HR: 09:15h
AN: S51A-05    [PDF]
TI: Spectral Analysis of Broadband Seismic Array Data, Tien Shan
AU: * Shamshy, S
EM: shakir@chevrontexaco.com
AF: ChevronTexaco Corp., 935 Gravier Street, Rm 1857, New Orleans, LA 70112 United States
AU: Pavlis, G L
EM: pavlis@indiana.edu
AF: Indiana University, Department of Geological Sciences, 1005 E. 10th Street, Bloomington, IN 47405 United States
AB: We used a spectral analysis method to examine amplitude variations of body waves recorded in the Tien Shan region of central Asia. We used broadband data from the Kyrgyz Network (KNET), Kazakhstan Network (KZNET), and from a set of temporary, PASSCAL stations operated from 1997-2000 we refer to as the Ghengis array. A spectral ratio method similar to that used by Wilson and Pavlis (2000) was employed, but with station AAK used as a reference instead of the array median. Spectral ratios were estimated for all teleseismic events and a larger, intermediate depth events from the Hindu-Kush region for all three-components of ground motion and total signal strength on all components. Results are visualized by maps of amplitude for various frequency bands and through the 4-D animation method introduced by Wilson and Pavlis (2000). Data from Hindu-Kush events showed amplitude variations as much as a factor of 100 across the study area with a strong frequency dependence. The largest variations were at the highest frequencies observed near 15 Hz. Stations in the northwestern part of the Tien Shan array show little variation in amplitude relative to the reference station, AAK. In the central and eastern part of the array, the amplitude estimates are significantly smaller at all frequencies. In contrast, for stations in the western Tien Shan near the Talas-Fergana Fault, and the southern Tien Shan near the Tarim Basin, the amplitude values become much larger than the reference site. The teleseismic data show a different pattern and show a somewhat smaller, overall amplitude variation at comparable frequencies. The northern part of the array again shows small variations relative to the reference stations. There are some amplifications in the southern stations of the array, especially in the Tarim Basin. The higher frequency observations that show large amplifications at stations in the Tarim Basin are readily explained by site effects due to the thick deposits of sediments beneath these stations. The difference and scale of amplitude fluctuations seen at longer periods, however, is difficult to explain. The fact that coherent, spatial patterns are seen in the data suggests there is some degree of focusing by lateral variations velocity beneath the array that might be used to constraint earth structure in future work.
DE: 7203 Body wave propagation
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2003 Fall Meeting