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Loveless, Jack

Interseismic deformation in Japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces. We use spherical linear block theory constrained by geodetic observations from densely spaced Global Positioning System (GPS) stations to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Japan-Kuril, Sagami, and Nankai subduction zones. The reference model developed in this paper consists of 20 blocks, produces a mean residual velocity magnitude of 1.84 mm/yr at 950 stations, and accounts for 96% of the observed interseismic deformation signal. We estimate fault slip rates in excess of 15 mm/yr along the Niigata-Kobe Tectonic Zone and Itoigawa-Shizuoka Tectonic Line through central Japan, confirming their hypothesized roles as major tectonic boundaries. Oblique convergence across the Nankai Trough is partitioned, with 3/4 of the ∼30 mm/yr of trench-parallel motion accommodated by strike-slip motion on the subduction interface and the remaining 1/4 accommodated by right-lateral slip on the Median Tectonic Line. In contrast, our models suggest negligible slip partitioning in eastern Hokkaido, where oblique slip on the Japan-Kuril subduction interface accommodates all of the trench-parallel component of relative plate motion. Inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes.

Geodetic observations of interseismic deformation provide constraints on the partitioning of fault slip across plate boundary zones, the spatial distribution of both elastic and inelastic strain accumulation, and the nature of the fault system evolution. Here we describe linear block theory, which decomposes surface velocity fields into four components: (1) plate rotations, (2) elastic deformation from faults with kinematically consistent slip rates, (3) elastic deformation from faults with spatially variable coupling, and (4) homogeneous intrablock strain. Elastic deformation rates are computed for each fault segment in a homogeneous elastic half-space using multiple optimal planar Cartesian coordinate systems to minimize areal distortion and triangular dislocation elements to accurately represent complex fault system geometry. Block motions, fault-slip rates, elastic coupling, and internal block strain rates are determined simultaneously using a linear estimator with constraints from both geodetically determined velocity fields and geologic fault-slip rate estimates. We also introduce algorithms for efficiently implementing alternative fault-network geometries to quantify parameter sensitivity to nonlinear perturbations in model geometry.

Geodetic data from the Global Navigation Satellite System (GNSS), and from satellite interferometric radar (InSAR) are revolutionizing how we look at instantaneous tectonic deformation, but the significance for long-term finite strain in orogenic belts is less clear. We review two different ways of analyzing geodetic data: velocity gradient fields from which one can extract strain, dilatation, and rotation rate, and elastic block modeling, which assumes that deformation is not continuous but occurs primarily on networks of interconnected faults separating quasi-rigid blocks. These methods are complementary: velocity gradients are purely kinematic and yield information about regional deformation; the calculation does not take into account either faults or rigid blocks but, where GNSS data are dense enough, active fault zones and stable blocks emerge naturally in the solution. Block modeling integrates known structural geometry with idealized earthquake cycle models to predict slip rates on active faults. Future technological advances should overcome many of today's uncertainties and provide rich new data to mine by providing denser, more uniform, and temporally continuous observations.

Elastic dislocation models of geodetic measurements above subduction zones have led to the identification of MW ≈ 6.0–7.2 slow slip events (SSEs) that release elastic strain over periods of days to months, but great (MW ≥ 8) SSEs have remained unidentified. We extrapolate observations of SSE duration and slip magnitude to show that slip velocity decreases with event magnitude and predict that the slip velocity of MW ≥ 8 SSEs is ≤50 mm/yr. The slip velocity for great SSEs may never exceed the plate convergence rate and thus never produce a reversal in trench perpendicular motion. Instead, geodetically constrained estimates of apparent partial elastic coupling on subduction zone interfaces worldwide may be direct observations of ongoing MW ≥ 8 silent earthquakes with durations of decades to centuries.