HR: 0800h
AN: S51A-0125    [Abstracts]
TI: Focal mechanisms from regional earthquakes in East Africa and refinements in stress patterns
AU: * Brazier, R A
EM: rab27@psu.edu
AF: Penn State University DuBois Campus, Mathematics Division, College Place, DuBois, PA 15801 United States
AU: Nyblade, A
EM: andy@geosc.psu.edu
AF: Penn State University, Geoscience Department, University Park, State College, PA 16802 United States
AB: Accurate event locations for eleven events with magnitude greater than 3.5 in Tanzania, East Africa have been obtained using data from the Tanzania Broadband Seismic Experiment. Focal mechanisms for most of these events have been determined by modeling P and SH polarities and amplitude ratios in a grid search method. Epicenters have been constrained by using P and S arrival times. Focal depths have been constrained by waveform modeling of regional and local depth phases. Most of the earthquakes occur in two areas along the western or eastern branch of the East African Rift. The first area is located between southern Lake Tanganyika and Lake Rukwa in the western branch, and the mechanisms in this region are characterized by strike-slip or oblique extension with nodal planes striking E-W to NW-SE. These events confirm that this part of the western rift is under transtension, as compared to the Lake Tanganyika and Rukwa rifts, where the stress field appears to be almost purely extensional. The second area for which we have obtained several new focal mechanisms is near the southern end of the eastern branch where the rift impinges on the eastern margin of the Tanzania craton. The focal mechanisms from this part of the rift have strike slip or normal motions but the orientations of the nodal planes are complex, consistent with the complicated orientation of the block faults. The focal depths for the events in the first area vary from 7 to 36 km, and in the second area, they vary from 11 to 34 km.
DE: 7230 Seismicity and seismotectonics
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting