11:00 AM - 11:15 AM
[SSS10-08] Distribution of amount of displacement along the Eastern Margin Fault System of Nara basin, Kinki District, Central Japan
Keywords:Active fault, Amount of displacement, Segmentation, Slip rate, Structural evolution
1. Introduction
The eastern margin fault zone of Nara Basin is 55 km in length, N-S strike, reverse active fault from Yamashina Basin (Kyoto City) to southern end of Nara Basin. It had been thought to be divided into two segments by discontinuity of fault geomorphology, the segment boundary is located near Joyo City. The presenter tries to make clear fault segmentation and structural evolution by comparison of features and the amount of displacement of various geological and geomorphological horizons.
2.Geological and geomorphological horizons along fault zone
Five horizons are recognizable from old to young. 1) Base of the Miocene and upper boundary of the underground basement rocks in Nara Basin. 2) Base of the Osaka Group (approximately 2.5 Ma) 3) Base of the lower subgroup of Osaka Group (approximately 1.7 Ma) 4) Ma 1 to Ma 3 layers in the Osaka Group (approximately 0.9 to 1.0 Ma) 5) Ma 9 layer in the Osaka Group and higher terraces (approximately 0.2 to 0.4 Ma).
3. Elevation (amount of displacement) of horizons along the fault zone
Fig. 1 shows elevation of the horizons both uplifting side (Daigo Mountains, Joyo and Ide Hills and Yamato Plateau) and sinking side (Yamashina Basin, Kidukawa Lowland and Nara Basin) of faults. This figure indicates followings. 1) the amount of displacement of the base of Osaka Group is minimum in the southern part of the Kidukawa Lowland (around Inooka, Kyotanabe City to Ide town). 2) The amount of displacement of the base of the Miocene is about 1000 m around the Nara Basin, however the most of it had caused before Early Quaternary (by deposition of the lower subgroup of Osaka Group). On the other hand, the amount of displacement around Uji area during middle to late Quaternary (since Ma 0) reaches 500 m or more.
These facts suggest that the eastern margin fault zone of the Nara Basin is subdivided into two segments at the Inooka hill (southern part of the Kizukawa Lowland).
4. Future study
The presenter hopes to make a database for estimating the late Quaternary fault activity by gathering much more data on the date of terraces and stratigraphy of subsurface geology.
Reference
The Headquaters for Earthquake Research Promotion (2001) Long-term evaluation on the southern part of the Eastern Margin active fault zone of the Kyoto and Nara Basins (the Eastern Margin Fault zone of the Nara Basin). (https://jishin.go.jp/regional_seismicity/rs_katsudanso/f075_kyoto_nara/)
The eastern margin fault zone of Nara Basin is 55 km in length, N-S strike, reverse active fault from Yamashina Basin (Kyoto City) to southern end of Nara Basin. It had been thought to be divided into two segments by discontinuity of fault geomorphology, the segment boundary is located near Joyo City. The presenter tries to make clear fault segmentation and structural evolution by comparison of features and the amount of displacement of various geological and geomorphological horizons.
2.Geological and geomorphological horizons along fault zone
Five horizons are recognizable from old to young. 1) Base of the Miocene and upper boundary of the underground basement rocks in Nara Basin. 2) Base of the Osaka Group (approximately 2.5 Ma) 3) Base of the lower subgroup of Osaka Group (approximately 1.7 Ma) 4) Ma 1 to Ma 3 layers in the Osaka Group (approximately 0.9 to 1.0 Ma) 5) Ma 9 layer in the Osaka Group and higher terraces (approximately 0.2 to 0.4 Ma).
3. Elevation (amount of displacement) of horizons along the fault zone
Fig. 1 shows elevation of the horizons both uplifting side (Daigo Mountains, Joyo and Ide Hills and Yamato Plateau) and sinking side (Yamashina Basin, Kidukawa Lowland and Nara Basin) of faults. This figure indicates followings. 1) the amount of displacement of the base of Osaka Group is minimum in the southern part of the Kidukawa Lowland (around Inooka, Kyotanabe City to Ide town). 2) The amount of displacement of the base of the Miocene is about 1000 m around the Nara Basin, however the most of it had caused before Early Quaternary (by deposition of the lower subgroup of Osaka Group). On the other hand, the amount of displacement around Uji area during middle to late Quaternary (since Ma 0) reaches 500 m or more.
These facts suggest that the eastern margin fault zone of the Nara Basin is subdivided into two segments at the Inooka hill (southern part of the Kizukawa Lowland).
4. Future study
The presenter hopes to make a database for estimating the late Quaternary fault activity by gathering much more data on the date of terraces and stratigraphy of subsurface geology.
Reference
The Headquaters for Earthquake Research Promotion (2001) Long-term evaluation on the southern part of the Eastern Margin active fault zone of the Kyoto and Nara Basins (the Eastern Margin Fault zone of the Nara Basin). (https://jishin.go.jp/regional_seismicity/rs_katsudanso/f075_kyoto_nara/)