Non-tectonic liquefaction-induced large surface displacements in the Aso Valley, Japan, caused by the 2016 Kumamoto earthquake, revealed by ALOS-2 SAR

Abstract

We constructed and analyzed full 3-D ground surface displacement field associated with the 2016 Kumamoto (Japan) earthquake using satellite radar images from ALOS-2. Displacements reflect not only tectonic crustal deformation caused by main earthquake faults but also non-tectonic surface deformations. The largest deformations in the earthquake sequence were found in the Aso Valley, along the NW outer rim of the Mt. Aso caldera. Large, independent surface deformations occurred in three areas with diameters of 500 m–2 km in the Aso Valley, and each area was horizontally displaced by more than 2 m to the NNW. These areas are underlain by thick lake-bottom deposits of saturated silt with low penetration resistance. As the direction of the displacements was parallel to the ground slope in each area, the strong seismic motion of the earthquake most probably induced liquefaction in the lake-bottom deposits and the ground surface slid horizontally along the slope on the liquefied silt. Because the areas of the large displacements are closely related to the thickness and shape of the lake-bottom deposits, amplification of the seismic wave in these deposits likely contributed to the liquefaction. A seismograph installed at the Aso Valley recorded co-seismic movement within several seconds of the main shock, indicating a possibility of the strong seismic motion amplifying the horizontal displacement of this area. On the SSE side of the deformed areas, numerous graben-like ruptures developed. As these ruptures are caused by surface tension during the large horizontal displacement, they are not earthquake fault traces and the process of this deformation is non-tectonic.