9:45 AM - 10:00 AM
[SSS14-04] Tectonic-geomorphological features and activity of the Onikobe Fault traced along the southwestern margin of the Onikobe Caldera, Miyagi, Japan
Keywords:Volcano tectonics, Caldera boundary faults
Calderas are volcanic depressions formed by magma chamber roof collapse. Active faults extend along the margins of some calderas, displacing post-caldera geomorphic surfaces[1]. Additionally, earthquakes associated with strong ground motion and crustal deformation often occur along caldera margins. Therefore, tectonic-geomorphological investigations on faults along caldera margins are crucial for seismic hazard assessment, as these faults can be seismogenic. However, detailed mapping of fault topography along caldera margins and evaluation of their activity remain limited. Here, we investigate the Onikobe Fault (OF)[2], a fault system along a caldera margin, to provide insights into seismic hazard assessment.
We used 1:15000 aerial photographs taken by GSI, and 5-m and 10-m-resolution DEMs derived from GSI to map fault topography and terraces (see Figure). Topographic profiles generated from the DEMs were used to measure vertical offsets of faults. We collected tephra samples from subaerial deposits and organic materials from terrace deposits to estimate terrace ages. Major element compositions of tephras were analyzed using a SEM-EDS system at the University of Tokyo to correlate with known tephras. Radiocarbon ages of organic materials were determined through AMS analysis at the Institute of Accelerator Analysis Ltd. Vertical offset rates along the fault were calculated by dividing vertical offsets by the estimated terrace ages.
The Onikobe Caldera (~12 km in diameter) formed in 0.2–0.3 Ma[3]. A resurgent dome developed in the central part of the caldera. The Eai River flows counterclockwise along the periphery of the dome, and outflows from the southern caldera margin. Terrace surfaces along this river are classified as Takahata (Th), Omori (Om), Hara (Hr) I–III, and Kuze (Kz), from older to younger. Confluent fans along the southwestern caldera rims are classified as Onikobe (Ok) I–VI, from older to younger.
The OF comprises ES–NS-trending scarps (8 km long) displacing Ok surfaces and NS-trending scarps (2 km long) displacing only the Th surface. These roughly correspond to the southern (SOF) and northern (NOF) parts of the OF, respectively[2]. Downhill-facing scarps are prominent along the SOF. Near its eastern end, uphill-facing scarps parallel to the downhill-facing ones form narrow grabens. Maximum vertical offsets are 9 m (OkII), 5 m (OkIII), 4 m (OkIV), and 0.4 m (OkVI). The NOF comprises uphill-facing scarps, with Ok and Th surfaces on the downthrown and upthrown sides, respectively. The maximum relative scarp height is ~25 m.
Naruko–Yanagisawa tephra (41–63 ka) was detected in the lower part of the aeolian deposit covering Th (Loc.1). Towada–Hachinohe tephra (15.5 ka) was detected in the lower part of the aeolian deposit covering OkII (Loc.2). Radiocarbon ages of organic materials in OkVI deposits are 460–311 calBP (Loc.3) and 488–424, 396–317 calBP (Loc.4). A previous study[4] reported the radiocarbon age of buried soil on OkIII, which we recalibrated to 6300–5574, 5528–5482 calBP. Consequently, terrace ages are estimated as 41–63 ka (Th), 15.5 ka (OkII), >5.5–6.3 ka (OkIII), and >0.3–0.5 ka (OkVI). Based on these estimates, vertical offset rates are calculated as ~0.6–1.3 m/kyr (SOF) and >0.4–0.6 m/kyr (NOF). The offset rate of the SOF is among the higher for active faults shorter than 10 km in Japan.
We clarified the tectonic-geomorphological features and activity of the OF. However, its paleoseismic history, slip per event, and fault plane dip directions remain unclear. Further studies will improve seismic hazard assessments and understanding of post-caldera geomorphological evolution.
This study was financially supported by the Tokyo Geographical Society.
[1] Sanjo and Sugai, 2023, Geomorphology, 440.
[2] Research Group for Active Faults of Japan, 1991, Active Faults in Japan (revised edition).
[3] Tsuchiya et al., 1997, The Geology of the Iwagasaki District.
[4] Omoto, 1992, Q. J. Geogr., 44.
We used 1:15000 aerial photographs taken by GSI, and 5-m and 10-m-resolution DEMs derived from GSI to map fault topography and terraces (see Figure). Topographic profiles generated from the DEMs were used to measure vertical offsets of faults. We collected tephra samples from subaerial deposits and organic materials from terrace deposits to estimate terrace ages. Major element compositions of tephras were analyzed using a SEM-EDS system at the University of Tokyo to correlate with known tephras. Radiocarbon ages of organic materials were determined through AMS analysis at the Institute of Accelerator Analysis Ltd. Vertical offset rates along the fault were calculated by dividing vertical offsets by the estimated terrace ages.
The Onikobe Caldera (~12 km in diameter) formed in 0.2–0.3 Ma[3]. A resurgent dome developed in the central part of the caldera. The Eai River flows counterclockwise along the periphery of the dome, and outflows from the southern caldera margin. Terrace surfaces along this river are classified as Takahata (Th), Omori (Om), Hara (Hr) I–III, and Kuze (Kz), from older to younger. Confluent fans along the southwestern caldera rims are classified as Onikobe (Ok) I–VI, from older to younger.
The OF comprises ES–NS-trending scarps (8 km long) displacing Ok surfaces and NS-trending scarps (2 km long) displacing only the Th surface. These roughly correspond to the southern (SOF) and northern (NOF) parts of the OF, respectively[2]. Downhill-facing scarps are prominent along the SOF. Near its eastern end, uphill-facing scarps parallel to the downhill-facing ones form narrow grabens. Maximum vertical offsets are 9 m (OkII), 5 m (OkIII), 4 m (OkIV), and 0.4 m (OkVI). The NOF comprises uphill-facing scarps, with Ok and Th surfaces on the downthrown and upthrown sides, respectively. The maximum relative scarp height is ~25 m.
Naruko–Yanagisawa tephra (41–63 ka) was detected in the lower part of the aeolian deposit covering Th (Loc.1). Towada–Hachinohe tephra (15.5 ka) was detected in the lower part of the aeolian deposit covering OkII (Loc.2). Radiocarbon ages of organic materials in OkVI deposits are 460–311 calBP (Loc.3) and 488–424, 396–317 calBP (Loc.4). A previous study[4] reported the radiocarbon age of buried soil on OkIII, which we recalibrated to 6300–5574, 5528–5482 calBP. Consequently, terrace ages are estimated as 41–63 ka (Th), 15.5 ka (OkII), >5.5–6.3 ka (OkIII), and >0.3–0.5 ka (OkVI). Based on these estimates, vertical offset rates are calculated as ~0.6–1.3 m/kyr (SOF) and >0.4–0.6 m/kyr (NOF). The offset rate of the SOF is among the higher for active faults shorter than 10 km in Japan.
We clarified the tectonic-geomorphological features and activity of the OF. However, its paleoseismic history, slip per event, and fault plane dip directions remain unclear. Further studies will improve seismic hazard assessments and understanding of post-caldera geomorphological evolution.
This study was financially supported by the Tokyo Geographical Society.
[1] Sanjo and Sugai, 2023, Geomorphology, 440.
[2] Research Group for Active Faults of Japan, 1991, Active Faults in Japan (revised edition).
[3] Tsuchiya et al., 1997, The Geology of the Iwagasaki District.
[4] Omoto, 1992, Q. J. Geogr., 44.