Precise three-dimensional mapping of the 2016 Kumamoto earthquake through the integration of SAR interferometry and offset tracking

Abstract

Measurement of precise three-dimensional surface displacements is really helpful to understand Earth's internal processes. In this point of view, many researches have been performed to retrieve three-dimensional surface displacements using three or more surface displacements maps from SAR, which represent different geometries respectively [1-6]. SAR interferometry, Multiple-aperture SAR interferometry or Offset tracking have been selectively employed to restore three-dimensional surface displacements occurring by earthquakes, volcanisms, glaciers or land subsidence [7-14]. However the limitations are still remain. SAR interferometry is difficult to measure large surface displacements. Although offset tracking is appropriate to measure large surface displacements, generally produce the surface displacements map of worse measurement precision than SAR interferometry [1]. Meanwhile, recent developments in SAR sensor and improvements in offset tracking have made it possible to complement each of these limitations [9]. In this study, we precisely measured the large surface displacements by employing the integration of SAR Interferometry and Offset tracking methods. Figure 1 show the shaded relief map for Kumamoto, Japan. Three interferometric pairs, which cover surface displacements occurring by the 2016 Kumamoto Earthquakes, were collected from ascending and descending paths of L-band ALOS PALSAR-2 datasets (asc1: 20151115_20160616; asc2: 20160211_20160602; dsc: 20160307_20160419) (Fig. 1). When the offset tracking map and integrated LOS displacements map are compared, integrated LOS map could observe surface displacements without unwrapping error, even in the decorrelated region of SAR interferometry because of the large surface displacements [15]. It enable us to extract the three-dimensional displacements without unmeasured area. Figure 2 shows three-dimensional surface-deformation field for the 2016 Kumamoto Earthquakes. The maximal surface displacements for east-west, north-south and up-down direction were −1.91, −1.57 and −2.49 m respectively. The root mean square errors of 1.76, 1.41 and 1.85 cm in the east, north and up directions, respectively, were estimated by comparing with the GPS measurements [16]. These results suggest that the integration method can be help to map the three-dimensional surface displacements even near the fault lines, which was impossible to observe by conventional SAR interferometry. Since it can be applied not only to earthquakes but also to large surface displacements of various factors, the observation method of this study will be produce more beneficial data to understand the Earth's internal processes.