Abstract
This study proposes a workflow that enables the accurate identification of earthquake-induced damage zones by using coherence image pairs of the Sentinel-1 satellite before and after an earthquake event. The workflow uses interferometric synthetic aperture radar (InSAR) processing to account for coherence variations between coseismic and preseismic image pairs. The coherence difference between two image pairs is useful information to detect specific disasters in a regional-scale area after an earthquake event. To remove background effects such as the atmospheric effect and ordinal surface changes, this study employs the two-step threshold method to develop the coseismic coherence difference (CCD) map for our analyses. Thirty-four Sentinel-1 images between January 2015 and February 2016 were collected to process 30 preseismic image pairs and two coseismic image pairs for assessing multiple types of disasters in Tainan City of southwestern Taiwan, where severe damages were observed after the Meinong earthquake event. The coseismic unwrapping phases were further calculated to estimate the surface displacement in east-west and vertical directions. Results in the CCD map agree well with the observations from post-earthquake field surveys. The workflow can accurately identify earthquake-induced land subsidence and surface displacements, even for areas with insufficient geological data or for areas that had been excluded from the liquefaction potential map. In addition, the CCD details the distribution of building damages and structure failures, which might be useful information for emergency actions applied to regional-scale problems. The conversion of 2D surface displacement reveals the complex behavior of geological activities during the earthquake. In the foothill area of Tainan City, the opposite surface displacements in local areas might be influenced by the axis activities of the Kuanmiao syncline.
Highlights
The Meinong earthquake struck southwestern Taiwan on 5 February 2016 at 07:57 Coordinated Universal Time (UTC)
When the coherence difference value between preseismic and coseismic image pairs was higher than the first threshold at the same pixel, we considered this pixel to be the candidate of a disaster area
This study proposed an efficient workflow of a two-step threshold method to remove the background effects and obtain coherence difference (CCD) pixels in Tainan City during the Meinong earthquake
Summary
The Meinong earthquake struck southwestern Taiwan on 5 February 2016 at 07:57 Coordinated Universal Time (UTC). The hypocenter of the ML 6.4 earthquake was located on 22.92◦ N and 120.54◦ E (Meinong district, Kaohsiung City) with a 16.7-km focal depth. The focal mechanism of this event was a thrust mechanism with two nodal planes striking in the north-south and the northwest-southeast directions (see Figure 1). This has been the most devastating earthquake in southwestern inland Taiwan after the 2010 Jiasiang earthquake. Serious damages were observed in Tainan City, which is about 35 km west of the epicenter, and the earthquake caused about 117 deaths, 551 injuries, 10 collapsed buildings and 247 seriously damaged buildings [1,2,3,4,5]. The Taiwan Earthquake Model team announced that this earthquake event did reduce the seismic hazard potential in southern Taiwan, but more than about 80% of the seismic hazard potential was not yet released [6]
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