HR: 0800h
AN: T11C-1267    [Abstracts]
TI: Localized weak zone and their role in generation of intraplate seismicity: Effects of weak zone geometry and rheology
AU: * JOSHI, A
EM: ajosh2@uky.edu
AF: University of Kentucky, Department of Mechanical Engineering, 101 Slone Building, Lexington, KY 40506 United States
AU: Kenner, S J
EM: shelley.kenner@uky.edu
AF: University of Kentucky, Department of Geological Sciences, 101 Slone Building, Lexington, KY 40506 United States
AB: In intraplate seismic zones (e.g. the New Madrid Seismic Zone, NMSZ, in the south-central United States) the source of stress that drives earthquake generation is very complex. One hypothesis proposes that geologically transient, short-lived bursts of earthquakes lasting 10's of thousands of years may results from perturbations of the local or regional stress field. This causes relaxation of a lower crustal weak zone which may drive repeated earthquakes. The number of earthquakes occurring in a given time is dependent on the geometry and rheology of the weak zone. In this study, we investigate the relevant parameter space as it affects the concentration of stress at the base of the seismogenic fault. Using finite element techniques which employ contact surfaces to model discrete faulting events and a maximum shear stress criteria evaluated at each node, variable stress drops are also considered. We find that model parameters representing the geometry and rheology (Maxwell, standard linear solid, and power-law) of weak zone are particularly important. Other parameters that are considered include earthquake stress drop, background tectonic stress, and maximum shear stress at failure. Results show that solutions are non-unique. With the addition of existing geological evidence, however, we can place some constraints on range of parameters which satisfy observations from the NMSZ. Initially, 2D antiplane models have been run to efficiently identify trends in the parameter space. This information is then used to guide the generation of 3D models, which are numerically costly but geologically more realistic. Some of the trends found using the 2D models as follows. When higher power-law exponents are used, the frequency of earthquakes is initially very high and the weak zone reaches a fully relaxed state relatively quickly. Thus, higher power-law exponents produce higher energy dissipation rates. As weak zone width increases, the total number of earthquakes occurring before the weak zone is totally relaxed also increases. There is an increase in the total time required for full relaxation as well. Using power-law rheologies in 2D, we can have 10-12 earthquakes in weak zones as narrow as 3 km. 3D models using the same weak zone width yield 5-6 earthquakes in first 3000 years. The duration of this earthquake sequence can be expanded by increasing the weak zone width. Thus, findings in 2D can be used to limit the number of numerically expensive 3D models which must be run to successfully define a parameter space which satisfies observations from the NMSZ.
DE: 7230 Seismicity and seismotectonics
DE: 8160 Rheology--general
DE: 8168 Stresses--general
DE: 7209 Earthquake dynamics and mechanics
DE: 3210 Modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting