Pohang Earthquake (15 Nov 2017 Mw 5.4) observed by Sentinel-1A/B SAR Interferometry


Co-seismic Descending DInSAR (looking from east to west)

Figure 1. DInSAR Image. One cycle of RGB means 2.8 cm increase of range. Sentinel-1B was descending and right-looking.


Figure 2. LOS (Line Of Sight) displacement map. Positive values indicate uprising/easting of the land surface while negative values are subsidence/westing.



Figure 3. A zoomed image of Fig. 2 near the epicenter. Red to yellow patches (>3cm uprising/easting) coincide with the mountains while cyan to green patches (<2 cm) are rice fields on alluvial plain after harvest. Beside a DEM error, one possible explanation is that alluvial plain could not be elastically restored fully due to the compaction thus subsidence after liquefaction (discussed with T. S. Kang at PKNU). Several cases of the soil subsidence around buildings have been confirmed by field investigation (J. H. Park and S. R. Lee at KIGAM). Slight change of displacement within the alluvial plain, especially near the mountain, might imply the varying depth of the alluvial plain (discussed with J. H. Park at KIGAM). 



Figure 4. Preliminary interpretation of the focal mechanism (discussed with T. S. Kang at PKNU, J. Rhie at SNU, and H. K. Lee at KNU). The location of the aftershock hypocenters has been modified slightly northwest following the discussion with J. H. Park at KIGAM.



Figure 5. Static slip distribution model for the Pohang earthquake obtained by inverting the InSAR data (modeled by S. G. Song at KIGAM).


Figure 6. Three dimensional surface displacement map calculated from the static slip distribution model in Fig. 5 (provided by S. G. Song at KIGAM). Note the lower-left image is the predicted descending InSAR displacement with LOS=(0.6329,  -0.1126, 0.7660) while the lower-right is the predicted ascending InSAR displacement with LOS=(-0.6262, -0.1451, 0.7660).


Google Earth KMZ


                The LOS (Line of Sight) unit vector of a right-looking radar in (x, y, z)=(E, N, UP) coordinates is given by

                     LOS (from surface to satellite)= ( - sin a * cos b,   sin a * sin b,   cos a ),

                    where     a: local incidence angle between z-axis and LOS, 

                                   b: satellite heading projected on the x-y plane measured clockwise from N.

                For example,

                            Descending, right looking : if a = 40 deg and b = 190.0847114240353 deg , then LOS = ( 0.6329,  -0.1126, 0.7660 ).

                            Ascending, right looking :  if a = 41 deg and b = -13.04524393024839 deg, then LOS = (-0.6391, -0.1481, 0.7547 ). 



SAR Data: Sentinel Data Hub






Software and Data Processing




                            Supplementary Figure 1. SNAP Graph Builder with SliceAssembly.


        SNAP Graph Builder:

            Sentinel-1B Descending Right-looking

            04Nov2017 – 16Nov2017 21:23:21 UST

            Slice Assembly: VV

            TOPSAR-Split: IW2, 6-15 bursts out of 20

            Back-Geocoding: SRTM 1Sec HGT (Auto Download)

            Interferogram: Subtract flat-earth phase and topographic phase

            CPU time < 1hr with Intel i7 / 32GB Memory



        SNAP-Snahpu subsequent processing:


            Mask Sea Area for Snaphu
                Raster -> Masks -> Land/Sea Mask: Mask out the sea, Use SRTM 3sec
                Image right click -> Spatial subset from view (optional)
                File -> Export -> Beam-DIMAP (if subset)

            Snaphu Export
                Radar -> Interferometric -> Unwrapping -> Snaphu Export
                Run snaphu in VMWare Ubuntu 14.04
                Execute command in snaphu.conf file.

            Snaphu Import

            Phase to Displacement
                Radar -> Interferometric -> Products -> Phase to Displacement

            Collocate the displacement into the interferogram
                Raster -> Geometric Operations -> Collocation
                Band Math: If coh < 0.3 then NaN else disp

            Terrain Geocoding
                Radar -> Geometric -> Terrain Correction -> Range-Doppler Terrain Correction

            Export to Google Earth KMZ
                Colour Manipulation
                Image right click -> Spatial Subset from view : (<20MB)
                Image right click -> Export View as Google Earth KMZ


IRIS Earthquake Resources

    Pohang Event

    Moment Tensor



Post-seismic Descending DInSAR Analysis

Figure 7. Post-seismic DInSAR and LOS displacement map (Sentinel-1B, descending right looking. 16-28 Nov 2017). No significant post-seismic displacements are observed. Most signals are considered as of atmospheric perturbation or soil moisture change during the 12 days after the main shock on 15 Nov 2017. Here are the KMZ files of DInSAR, LOS displacement, and LOS displacement  with coherence cut of 0.2.



Co-seismic Ascending DInSAR (looking from west to east)


(a) DInSAR (IW2)


(b) A zoomed up image of (a) DInSAR near the epicenter


(c) LOS displacement map near the epicenter.  Ascending, right looking :  if a = 41 deg and b = -13.04524393024839 deg, then LOS = (-0.6391, -0.1481, 0.7547 ). 

Figure 8. Co-seismic DInSAR and LOS displacement map obtained from Sentinel-1A, ascending right looking, 10 Nov - 4 Dec 2017 InSAR pair. Similar displacement patterns (~3 cm) are observed near the epicenter when compared with the descending orbit, although very low coherence poses difficulties in phase unwrapping or interpretation due to the 24-days of temporal decorrelation, atmospheric delay, and/or soil moisture change. Here are the KMZ files of DInSAR, LOS displacement, LOS displacement > Coherence 0.2, and LOS displacement > Coherence 0.3.  To extract data you can download the black(-0.1 m) and white(0.1 m) KMZ files of LOS displacement, LOS displacement > Coherence 0.2, and LOS displacement > Coherence 0.3. You may also find that a coherence image in black(0) and white(1) and local incidence angle in black (40 deg) and white(42 deg) KMZ files are useful for slip displacement modeling.



                            Supplementary Figure 2. SNAP Graph Builder with Land-Sea-Mask (without SliceAssembly).


All rights reserved by Prof. Hoonyol Lee, Division of Geology and Geophysics, Kangwon National University.

Last updated on 2017/12/11 杉推析 09:42:11 +0900