HR: 0800h
AN: S51C-1023    [Abstracts]
TI: Earthquakes Generated From Bedding-Plane Reverse Faults of the Seattle fault Zone: the Roof Thrust is not Passive
AU: * Kelsey, H M
EM: hmk1@humboldt.edu
AF: Dept. of Geology, Humboldt State University, Arcata, CA 95521 United States
AU: Sherrod, B
EM: bsherrod@ess.washington.edu
AF: U. S. Geological Survey, Dept. of Earth and Space Sciences, Box 351310, University of Washington, Seattle, WA 98195 United States
AB: The north-side-up reverse faults that splay off the roof thrust of the Seattle fault zone are seismogenic on their own because detailed investigations of uplifted wave-cut platforms demonstrate that the splay faults generate localized abrupt uplift at times when there was no regional coseismic uplift of Puget lowland shorelines. A critical question for understanding the seismic behavior of the Seattle fault zone is how the north-side-up reverse faults interact with the roof thrust/floor thrust. Brocher et al. (2004) suggest that the Seattle fault zone is a passive roof duplex (a 'wedge thrust') with the north-side-up reverse faults being passive splay faults off the roof of the wedge thrust. Undoubtedly a regional uplift, such as that of 1,100 years ago, involved an earthquake that nucleated at depth involving slip on both the floor and the roof thrusts. Such an earthquake triggered slip on some of the north-side-up, bedding plane reverse faults that splay off the roof thrust. At critical sites where the north-side-up reverse faults intersect the regionally uplifted 1,100 year old shore platform, we observe that the 1,100 year ago earthquake regionally uplifted the entire shoreline of the Seattle uplift but that an earthquake only hundreds of years earlier only uplifted limited areas immediately (within hundreds of meters) north of the reverse faults. We infer that the Seattle fault zone can both rupture in earthquakes that produce regional uplift (with or without surface displacement on the reverse splay faults) and produce earthquakes that rupture the splay faults with no rupture of the underlying floor thrust and little or no rupture of the roof thrust. Therefore, the roof of the wedge thrust is not passive as originally proposed but rather can be a point of nucleation of splay fault rupture that propagates to the surface on bedding planes. Our results thus imply that flexural slip faults along bedding planes can be rooted in deeper low angle structures and that these splay faults can release strain in earthquakes at times that the deeper faults do not.
DE: 7221 Paleoseismology (8036)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005