講演情報
[U15-P02]SAR衛星データを活用した副断層の被害状況把握★招待講演
*小俣 雅志1、杉本 惇1、足達 健人1、渋谷 典幸1、渡邉 和輝1 (1.株式会社パスコ)
キーワード：
干渉SAR解析、副断層、微小変位、地表地震断層
Disaster damage tends to concentrate in areas where surface ruptures and other forms of ground deformation occur. Accordingly, immediately after an event, it is essential to identify, from within the wide affected area, those surface deformation zones. In this study, using the 2016 Kumamoto Earthquake as the primary case example, we present instances of extracting coseismic surface deformation.
During the 2016 Kumamoto Earthquake, the GSI (GSI, 2016) and others employed interferometric SAR (InSAR) analysis using ALOS-2 data to detect coseismic crustal deformation, demonstrating that significant displacement occurred along the Futagawa-Hinagu Fault Zone. Field investigations subsequently confirmed surface ruptures whose locations corresponded closely with the distribution of the Futagawa-Hinagu Fault Zone depicted in existing active fault maps, and these rupture traces coincided with the major discontinuities in the interferometric fringes derived from the InSAR analysis. In contrast, Fujiwara et al. (2016) reported that small discontinuities in the interferometric fringes were observed at locations distant from the main fault trace and suggested that these subtle discontinuities were attributable to surface ruptures.
Omata et al. (2016) proposed an LC-InSAR visualization (Patent No. JP 6889993) that simultaneously displays interferometric fringes and low coherence derived from interferometric SAR analysis, demonstrating that locations exhibiting both phase discontinuities and low coherence values may indicate the presence of minute displacement. We call these locations characterized by phase discontinuity and low coherence values phase discontinuity lines. Subsequent field investigations were conducted at sites distant from the main fault trace using the LC-InSAR images. These surveys revealed that where linear zones of phase discontinuity lines, either minute displacements had occurred or ground deformation was present that was too subtle to be readily identified as a surface rupture (Omata et al., 2017). Minute deformation observed along these phase discontinuity lines was interpreted as being suggestive of distributed faults, and trench investigations were therefore conducted at ten sites crossing these lines. The trenching results revealed that, at nine of the sites, the trench walls preserved not only deformation structures produced by the 2016 Kumamoto Earthquake but also evidence of fault activity predating the event. These findings suggest that the distributed faults identified in this study may have repeatedly ruptured in association with past earthquakes along the Futagawa-Hinagu Fault Zone (Omata et al., 2023). Phase discontinuity lines occurring at locations distant from the main fault trace have also been documented in overseas earthquakes, including the Ridgecrest Earthquake, and field investigations have confirmed that coseismic surface deformation occurred along these features (Adachi et al., in this session).
Surface deformation generated by distributed faults occurring at locations distant from the main fault trace should be regarded not as exceptional cases but as phenomena that occur universally during large earthquakes. Surface displacement along distributed faults has also been documented at sites far from the main fault trace, where such displacement has caused residential and infrastructure damage. Predicting the locations of these distributed faults in advance is extremely difficult. Nevertheless, analyses of SAR satellite data have revealed that damage can occur even along distributed fault traces that were previously considered unpredictable. Although such information becomes available only after the occurrence of an earthquake, the use of satellite data enables the identification of damage related surface deformation zones across wide areas. This, in turn, provides a basis for mitigating further damage during post-disaster response.
This study was conducted as part of the contract research in FY 2019-2023, sponsored by the Secretariat of Nuclear Regulation Authority, Japan.
During the 2016 Kumamoto Earthquake, the GSI (GSI, 2016) and others employed interferometric SAR (InSAR) analysis using ALOS-2 data to detect coseismic crustal deformation, demonstrating that significant displacement occurred along the Futagawa-Hinagu Fault Zone. Field investigations subsequently confirmed surface ruptures whose locations corresponded closely with the distribution of the Futagawa-Hinagu Fault Zone depicted in existing active fault maps, and these rupture traces coincided with the major discontinuities in the interferometric fringes derived from the InSAR analysis. In contrast, Fujiwara et al. (2016) reported that small discontinuities in the interferometric fringes were observed at locations distant from the main fault trace and suggested that these subtle discontinuities were attributable to surface ruptures.
Omata et al. (2016) proposed an LC-InSAR visualization (Patent No. JP 6889993) that simultaneously displays interferometric fringes and low coherence derived from interferometric SAR analysis, demonstrating that locations exhibiting both phase discontinuities and low coherence values may indicate the presence of minute displacement. We call these locations characterized by phase discontinuity and low coherence values phase discontinuity lines. Subsequent field investigations were conducted at sites distant from the main fault trace using the LC-InSAR images. These surveys revealed that where linear zones of phase discontinuity lines, either minute displacements had occurred or ground deformation was present that was too subtle to be readily identified as a surface rupture (Omata et al., 2017). Minute deformation observed along these phase discontinuity lines was interpreted as being suggestive of distributed faults, and trench investigations were therefore conducted at ten sites crossing these lines. The trenching results revealed that, at nine of the sites, the trench walls preserved not only deformation structures produced by the 2016 Kumamoto Earthquake but also evidence of fault activity predating the event. These findings suggest that the distributed faults identified in this study may have repeatedly ruptured in association with past earthquakes along the Futagawa-Hinagu Fault Zone (Omata et al., 2023). Phase discontinuity lines occurring at locations distant from the main fault trace have also been documented in overseas earthquakes, including the Ridgecrest Earthquake, and field investigations have confirmed that coseismic surface deformation occurred along these features (Adachi et al., in this session).
Surface deformation generated by distributed faults occurring at locations distant from the main fault trace should be regarded not as exceptional cases but as phenomena that occur universally during large earthquakes. Surface displacement along distributed faults has also been documented at sites far from the main fault trace, where such displacement has caused residential and infrastructure damage. Predicting the locations of these distributed faults in advance is extremely difficult. Nevertheless, analyses of SAR satellite data have revealed that damage can occur even along distributed fault traces that were previously considered unpredictable. Although such information becomes available only after the occurrence of an earthquake, the use of satellite data enables the identification of damage related surface deformation zones across wide areas. This, in turn, provides a basis for mitigating further damage during post-disaster response.
This study was conducted as part of the contract research in FY 2019-2023, sponsored by the Secretariat of Nuclear Regulation Authority, Japan.