9:45 AM - 10:00 AM
[SCG50-04] Visualization of crustal deformation in a high-strain shear zone by geodetic, geomorphologic, and geologic approaches
★Invited Papers
Keywords:High-strain shear zone, GNSS observation, Lineament, Multiple inverse method, Southern Kyushu
In Japanese Islands, most active faults were activated millions to 500,000 years ago, and there is no known fault emerged 100,000 years ago or later (Doke et al., 2012, Active Fault Res.). However, it is probable that newborn faults could not be identified by previous surveys, because tectonic landform attributed to such faults is likely unclear.
Owing to a nationwide GNSS network installed since mid-1990s, several high-strain shear zones defined as concentrated regions of strain rates have been recognized (Sagiya et al., 2000, PAGEOPH; Nishimura and Takada, 2017, EPS). Newborn faults are likely concealed in such high-strain shear zones.
As a case study to visualize crustal deformation induced by concealed faults and related structures, we applied integrated approaches of geodesy, geomorphology, and geology to a high-strain shear zone in southern Kyushu, Japan. Previous GNSS data analyses have suggested an occurrence of a sinistral, E-W trend high-strain shear zone in the study area (Nishimura and Hashimoto, 2006, Tectonophysics; Wallace et al., 2009, Geology). Our GNSS observation for recent 4.7 years by a network composed of 10 stations revealed that locking depth and slip velocity of the deeper extension of the fault were estimated to be 8 km and left-lateral slip of 10 mm/year.
In the geomorphological approach, active faults and lineaments in southern Kyushu were mapped by aerial photograph interpretation (Goto et al., 2020, JAEA-Research 2020-013). Whereas there are few active faults with tectonic landforms, concentration of E-W trend lineaments is identified in northwestern Kagoshima, near the center of the sinistral high-strain shear zone detected by the GNSS observation and along the E-W trend aftershock distribution by the 1997 Northwestern Kagoshima Earthquakes (Faculty of Science, Kagoshima Univ., 1997, Rep. CCEP). Based on the aerial photography, we conducted field survey in the area of 20 × 20 km, including the concentrating area of E-W trend lineaments.
The field survey showed that minor faults were common in the study area, though occurrences of crush zones including fault gouge, breccia, and cataclasite were sparse and not continuous. Thus, we estimated stress field from slip data of the minor faults, and compared it with the stress field corresponding to the sinistral high-strain shear zone. First, present stress field in the study area was calculated by inversion from focal mechanisms of microearthquakes (n=181) observed from 06/2002 to 12/2015, using the method by Michael (1987, JGR). The calculated best-fit stress is NE-SW trending σ1 and NW-SE trending σ3, consistent with the sinistral high-strain shear zone. Next, we calculated the direction of maximum shear stress when the present best-fit stress affects each minor fault. Then we further calculated misfit angles between real slip directions of minor faults and the calculated directions of maximum shear stress. Spatial distribution of minor faults with small misfit angle (<30°) shows that the densest cluster occurs in the concentrating area of E-W trend lineaments along the aftershock distribution by the 1997 Earthquakes. In addition, the second densest cluster is also E-W trend and occurs in 4 km north of the densest cluster.
In addition, we applied the multiple inverse method (Yamaji, 2000, JSG; Sato et al., 2017, J. Geol. Soc. Jpn.) to the slip data of minor faults to determine the orientations of paleostress fields. The calculated stress field in the densest cluster is NW-SE trending σ3 showing sinistral strike-slip to normal sense for the E-W trend shear zone. The stress field in the second cluster is somewhat different from the densest cluster, but partly includes sinistral sense NE-SW trending σ1. These findings suggest that the subsurface deformation is localized rather than dispersed in the sinistral high-strain shear zone estimated from the GNSS observation.
This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (FYs 2018-2020)”.
Owing to a nationwide GNSS network installed since mid-1990s, several high-strain shear zones defined as concentrated regions of strain rates have been recognized (Sagiya et al., 2000, PAGEOPH; Nishimura and Takada, 2017, EPS). Newborn faults are likely concealed in such high-strain shear zones.
As a case study to visualize crustal deformation induced by concealed faults and related structures, we applied integrated approaches of geodesy, geomorphology, and geology to a high-strain shear zone in southern Kyushu, Japan. Previous GNSS data analyses have suggested an occurrence of a sinistral, E-W trend high-strain shear zone in the study area (Nishimura and Hashimoto, 2006, Tectonophysics; Wallace et al., 2009, Geology). Our GNSS observation for recent 4.7 years by a network composed of 10 stations revealed that locking depth and slip velocity of the deeper extension of the fault were estimated to be 8 km and left-lateral slip of 10 mm/year.
In the geomorphological approach, active faults and lineaments in southern Kyushu were mapped by aerial photograph interpretation (Goto et al., 2020, JAEA-Research 2020-013). Whereas there are few active faults with tectonic landforms, concentration of E-W trend lineaments is identified in northwestern Kagoshima, near the center of the sinistral high-strain shear zone detected by the GNSS observation and along the E-W trend aftershock distribution by the 1997 Northwestern Kagoshima Earthquakes (Faculty of Science, Kagoshima Univ., 1997, Rep. CCEP). Based on the aerial photography, we conducted field survey in the area of 20 × 20 km, including the concentrating area of E-W trend lineaments.
The field survey showed that minor faults were common in the study area, though occurrences of crush zones including fault gouge, breccia, and cataclasite were sparse and not continuous. Thus, we estimated stress field from slip data of the minor faults, and compared it with the stress field corresponding to the sinistral high-strain shear zone. First, present stress field in the study area was calculated by inversion from focal mechanisms of microearthquakes (n=181) observed from 06/2002 to 12/2015, using the method by Michael (1987, JGR). The calculated best-fit stress is NE-SW trending σ1 and NW-SE trending σ3, consistent with the sinistral high-strain shear zone. Next, we calculated the direction of maximum shear stress when the present best-fit stress affects each minor fault. Then we further calculated misfit angles between real slip directions of minor faults and the calculated directions of maximum shear stress. Spatial distribution of minor faults with small misfit angle (<30°) shows that the densest cluster occurs in the concentrating area of E-W trend lineaments along the aftershock distribution by the 1997 Earthquakes. In addition, the second densest cluster is also E-W trend and occurs in 4 km north of the densest cluster.
In addition, we applied the multiple inverse method (Yamaji, 2000, JSG; Sato et al., 2017, J. Geol. Soc. Jpn.) to the slip data of minor faults to determine the orientations of paleostress fields. The calculated stress field in the densest cluster is NW-SE trending σ3 showing sinistral strike-slip to normal sense for the E-W trend shear zone. The stress field in the second cluster is somewhat different from the densest cluster, but partly includes sinistral sense NE-SW trending σ1. These findings suggest that the subsurface deformation is localized rather than dispersed in the sinistral high-strain shear zone estimated from the GNSS observation.
This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (FYs 2018-2020)”.