The M7 2016 Kumamoto, Japan, Earthquake: 3D coseismic deformation from differential topography

Chelsea P. Scott, Ramon Arrowsmith, Lia J. Lajoie, Ed Nissen, Tadashi Maruyama, & Chiba Tatsuro

Submitted August 15, 2017, SCEC Contribution #7781, 2017 SCEC Annual Meeting Poster #099

The April 2016 Kumamoto earthquake sequence ruptured the Hinagu and Futagawa faults on the Kyushu Island of SW Japan. Our work concentrates on the M7 mainshock, which ruptured 30 km of fault length with an oblique strike-slip mechanism. We present 3D near-fault displacements determined from the differencing of high-resolution topography acquired before and after the mainshock. We jointly invert differential topography and InSAR displacements for distributed fault slip to examine the mechanical properties of the shallow fault zone.

Differential topography provides spatially dense constraints on the near-field coseismic deformation, which complement field measurements of fault slip and far-field InSAR displacements. We use a windowed implementation of the Iterative Closest Point (ICP) algorithm to calculate the full 3D coseismic displacement field with a spatial resolution of 50 m from airborne lidar data with an average shot density of 3 points/m2. We develop an approach for computing the displacement uncertainty, and demonstrate that the performance of the ICP method depends on the local topographic roughness. Errors range from 10 cm over higher relief and forested areas to 25 cm over relatively flat agricultural lands.

The Kumamoto earthquake ruptured several fault strands with a maximum slip of 2.2 m. The distributed off-fault deformation concentrated within 1 km of the fault trace suggests a shallow depletion of fault slip. We examine two approaches for quantifying depth-dependent variations in slip. First, we calculate differential ICP displacements at increasing apertures across the fault. Second, we conduct a joint-inversion of the differential topography and InSAR displacements for fault geometry and distributed slip. Both approaches suggest that fault slip is depleted in the upper 100 m of the crust relative to 500 m depth. A comparison of the modeled slip to published field measurements suggests an additional depletion of slip from 100 m depth to the surface. The high surface strains of 1 to 4% in the ~100 m surrounding the fault suggest that the elastic strength of rocks is exceeded in the fault volume and that the depleted fault slip is accommodated by off-fault folding and fracturing at a finer scale than our windowed analysis. Differential topography and InSAR data can resolve slip throughout the seismogenic zone and are indicative of the physical processes that control the transmission of slip from the shallow crust to the surface.

Key Words
Earthquake deformation, differential topography, InSAR

Citation
Scott, C. P., Arrowsmith, R., Lajoie, L. J., Nissen, E., Maruyama, T., & Tatsuro, C. (2017, 08). The M7 2016 Kumamoto, Japan, Earthquake: 3D coseismic deformation from differential topography. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Tectonic Geodesy