The devastating March 11, 2011 Tohoku-Oki earthquake and its tsunami caused more than 15,000 fatalities and severe damage in NE Japan. The densest geodetic and seismic networks in Japan made this earthquake the best-recorded event ever. Seafloor geodetic stations recorded over 10s of m horizontal displacements during the earthquake. High-resolution seismic bathymetry data indicate that the rupture reached the trench. Model inversion studies indicate a slip distribution of up to ~50 m over a remarkable compact area of 400 km along strike and 200 km wide. Coseismic and postseismic crustal deformation recorded at unprecedented high spatial and temporal resolution and precision allow the researchers to further their understanding of the rheological structure of the Earth and the subduction zone processes. In particular, the high quality geodetic data provide a unique opportunity to constrain the three-dimensional (3D) rheology of the upper mantle and lower crust and the evolution of transient slip on the megathrust. The scope of the project will be broad enough such that results will be applicable to other subduction margins (Sumatra, Chile, Alaska and Cascadia) where deformation is currently at various stages of the subduction earthquake cycle.
This project will examine the postseismic deformation of the 2011 Tohoku-Oki earthquake. In previous investigations, it has been a challenge to separate the contributions of afterslip on the megathrust from viscoelastic relaxation of the earthquake-induced stresses in the upper mantle. Effects of the complex 3D rheology of convergent margins on subduction zone earthquake deformation have yet to be better understood. In the proposed research, we seek to understand: (a) What is the distribution and evolution of the afterslip on the megathrust following a giant earthquake? (b) How does the 3D rheology of the Earth control the postseismic crustal deformation? (c) What are the rheological properties of the oceanic and continental upper mantle? In this work the researchers will benefit from access to the wealth of Global Positioning System (GPS) data recorded by the Japan national network as well as regional stations across East Asia. They will also constrain afterslip using repeating earthquakes. A 3D finite element code will incorporate complex subduction slab geometry and advanced mantle rheology in the Earth.
Postseismic deformation refers to transient motions observed that immediately follow very large earthquakes. Observing these motions is important because postseismic deformation provides us a rare opportunity to study how the Earth reacts to a large change in forces applied within it, and we can use this to study the mechanical properties of fault zones and the material that surrounds them. This in turn gives us a better understanding of the forces that drive plate tectonics and ultimately cause earthquakes. Despite getting a lot of attention over the last decade or two, many aspects of postseismic deformation remain mysterious and subject to disagreement. There are several reasons for that. Perhaps the biggest is that there are multiple physical mechanisms that can contribute to the postseismic signal, and for each mechanism there are some parameters that must be determined empirically. This sets up tradeoffs between the mechanisms, and all of the physical models must be correct before we can get reliable estimates of any of the parameters. To make things more interesting, some second-order effects can be important and ignoring them can further bias our inferences on parameter values or even which physical mechanisms are important. We used displacements of GPS sites in Japan and the surrounding area to study postseismic deformation following the 2001 earthquake off of northern Japan. This work has focused on whether rheological variations and effects like poroelastic relaxation cause significant impacts on the postseismic models inferred for this earthquake. Our work thus far has put bounds on the size of these impacts, and suggests that some of these variations (in particular, the presence of a low viscosity zone beneath the volcanic arc) improve the fit to the data significantly and must be considered in deriving the optimal postseismic model. We are now finishing an evaluation of how this impacts our estimate of the best postseismic model. This work has supported the postdoctoral studies of Dr. Yan Hu, a postdoctoral researcher at UC Berkeley. This gives Dr. Hu the opportunity to develop his record of research accomplishments and continue his academic career while he seeks a full-time permanent position. The work will also contribute to the understanding of the earthquake cycle and earthquakes in Japan and other subduction zones.
