5:15 PM - 7:15 PM
[SSS14-P14] High-Density Gravity Survey for Estimating Accurate Fault Location on the Nishiyama Fault Zone and the Umi Fault, Fukuoka Prefecture, Japan
Keywords:Nishiyama Fault Zone, Umi Fault, Gravity Survey
In order to predict future earthquakes caused by active faults, it is necessary to know the existence and exact location of the fault and its past activity history. The location of an active fault is usually estimated by aerial photograph interpretation and topographic analysis using DEM data. However, it is difficult to estimate the location of a fault from its topographic features if it has been active for a long period of time or if significant erosion or burial has occurred since the last activity.
Five active fault zones run from northwest to southeast, mainly in left lateral strike-slip displacement in Fukuoka prefecture (Earthquake Research Committee, the Headquaters for Earthquake Research Promotion: HERP, 2013). Of these, the southeastern segment of the Kego fault zone is distributed just beneath the urban area of Fukuoka city and is feared to be the site of the next earthquake in near future. To the east of the Kego fault zone, the parallel Umi and Nishiyama fault zones are distributed. According to ‘the Long-Term Evaluation of Active Faults in the Kyushu Region’ published by the HERP (2013), the exact timing of the last earthquake event, recurrence interval, and the future earthquake potential of the Nishiyama fault zone are unknown. For the Umi fault, accurate data on past fault activity is unclear, and the exact location of the fault northwest part of the fault (just below the Fukuoka City) is also unknown.
The Geological Survey of Japan (GSJ), AIST, conducted geomorphological survey, S-wave seismic reflection survey and high-densitygravity survey to estimate the exact location of the fault in the Nishiyama segment of the Nishiyama Fault Zone and the northwest blind part of the Umi Fault. The results of the high-density gravity survey are reported in this presentation. These surveys were carried out as part of the ‘Project for the development of high-precision digital geological information for disaster prevention and mitigation’.
Understanding the subsurface structure by geophysical surveys is particularly important when it is difficult to estimate the location of faults based on topography. Recent improvements in the accuracy of gravimeters and advances in satellite positioning technology have made it possible to measure gravity on the order of microgals. By measuring such dense gravity data at intervals of a few meters to 200 m, it has become possible to detect small-scale structures around active faults that could not be detected in the past (e.g. Sumita et al., 2011, Research and Development Bureau, Ministry of Education, Culture, Sports, Science and Technology and Kyushu University, 2014).
In this study, we conducted 559 gravity measurements using Scintrex relative gravimeters (CG-5 and CG-6) in the Nishiyama section of the Nishiyama Fault Zone and around the Umi Fault at intervals of 10 to 150 m. GNSS receivers with a measurement accuracy of within 10 cm were used to survey the positions of the measurement points. After performing various corrections to the acquired data and existing gravity data, various filter analyses were performed on the obtained Bouguer anomalies, and the gravity base structure and fault locations were estimated using several model calculation methods, including a two-layer structure model and a density structure model.
The presentation will discuss the results of gravity structure extraction by first-order horizontal and vertical differentiation of Bouguer anomalies, second- and third-order tilt-plane residual gravity anomalies, the results of three-dimensional two-layer structure analysis, and the correspondence between these and topography and geological structures.
Five active fault zones run from northwest to southeast, mainly in left lateral strike-slip displacement in Fukuoka prefecture (Earthquake Research Committee, the Headquaters for Earthquake Research Promotion: HERP, 2013). Of these, the southeastern segment of the Kego fault zone is distributed just beneath the urban area of Fukuoka city and is feared to be the site of the next earthquake in near future. To the east of the Kego fault zone, the parallel Umi and Nishiyama fault zones are distributed. According to ‘the Long-Term Evaluation of Active Faults in the Kyushu Region’ published by the HERP (2013), the exact timing of the last earthquake event, recurrence interval, and the future earthquake potential of the Nishiyama fault zone are unknown. For the Umi fault, accurate data on past fault activity is unclear, and the exact location of the fault northwest part of the fault (just below the Fukuoka City) is also unknown.
The Geological Survey of Japan (GSJ), AIST, conducted geomorphological survey, S-wave seismic reflection survey and high-densitygravity survey to estimate the exact location of the fault in the Nishiyama segment of the Nishiyama Fault Zone and the northwest blind part of the Umi Fault. The results of the high-density gravity survey are reported in this presentation. These surveys were carried out as part of the ‘Project for the development of high-precision digital geological information for disaster prevention and mitigation’.
Understanding the subsurface structure by geophysical surveys is particularly important when it is difficult to estimate the location of faults based on topography. Recent improvements in the accuracy of gravimeters and advances in satellite positioning technology have made it possible to measure gravity on the order of microgals. By measuring such dense gravity data at intervals of a few meters to 200 m, it has become possible to detect small-scale structures around active faults that could not be detected in the past (e.g. Sumita et al., 2011, Research and Development Bureau, Ministry of Education, Culture, Sports, Science and Technology and Kyushu University, 2014).
In this study, we conducted 559 gravity measurements using Scintrex relative gravimeters (CG-5 and CG-6) in the Nishiyama section of the Nishiyama Fault Zone and around the Umi Fault at intervals of 10 to 150 m. GNSS receivers with a measurement accuracy of within 10 cm were used to survey the positions of the measurement points. After performing various corrections to the acquired data and existing gravity data, various filter analyses were performed on the obtained Bouguer anomalies, and the gravity base structure and fault locations were estimated using several model calculation methods, including a two-layer structure model and a density structure model.
The presentation will discuss the results of gravity structure extraction by first-order horizontal and vertical differentiation of Bouguer anomalies, second- and third-order tilt-plane residual gravity anomalies, the results of three-dimensional two-layer structure analysis, and the correspondence between these and topography and geological structures.