THE M7 2016 KUMAMOTO, JAPAN, EARTHQUAKE: 3D COSEISMIC DEFORMATION FROM DIFFERENTIAL TOPOGRAPHY (Invited Presentation)

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 that 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. These results offer unusual insight into the fault zone geometry, coseismic slip, and the mechanical and rheological 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 lidar data with an average shot density of 3 points/m². We develop an approach for computing 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 as it propagated from the SW to the NE. We calculate differential ICP displacements that straddle the fault zone as a metric for fault slip in the upper tens of meters of the crust. This analysis suggests an elliptical along-strike slip envelope with a maximum slip that exceeds 2 m in the shallow crust several kilometers from the Kumamoto airport. We compare the differential ICP displacements to published fault slip measurements made in the field. The slip values from these two datasets are similar along fault segments with higher slip, yet the ICP displacements exceed the field measurements where slip is lower. This is consistent with a magnitude- dependent depletion of fault slip in the upper tens of meters of the crust, which is indicative of the fault processes that control the transmission of fault slip from the very shallow crust to the Earth’s 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. Likely, the depleted fault slip is accommodated as off-fault folding and fracturing.