The Geodetic Signature of the Earthquake Cycle at Subduction Zones: Model Constraints on the Deep Processes

Abstract

Recent megathrust events in Tohoku (Japan), Maule (Chile), and Sumatra (Indonesia) were well recorded. Much has been learned about the dominant physical processes in megathrust zones: (partial) locking of the plate interface, detailed coseismic slip, relocking, afterslip, viscoelastic mantle relaxation, and interseismic loading. These and older observations show complex spatial and temporal patterns in crustal deformation and displacement, and significant differences among different margins. A key question is whether these differences reflect variations in the underlying processes, like differences in locking, or the margin geometry, or whether they are a consequence of the stage in the earthquake cycle of the margin. Quantitative models can connect these plate boundary processes to surficial and far‐field observations. We use relatively simple, cyclic geodynamic models to isolate the first‐order geodetic signature of the megathrust cycle. Coseismic and subsequent slip on the subduction interface is dynamically (and consistently) driven. A review of global preseismic, coseismic, and postseismic geodetic observations, and of their fit to the model predictions, indicates that similar physical processes are active at different margins. Most of the observed variability between the individual margins appears to be controlled by their different stages in the earthquake cycle. The modeling results also provide a possible explanation for observations of tensile faulting aftershocks and tensile cracking of the overriding plate, which are puzzling in the context of convergence/compression. From the inversion of our synthetic GNSS velocities we find that geodetic observations may incorrectly suggest weak locking of some margins, for example, the west Aleutian margin.

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

“…Geological information bridging the seismic cycle with a fraction of Myr (e.g., 10 2 to 10 5 yr) may come from systematic investigation of terrace uplift (e.g., SAmerica: Bookhagen et al, 2006;Saillard et al, 2009) and long-term trends in mechanical coupling (e.g., across the Andes: Oncken et al, 2006). Geodynamic modelling (e.g., impact of plateau and fore-arc uplift: Vogt and Gerya, 2014a;Sobolev and Muldashev, 2017;Govers et al, 2018) may test which conditions and rheologies, in the context of steady-statelong-term coupling with recurrent seismicity, trigger detachment instabilities on the 10 5 to 10 6 yr scale.…”

The Accumulation of Slip Deficit in Subduction Zones in the Absence of Mechanical Coupling: Implications for the Behavior of Megathrust Earthquakes

“…Three‐dimensional meshes are generated with Gmsh (Geuzaine & Remacle, ) and convergence tests demonstrate that our results are insensitive to further mesh refinement. We use the finite element modeling platform GTECTON (version 2017.1; Govers & Wortel, ; Govers & Wortel, ; Govers et al, ) compiled with PETSc 3.4.2 (http://www.mcs.anl.gov/petsc) to solve the static mechanical equilibrium equations.…”

“…Threedimensional meshes are generated with Gmsh (Geuzaine & Remacle, 2009) and convergence tests demonstrate that our results are insensitive to further mesh refinement. We use the finite element modeling platform GTECTON (version 2017.1; Govers & Wortel, 1993;Govers & Wortel, 2005;Govers et al, 2018) Two types of boundary conditions are imposed on the plate interface: locked (i.e., zero slip allowed) and unlocked (i.e., free to slide with no resistance). We define locked sections as rectangular areas on the megathrust with one side oriented parallel to strike (the along-strike dimension is the fault length) and the other along dip (the fault width) for straightforward comparison with the solutions of Okada (1992; Figure 2).…”

Precursory Stress Changes and Fault Dilation Lead to Fault Rupture: Insights From Discrete Element Simulations

“…Subduction zone earthquake cycle modeling (Avouac et al, 2015;Govers et al, 2018) provides useful insights into many phenomena that are observed seismologically and geodetically, however are often oversimplified by a planar fault surface. Numerical models allow for the simultaneous monitoring of stresses and kinematics, and therefore serve as a useful tool to investigate potential earthquake precursors.…”

“…Numerical models allow for the simultaneous monitoring of stresses and kinematics, and therefore serve as a useful tool to investigate potential earthquake precursors. Subduction zone earthquake cycle modeling (Avouac et al, 2015;Govers et al, 2018) provides useful insights into many phenomena that are observed seismologically and geodetically, however are often oversimplified by a planar fault surface. Particle dynamics models, such as the discrete element method (DEM), offer a unique approach, wherein particle assemblages interact according to simplified contact laws, yielding complex kinematic and mechanical behaviors within granular, heterogeneous media.…”