Pollen fossil assemblages have the potential to reflect local landform change caused by coseismic subsidence. We estimated the intervals of faultings events of the Sekidosan fault, Ouchigata fault zone with the results of pollen analysis from a horizon that has low resolution detecting seismic events.
The data for pollen analysis were collected on the Mijiro drilling core no.2 obtained in 2004 by the Active Fault Research Center of Japan.
The pollen analysis reconstructed the past local vegetation between about 8,900 cal yrs BP and 5,100 cal yrs BP.Three local pollen zones were recognized from the result of the analysis. There zones are I : 8,200-7,800 cal yrs BP, II : 7,500-6,100 cal yrs BP and III : 5,900-5,300 cal yrs BP. The zone II includes two sub pollen zones.
In particular, the pollen assemblage changes of zones II and III shows some oscillations that were caused by the change of the moisture on the surface. The taxa were classified into 3 groups (F, W, D). F is the index of water flow from the mountains around the plain. This is composed of riparian forest taxa. W is the index of moistening of the surface. This is composed of Monolete. D is the index of relative drying. This is composed of the absolute number of Alnus. These indices changed simultaneously and swiftly during 6,100 and 5,930 cal yrs BP. We call this event as PEv (Pollen Event) 1.
We directed our attention to the oscillations of W and D, in particular. They have the contrary oscillations at the same time and they may show the change originated in the same events. We can see similar oscillations twice before PEv1.
The interval of the second faulting event from the last and PEv1 is about 1,100-2,500 years. This is almost the same as the faulting intervals during the past 5,000 years 1,400-2,300 years. This is a potential that PEv was caused by very local environmental change with faulting.
The data of pollen analysis can show the changes of the past depositional environment in the sediments that has too low resolution to determine faulting events. This study suggests the potential of pollen analysis as one of the useful method for restration of paleo-environment.
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