The M7 2016 Kumamoto, Japan, Earthquake: Surface Strain in the Fault Damage Zone and Shallow Fault Slip Revealed with Near-Field Geodetic Imagery
Abstract
The differencing of high-resolution topography acquired before and after an earthquake measures the 3D deformation field along and adjacent to the surface rupture and fills a near-fault gap in geodetic data. Observations of the fresh surface rupture, on-fault slip, and the distributed off-fault deformation in the shallow crust facilitate understanding how processes active in a single earthquake influence fault zone structure over multiple earthquake cycles. Earthquake slip inversions constrained with near- and far-field displacements quantify coseismic slip throughout the seismogenic zone and are necessary to indicate mechanical controls on slip propagation and off fault strains.
Our work on the April 2016 M7 Kumamoto earthquake addresses these topics. The earthquake ruptured the Hinagu and Futagawa faults on the Kyushu Island of southwestern Japan with an oblique strike-slip mechanism and surface slip exceeding 2 m. Applying the Iterative Closest Point (ICP) algorithm to high resolution lidar topography, we measure the 3D surface displacement field at a 50 m spatial resolution. 36 ± 29% and 62 ± 32% of the horizontal and vertical deformation, respectively, was accommodated off the principal fault trace. Horizontal strains of up to 0.03 suggest that the approximate elastic strain limit was exceeded over a 250m width in many locations along the rupture. These observations illustrate the permanent damage to the fault zone accumulated during the single earthquake. We developed a lidar topography-optical-InSAR distributed-slip earthquake source inversion. The biggest challenge was developing a fault geometry consistent with complexities in the fault trace yet simple enough for the inversion to be computationally feasible. We calculate a Mw6.97 for the earthquake.The maximum slip of 5.6 m occurs between 2 and 4 km depth, and there is a 40% slip depletion at the surface. The missing slip along the primary fault surface is likely accommodated as distributed and off-fault deformation. Ultimately, slip models that include the complexity in the fault geometry and crustal rheology observed in near-fault imagery will push our understanding of how shallow fault zone structure and rock properties guide slip propagation.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T42B..05S
- Keywords:
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- 8155 Plate motions: general;
- TECTONOPHYSICSDE: 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICSDE: 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS