Dynamic links between short-term deformation and long-term tectonics: a finite element study
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Geodetic and geological data for crustal deformation are significantly different in many places. This discrepancy may indicate different strain rates in the geological past, or simply reflect the timescale-dependent behavior of fault movement. By using 2-D and 3-D finite element models with viscoelastoplastic rheology, I have explored the spatial-temporal strain and stress evolution during seismic cycles, and linked these results to long-term tectonics. In the central Andes, I found that much of the GPS-measured crustal shortening can be explained by interseismic and postseismic viscoelastic strains; the apparently discordant GPS and geological data can be reconciled in the geodynamic model with a weakened crust in the Subandes, which allows accumulation of plastic strain that leads to mountain building. In Cascadia, I showed that the critical condition for producing permanent crustal shortening, hence mountain building, is for the plastic yield strength of the plate interface to be higher than that of the overriding plate. Strong trench coupling and a weak lithosphere explain the Andean mountain building, whereas weak trench coupling in Cascadia allows short-term crustal shortening to be restored periodically by trench earthquakes and aseismic slips. In eastern Tibetan Plateau, I found that Coulomb stress change pattern is affected by including previous major earthquakes in the model; interseismic locking on the Xianshuihe fault can increase extra Coulomb stress on the Longmen Shan fault up to ~50 Pa/yr, and hence ~0.001 MPa since last big event in 1981 on the Xianshuihe fault.
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