Abstract
We identified and analyzed surface displacements associated with the 2018 Hokkaido Eastern Iburi Earthquake in northern Japan using satellite radar interferograms from the Advanced Land Observing Satellite 2. The data generally show elastic deformation caused by the main earthquake as well as numerous complex surface displacements that cannot be explained by the motion of the seismic source fault. We identified three distinct phenomena: linear surface displacements representing secondary earthquake faults west of the epicenter, surface deformation caused by liquefaction in urban and coastal areas, and coherence changes in the interferograms due to landslides in mountainous areas and liquefaction in urban areas. The linear surface displacements show reverse fault motion with low dip angles and appear to be a geographic extension of known active faults; however, it is unlikely that these displacements were directly connected to the source fault of the main earthquake. Although there is no evidence that they generated strong seismic waves at the time of the main earthquake, there is a possibility that they represent active fault traces and could be the sources of large earthquakes in the future. Therefore, such linear surface displacements can be used to identify potentially dangerous hidden active faults. The interferograms reveal that liquefaction in urban areas occurred in low areas artificially filled during past residential development. Coherence-change maps drawn from the interferograms were useful for detecting liquefaction, but their high sensitivity limited their application for landslide detection in mountainous areas; the phase noise deviation method was more practical for purposes such as rapid response or mitigation. Our methods have the potential to allow improved mapping of local hazards in other areas and can be applied to urban planning and/or safety assessments.
Highlights
The 2018 Hokkaido Eastern Iburi Earthquake (Mw 6.6), which occurred on September 6, 2018, in northern Japan, caused crustal deformation along with a large number of landslides and sediment-related disasters; the depth of the main earthquake’s hypocenter was ~ 35 km (Earthquake Research Committee 2018) and no exposed fault has been identified
We used satellite synthetic aperture radar (SAR) interferometry to identify and interpret surface displacements associated with the 2018 Hokkaido Eastern Iburi Earthquake with the following conclusions: (1) Linear surface displacements appeared west of the epicenter
(2) The linear surface displacements were located along the geographic extension of known active faults; structural discontinuities in the basement were found just beneath them by previous seismic exploration
Summary
The 2018 Hokkaido Eastern Iburi Earthquake (Mw 6.6), which occurred on September 6, 2018, in northern Japan, caused crustal deformation along with a large number of landslides and sediment-related disasters; the depth of the main earthquake’s hypocenter (obtained from the seismic wave) was ~ 35 km (Earthquake Research Committee 2018) and no exposed fault has been identified. ALOS-2 has two main advantages for observations of such surface displacement: the wavelength of its emitted microwaves and its capability to observe multiple off-nadir angles. With regard to the former, L-band SAR interferometry. (InSAR) is generally advantageous for detecting surface displacements, even in vegetated areas, owing to its high coherence as compared with C- or X-band microwaves (Morishita and Hanssen 2015; Rosen et al 1996). This capability enables the capture of widespread crustal deformation, even in vegetated mountainous areas. ALOS-2 InSAR data are an effective tool for estimating the displacement and other surface phenomena caused by earthquakes
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