[S-SS13] Active faults and paleoseismology

[SSS13-07] Fault displacement and seep activity at the western flank of the Miura Knoll, Sagami Bay

★Invited Papers

*Juichiro Ashi¹, Ayanori Misawa², Toshiya Kanamatsu³, Ken Ikehara² (1.UTokyo, 2.AIST, 3.JAMSTEC)

Keywords:active fault, Kanto Earthquake, cold seep

Great earthquakes, such as 1703 Genroku Earthquake and 1923 Taisho Earthquake, repeatedly occurred along the Sagami Trough and caused devastating damage to the southern Kanto region. On the eastern Sagami Bay, focal region of the Taisho Earthquake, seafloor knolls trend NW-SE direction, and an active fault was found along the Sagami Tectonic Line (Kimura, 1973, Kagaku) connecting the foot of the knolls (Ohkochi, 1990, Journal of Geography). Seismic reflection surveys have revealed deep plate boundary faults (Sato et al., 2010, Kagaku) and the relatively shallow structures of the faults branched from them (Yamashita et al., 2013, JAMSTEC-R; No et al., 2014, EPS). However, structures and sample-based age data from very surface of the seafloor are not sufficient to discuss recent faulting activity. Shallow structures are generally studied by a shipboard sub-bottom profiler (SBP) using high-frequency acoustic waves, but the waves scatter in complex topography, making it difficult to detect structures. We installed an acoustic source and receivers on the ROV NSS, and conducted a deep-towed SBP survey during the KH-10-3 cruise, and discovered a fault that reaches the seafloor at the base of the western slope of Miura Knoll. Moreover, the location of the fault was identified by SBP, and a piston core sample was successfully collected on the hanging wall of the fault during the KH-16-5 cruise. The sample includes six coarse-grained layers composed of mudstone granules in silt layers. The upper most layer is dominant in shell fragments and existence of a bivalve shell of Calyptogena soyoae suggests ancient biological colonies associated with methane seepage along the fault. Moreover, video observation of the seafloor by the ROV NSS confirmed the distribution of Calyptogena colonies along the base of the slope, and discovered the presence of living Bathymodiolus mussels during the KH-19-5 cruise. The shell layer observed in the piston core samples were deposited at such a slope base and are interpreted to be currently located on the hanging wall due to faulting activity. In addition, several thin layers of fine-grained turbidites, which are difficult to deposit on the present-day steep slope, are found below the shell beds, suggesting that the strata were deposited before the uplift by fault activity. Although similar fine-grained turbidite is also present in piston core samples taken from the gentle slope of the foot wall of the fault, samples have not been obtained to depths enable to correlate the sedimentation ages. Therefore, we extrapolated the depth of the strata of the same age (about 16.5 ka) as the fine-grained turbidite below the shell layers by using radiocarbon ages of planktonic foraminifera, and estimated that the two layers were displaced vertically by about 15 meters across the fault. Based on the ages of the strata above and below the upper most shell layer (approximately 11 ka and 15 ka), and assuming that displacement has accumulated since the shell layers were deposited to the present, there is a possibility that the fault is classified as class A in activity.