10:45 AM - 11:00 AM
[SSS16-07] Surface Deformation before and after the 2016 Kumamoto Earthquake Detected by SAR
Keywords:ALOS-1/2, Kumamoto Earthquake, postseismic deformation, SAR, Sentinel-1, Surface deformation
We applied time series analysis to ALOS-1 images during 2007 - 2011 with StaMPS. However, we could not detect significant deformation around the Futagawa-Hinagu faults and in the Kumamoto plain.
For the purpose of study of postseismic deformation, we collected ALOS-2 and Sentinel-1 images that were acquired from both descending and ascending orbits till the end of 2018, and analyzed them with Gamma(R) software. We detect at least 6 spots of increase of Line of sight (LOS) in and around the active fault traces in ALOS-2 descending interferograms. Three of them are located close to surface traces of the Futagawa-Hinagu faults or their extension. The maximum LOS change exceeds 10 cm till August 2018. This deformation is much larger than postseismic ddisplacements observed with GNSS after other inland earthquakes in Japan. Converting LOS changes into quasi-EW and vertical components, we found that westward movement is dominant on the SE side of the Futagawa-Hinagu faults.
We also notice a zone of LOS increase of 6 cm or more that extends NW-ward in the Kumamoto city from the junction of Futagawa and Hinagu faults. We observed coseismic subsidence in the same area in coseismic interferograms and there are several reports of small surface ruptures. Considering the location and timing, this off-fault LOS increase may be related to stress release by this event. The fifth one is in northern part of the Aso caldera, which also exceeds 6 cm. The last is found in the Kikuchi city, more than 10 km north of the Futagawa fault. This zone is extended in the direction of WNW-ESE along a channel.
Sentinel-1 interferograms also show similar distribution of displacements in spite of low coherence.
Changes in LOS increase across the surface fault traces are so sharp and nonsymmetrical. It may be difficult to explain all these deformations with not only afterslip on a couple of faults. This area is covered with thick pyroclastic flow and marine or non-marine deposits. Because there may be a weak correlation between the distribution of these deposits and postseismic deformation, subsurface heterogeneous structure may affect spatial distribution of postseismic deformation.