[SSS08-P11] Review on ca. 3 ka Event of Kozu-Matsuda Fault Zone inferred from CNS elemental and Pollen analyses in southern Ashigara Plain, central Japan
Keywords:Kozu-Matsuda Fault Zone, Ashigara Plain, CNS elemental analysis, pollen analysis, Off-fault paleoseismology, Holocene
This study focuses on the ca. 3 ka paleoseismic event of the Kozu-Matsuda Fault Zone (Yamazaki and Mizuno 1999). This event was suggested by drastically increasing of a brackish water diatom species around 3 ka. Nevertheless, it is necessary to review about this paleoenvironmental change, because diatom fossil assemblage in this event layer includes abundant freshwater species. Therefore, we conducted CNS elemental and pollen fossil analyses on two core samples (TJ11-1, and GS-ASG-5) taken in the southern Ashigara Plain for reviewing the ca. 3 ka event.
TJ11-1 core, 15.2 m long, was drilled by ERI of the University of Tokyo in 2011 (ERI, 2012). A radiocarbon age from 5.28 m depth was measured to be 2,346-2,677 cal BP. In addition, a total of two tephra layers were found; Fuji-Zunasawa tephra (F-Zn, 2.5-2.8 ka, Machida and Arai 2004) in 5.89-5.91 m depth, and Amagi-Kawagodaira tephra (KgP, 3,126-3,145 cal BP, Machida and Arai 2004) in 6.71-6.74 m depth. Pollen fossil assemblage shows that Gramineae and Cyperaceae increased drastically with some emergent plant pollens such as Typha and Alisma in 6.0 to 6.2 m depth, between F-Zn and KgP tephra layers. This indicates that wetland area expanded around coring site in ca. 3 ka.
GS-ASG-5 core, 15.0 m long, was obtained by GSJ of AIST in 2016 (Sato et al. 2017). A total of three tephra layers were found in this core sample; Gotenba-mud flow (Gmf, 2.5 ka, Machida 1964) in 4.00-4.12 m depth, F-Zn in 4.48-4.52 m depth, and KgP in 5.3 m depth relatively. A radiocarbon age from 4.73-4.74 m depth was measured to be 2,355-2,700 cal BP. Result of CNS elemental analysis shows that higher TS value and lower C/N ratio value occurred in 4.7 to 5.0 m depth, between F-Zn and KgP tephra layers. This suggest that sea water inundation around coring site occurred around 3 ka.
TJ11-1 and GS-ASG-5 cores shows synchronous paleoenvironmental change around 3 ka, between F-Zn and KgP tephra layers, despite ~2 km away. This environmental change suggests coseismic subsidence occurred in ca. 3 ka rather than riverine erosion and eustatic sea-level rise. Based on tephrochronological evidences, this coseismic event is corresponding to the ca. 3ka event of Yamazaki and Mizuno (1999). Therefore, we concluded that Kozu-Matsuda Fault Zone probably slipped around 3 ka, between F-Zn and KgP eruption.
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*English translation from the original written in Japanese.