HR: 15:25h
AN: T13D-08 INVITED     [Abstracts]
TI: Combined Study of the 1992 Landers, 1999 Hector Mine, and 2002 Denali Postseismic Deformations: In Search of a Common Lithospheric Rheology
AU: * Freed, A M
EM: freed@purdue.edu
AF: Dept. of Earth & Atmospheric Sciences, Purdue Univeristy, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Burgmann, R
EM: burgmann@seismo.berkeley.edu
AF: Dept. of Earth & Planetary Science, Univ. of California, Berkeley, 389 McCone Hall, Berkeley, CA 94720 United States
AB: Large earthquakes represent rock deformation experiments in which sudden stress changes trigger observable postseismic surface deformations that can be used to infer rheological properties of the lithosphere. By using geodetic surface measurements as constraints on finite element models, such experiments seek to understand the relative contributions of postseismic viscous flow, poroelastic rebound, and afterslip, the prime candidate processes of postseismic relaxation. The major challenge in such studies is that of non-uniqueness. All three mechanisms can induce similar surface deformations in certain regions making it difficult to distinguish which mechanism is truly dominant. This is especially true if the quantity and quality of observable constraints is limited, as is often the case even for events with reasonable GPS coverage. The effort of finding a unique postseismic solution is also complicated by the fact that formally optimized afterslip models, which lack physical constraints, can usually be found to explain most horizontal GPS data sets. The key toward uniqueness in postseismic solutions lies in improving the number of constraints available for a study and in recognizing key diagnostic features in the spatial or temporal patterns of these data. GPS and InSAR data sets can be combined, but often one of these is not available or of poor quality. One can also take advantage of earthquake sequences, such as the 1992 Landers/1999 Hector Mine sequence, in that a single rheologic model must be able to explain postseismic surface deformations following both events, as their close proximity implies that they share the same lithosphere. However, geodetically observed earthquake sequences are rare. We investigate a new approach to postseismic studies, that of considering multiple, unrelated earthquakes in a single parameter study. We hypothesize that the lithosphere is not as heterogeneous as generally assumed. For example, it is thought that lower crustal flow is dominant in some regions after an earthquake, deeper mantle flow in others. We theorize that such findings are potentially the result of analysis bias, as many postseismic analyses do not consider all potential depths of flow or mechanisms. We suspect that in a robust study one might find that most postseismic responses to earthquakes are controlled by the same mechanism (or combination of mechanisms) operating at consistent depths with similar stress dependence, though varying according to local heat flow, crustal thickness, and lithology. We begin to test this hypothesis by attempting to explain postseismic deformation associated with the 1992 Landers, 1999 Hector Mine, and 2002 Denali earthquakes with similar mechanical models, for example a combination of powerlaw flow in the deeper lithospheric mantle (50 to 100 km deep) and shallow poroelastic rebound. Finding a consistent mechanical model for all three earthquakes may indicate that the lithosphere response relies on the same mechanisms and occurs in a similar mode for different events subject to different tectonic domains. Such a finding would spur further testing by adding additional earthquakes to the study. On the other hand, failure to find a consistent mechanical model to explain all three postseismic events will serve to highlight gross heterogeneities in lithospheric rheology.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3902 Creep and deformation
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting