HR: 08:45h
AN: G21D-04    [Abstracts]
TI: Far-Reaching Transient Motions After Mojave Earthquakes Require Broad Mantle Flow Beneath a Strong Crust
AU: * Freed, A M
EM: freed@purdue.edu
AF: Dept. of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, United States
AU: Burgmann, R
EM: burgmann@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, Univ. of California, Berkeley, Berkeley, CA 94720, United States
AU: Herring, T A
EM: tah@mit.edu
AF: Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, United States
AB: The variation of strength with depth of continental lithosphere continues to be much debated: is a weak viscously deforming lower crustal layer sandwiched between strong upper-crustal and upper-mantle layers or is the upper mantle simply weaker than the lower crust? It also continues to be debated whether continental deformation at depth occurs along discrete, strain-weakened shear zones or is broadly distributed in viscously deforming layers. Because of the difficulty to directly determine viscoelastic strength and the degree of localization of deformation, there is no consensus on either issue. A useful approach for inferring the strength of the lithosphere is to utilize earthquakes as large rock deformation experiments where coseismic stress changes induce a variety of postseismic responses, including afterslip, poroelastic rebound, and viscoelastic relaxation. However, given the limited spatial and temporal resolution of postseismic observations, it has proven difficult to sort out the relative postseismic contributions of each mechanism, let alone to determine how viscosity varies as a function of depth. Here we utilize 7 years of observed postseismic transient surface displacements following the 1999 M7.1 Hector Mine quake in southern California as recorded at an extensive array of continuous GPS stations that demonstrate a previously unrecognized broad pattern of transient deformation throughout southern California and into Nevada, more than 200 km from the epicenter. This first of a kind far-field view (over 4 rupture lengths) of a postseismic deformation field following a strike-slip earthquake, allows us to much more uniquely determine the mechanism responsible for this broad deformation pattern. We use a 3-D viscoelastic finite element model of the region that incorporates both the Hector Mine rupture and the nearby 1992 M7.3 Landers earthquake that is still influencing transient displacements. We considered a wide range of possible viscoelastic structures ranging from viscosity being uniform with depth below 20 km depth, to structures where viscosity decreases rapidly with depth (as might be expected due to increasing temperatures). The best models are those where the viscosity below 40 km depth is an order of magnitude or more lower than the viscosity of mantle above and two orders of magnitude less than the viscosity of the lower crust. We are able to rule out significant contributions of localized afterslip below the seismogenic zone to far-field postseismic deformation, as well as a significant contributions from poroelastic rebound. Thus, we are able to determine that the pattern of broad postseismic transients following these Mojave Desert earthquakes can only be explained by viscoelastic flow in a region of the mantle 100s of km wide and below a depth of 40 km. This result enables two robust conclusions regarding the nature of lithospheric strength in this region to be reached: the mantle is weaker than the lower crust, and flow occurs over a wide region of mantle as opposed to within a narrow shear zone beneath the fault. Considering the broad region of mantle sampled by this study, it is possible that such a model may be appropriate for much of western North America.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Geodesy [G]
MN: 2007 Fall Meeting