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Crustal Deformation During Co- and Postseismic Phases of the Earthquake Cycle Inferred from Geodetic and Seismic Data

  • Author(s): Huang, Mong-Han
  • Advisor(s): Bürgmann, Roland
  • et al.
Abstract

The work presented in my dissertation focuses on the crustal deformation during the co- and postseismic periods in earthquake cycles. I use geodetic and seismic data to constrain and better understand the behavior of the earthquake source during the coseismic period. For the postseismic period, I use geodetic data to observe the surface displacements from centimeter-scale to millimeter-scale from an Mw 7.9 and Mw 6.9 event, respectively. I model different mechanisms to explain the postseismic deformation and to further constrain the crustal and upper mantle rheology.

For the coseismic earthquake source study, I explore the source of the 2010 Mw 6.3 Jia-Shian, Taiwan earthquake. I develop finite-source models using a combination of seismic data (strong motion and broadband) and geodetic data (InSAR and GPS) to understand the rupture process and slip distribution of this event. The main shock is a thrust event with a small left-lateral component. Both the main shock and aftershocks are located in a transition zone where the depth of seismicity and an inferred regional basal detachment increases from central to southern Taiwan. The depth of this event and the orientation of its compressional axis suggest that this event involves the reactivation of a deep and weak pre-existing NW-SE geological structure.

The 1989 Mw 6.9 Loma Prieta earthquake provides the first opportunity since the 1906 San Francisco (Mw 7.9) earthquake to study postseismic relaxation processes and estimate rheological parameters in the region with modern space geodetic tools. The first five years postseismic displacements can be interpreted to be due to aseismic right-oblique fault slip on or near the coseismic rupture, as well as thrusting up-dip of the rupture within the Foothills thrust belt. However, continuing transient surface displacements (≤ 5 mm/yr) until 2002 revealed by PSInSAR and GPS in the northern Santa Cruz Mountains may indicate a longer-term postseismic deformation. I model the viscoelastic relaxation of the lower crust and upper mantle following the Loma Prieta earthquake to explain the surface displacement. A 14-km-thick lower crust (16 - 30 km depth) viscosity of > 1019 Pa s and an upper mantle viscosity of ~1018 Pa s best explain the geodetic data. The weak upper mantle viscosity in this area is in good agreement with upper mantle rheology in southern California (0.46 - 5 × 1019 Pa s) using a similar approach from studying the postseismic deformation following the 1999 (Mw 7.1) Hector Mine earthquake.

Periods of accelerated postseismic deformation following large earthquakes reflect the response of the Earth's lithosphere to sudden coseismic stress changes. I investigate postseismic displacements following the 2008 Wenchuan (Mw 7.9), China earthquake in eastern Tibet and probe the differences in rheological properties across the edge of the Tibetan Plateau. Based on nearly two years of GPS and InSAR measurements, I find that the shallow afterslip on the Beichuan Fault can explain the near-field displacements, and the far-field displacements can be explained by a viscoelastic lower crust beneath Tibet with an initial effective viscosity of 4.4 × 1017 Pa s and a long-term viscosity of 1018 Pa s. On the other hand, the Sichuan Basin block has a high-viscosity upper mantle (> 1020 Pa s) underlying an elastic 35-km-thick crust. The inferred strong contrast in lithospheric rheologies between the Tibetan Plateau and the Sichuan Basin is consistent with models of ductile lower crustal flow that predict maximum topographic gradients across the Plateau margins where viscosity differences are greatest.

With additional 6-year-long continuous GPS measurements deployed in the eastern Tibetan Plateau and the Sichuan Basin, viscoelastic relaxation models with the same geometry setups suggests Tibetan lower crust with an initial effective viscosity of 9 × 1017 Pa s and steady-state viscosity of 1019 Pa s. I also use the laboratory experiments derived power law flow model to fit the postseismic deformation. The viscosity estimated from this model varies with material parameters (e.g. grain size, water content, etc.) as well as environmental parameters (temperature, pressure, background strain rate, etc.). The diffusion creep refers to the power law flow mainly controlled by the mineral grain size, and the dislocation creep refers to it mainly controlled by the background stress level.For a diffusion creep type of power law flow, a Tibetan crust composed of wet feldspar (water content = 1000 H/106Si; grain size = 1 - 4 mm) and upper mantle composed of wet olivine (water content = 200 H/106Si; grain size = ~2 mm) can predict the 6-year-long poseismic time series well. This result roughly agrees with rock mechanics laboratory experiments.The channel flow model predicts the plateau margins are steepest where the viscosity of the surrounding blocks are highest. The low viscosity in the Tibetan lower crust and the contrasting rheology across the plateau margin derived from postseismic deformation are consistent with the channel flow model.

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