Evidence of Late Quaternary Fault Activity from Pollen Fossil Assemblages and Estimate of the Coseismic Events, Central Japan

Abstract:

The data of pollen analysis can show the changes of the past depositional environment in the sediments. Coseismic subsidence and subsequent geomorphic changes affect local vegetation so there is a possibility that pollen records show associated environmental changes. We examined the relationship between Late Quaternary pollen assemblage changes and local geomorphic development on three active fault zones in Japan. We analyzed two areas of the Itoigawa-Shizuoka tectonic line (ISTL) active fault system in central Japan to accumulate the pollen analytical data that have close relationships to paleoseismic events. The ISTL that extends for about 150 km is one of the most active and important fault systems in Japan. The Kamishiro fault is a west-dipping reverse fault on the northern part of the ISTL with an average sliprate of 1.5 mm/yr. The Chino fault is a left-lateral strike slip fault on the central ISTL that characterized by a high average slip reaching 10 mm/yr. The Kamishiro pollen samples were collected from trench walls, and the Chino samples were from a drill core. We can correlate pollen changes and paleoseismic events exactly. The pollen assemblage variations from these two sites show some oscillations that were caused by hydrologic change on surface. The taxa were classified into three groups: F, W, and D. F is the index of flow from the mountains around the plain, W is the index of moistening of the surface. D is the index of relative drying. F is composed of riparian forest taxa. W and D are taxa affected by hydrologic change, and differ within the areas. This is caused by the difference of climate and regional vegetation. We directed our attention to the oscillations of W and D. They have some contrary and synchronous oscillations which may indicate that they show change from the same events. It is concluded that these changes observed in the W and D pollen assemblages reflect the coseismic subsidence. Using these pollen data, we tried to estimate the intervals of faulting events of the Sekidosan fault. It is a reverse fault in the southern part of Noto peninsula and it has 0.63-0.89 mm/yr average slip rate. We analyzed the horizon of the drilling core that has too low resolution for geomorphological method to estimate coseismic event. The result suggests the potential of pollen analysis as a useful method for paleoseismology.