Abstract
By applying a pixel offset analysis using RADARSAT-2 SAR data to an inland crustal earthquake that occurred on Bohol Island, Philippines on 15 October 2013, we succeeded in mapping a ground displacement associated with the earthquake. The most concentrated crustal deformation with ground displacement exceeding 1 m is located in the northwest part of the island. The crustal deformation is zonally distributed and extends a length of approximately 50 km in the ENE–WSW direction. The ground in the mountainous area moved toward the satellite, while the ground in the northern coastal zone moved away from the satellite. A clear displacement discontinuity with a length of about 5 km, probably corresponding to earthquake surface faults, can be identified in the northeastern region. Our fault model consisting of two rectangular planes shows nearly pure reverse-fault motion on south-southeast-dipping planes with moderate dip angles. A local rupture occurs in the northeast at shallow depths and produces surface ruptures. By applying an additive color process using SAR amplitude images, significant changes in backscatter intensity were detected along the coast from Maribojoc to Loon; these changes suggest that the seafloor uplifted and the shoreline resultantly shifted seaward. The area showing the shoreline change is in good spatial agreement with the locally distributed large ground uplift predicted from our fault model. We identified a good correlation between the ground upheaval produced by the reverse-fault motion and elevation in the mountainous area, which is consistent with the idea that repeated historical reverse faulting developed the present-day topography.
Highlights
A devastating inland earthquake with moment magnitude (Mw) of 7.1 struck Bohol Island, Philippines on 15 October 2013 [1]
Within the Philippine islands, which straddle a region of complex tectonics, Bohol Island is located in the Sunda block, beneath which the Philippine Sea plate subducts from the east westward
We reduced the artifact by applying an elevation-dependent correction incorporating NASA Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data with a 3-arcsec resolution [16] in the same manner as used by Kobayashi et al [13]
Summary
A devastating inland earthquake with moment magnitude (Mw) of 7.1 struck Bohol Island, Philippines on 15 October 2013 [1]. GPS data analyses and focal mechanism solutions suggest that the island is regionally subjected to E–W to NW–SE compression [2,3]. The Philippines have suffered from considerable historic seismic activity, mainly along the subduction zones and the Philippine fault zone (Figure 1), but the seismicity in and around Bohol Island has been relatively low [4]. The largest previous earthquake that had occurred during the past several decades was the M6.8 event that occurred east of the island in 1990 (Figure 1), and no seismic events with a magnitude exceeding 7 had occurred in the crust. The existence of the East Bohol fault in the southeast part of Bohol Island has been well known, the 2013 seismic event did not involve this fault but occurred along a previously undiscovered fault (hereafter called North Bohol fault)
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