Abstract
On 15 November 2017, liquefaction phenomena were observed around the epicenter after a 5.4 magnitude earthquake occurred in Pohang in southeast Korea. In this study, we attempted to detect areas of sudden water content increase by using SAR (synthetic aperture radar) and optical satellite images. We analyzed coherence changes using Sentinel-1 SAR coseismic image pairs and analyzed NDWI (normalized difference water index) changes using Landsat 8 and Sentinel-2 optical satellite images from before and after the earthquake. Coherence analysis showed no liquefaction-induced surface changes. The NDWI time series analysis models using Landsat 8 and Sentinel-2 optical images confirmed liquefaction phenomena close to the epicenter but could not detect liquefaction phenomena far from the epicenter. We proposed and evaluated the TDLI (temporal difference liquefaction index), which uses only one SWIR (short-wave infrared) band at 2200 nm, which is sensitive to soil moisture content. The Sentinel-2 TDLI was most consistent with field observations where sand blow from liquefaction was confirmed. We found that Sentinel-2, with its relatively shorter revisit period compared to that of Landsat 8 (5 days vs. 16 days), was more effective for detecting traces of short-lived liquefaction phenomena on the surface. The Sentinel-2 TDLI could help facilitate rapid investigations and responses to liquefaction damage.
Highlights
On 15 November 2017, a magnitude 5.4 earthquake occurred in Pohang in southeast Korea (Figure 1)
It was concluded that the change in surface water content due to liquefaction indicated on farmland did not greatly affect the decorrelation
Conventional NDWI techniques have been developed for the detection of water in vegetation or open water, which is limited to detecting changes to surfaces that underwent liquefaction
Summary
On 15 November 2017, a magnitude 5.4 earthquake occurred in Pohang in southeast Korea (Figure 1). Previous studies used satellite and unmanned aerial photographs to investigate various surface deformation phenomena, prior to field surveys, after the Pohang earthquake. They confirmed secondary ground deformations, such as ground cracks and rock falls around the epicenter, as well as about 600 sand blows related to soil liquefaction (Figure 1). Optical satellite images have a relatively high spatial resolution compared to SAR images and can directly detect liquefaction phenomena caused by the earthquake. This study evaluates the use of optical and SAR images for detecting earth surface liquefaction caused by the Pohang earthquake, as well as the performance of analytical techniques. No seismic liquefaction studies have used Sentinel-2 data at the time of publication
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