comparing the estimates of t* to the residual travel times estimated by Ritsema et. al., (1998), we observe that the t* values and residual travel times are not always correlated. Also, the observed t* values from P wave data are anti-correlated to the observed S wave data for stations within the craton. We are planning to compute forward models in order to isolate elastic from anelastic effects on our t* measurements, and to place constraints on upper mantle Q values.
DE: 7218 Lithosphere and upper mantle
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2003 Fall Meeting