The 16 April earthquake ruptured a northern part of the Hinagu fault and the Futagawa fault and extended northeast to a previously unknown 5-km-long NE-trending strike-slip fault within the Aso Caldera. The 16 April 2016 Mw = 7.0 (Mjma = 7.3) Kumamoto earthquake struck the cities of Kumamoto, Mashiki, Nishihara and Minami-Aso from the Kumamoto plain to the western edge of Aso Caldera in central Kyushu, and brought significant damage to buildings, killing 50 people (Fig. It provides an important insight into scale- and depth-dependent stress heterogeneity and an implication to a proper estimate of seismic hazard in complex and broad multiple fault strands. The Kumamoto case, with detailed geological observations and geophysical models, would be the second significant slip-partitioned earthquake around the globe. Our simple dislocation model demonstrates that this bifurcation into pure strike-slip and normal faults likely occurs for optimally oriented failure near the surface. Together with geodetic and seismic inversions of subsurface fault slip, we present a schematic structural model where oblique motion occurred on a northwest-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. The locations and slip motions of the 2016 rupture are also manifested as interferogram fringe offsets in InSAR images. The maximum amount of coseismic throw on the Idenokuchi fault is ~2 m, which is nearly equivalent to the maximum slip on the strike-slip rupture. One of the noteworthy features we observed in the field are ~10-km-long segmented normal fault scarps, dipping to the northwest, along the previously mapped Idenokuchi fault, 1.2–2.0 km south of and subparallel to the Futagawa fault. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic slip. This included a previously unknown 5-km-long fault within the Aso Caldera, central Kyushu. As opposed to the idealized linear movement of a strike-slip fault, the team demonstrated what geologists knew in theory, that shear strain has several stages before final movement along the fault.An ENE-trending ~30-km-long surface rupture emerged during the Mw = 7.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu faults. This is a similar trait we see in many systems on Earth, from rivers finding the easiest path to lower elevations to mammals taking the easiest path from point A to point B.Īs the faults propagate, the team measured how strain is transferred to different parts of the fault, a process that in real life takes millions of years and across many miles. The team found that the faults develop through a "Lazy Earth" hypothesis, whereby the fault propagation takes the easiest path. In one boundary condition, there is a pre-existing fault along the two slabs, in another there is a pre-existing displacement beneath the clay slabs, and in the last example the displacement is in a wider shear zone.Īfter the models were setup, the team moved the two clay slabs in opposite directions in order to measure minute changes as the strike-slip faults developed. After creating two slabs of this kaolin clay, the team setup several boundary conditions on which to test the development of the strike-slip fault. The team made sure the length to depth was scaled appropriately to mimic that on Earth and with the correct viscosity. Hatem built a miniature model of the Earth's crust using kaolin clay.
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