Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Holocene Activity of the Chichio Fault, the Median Tectonic Line Active Fault System, Southwest Japan, based on Trenching Studies
Atsumasa OKADAHiroyuki TSUTSUMI
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1997 Volume 106 Issue 5 Pages 644-659

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Abstract

The Chichio fault, a segment of the Median Tectonic Line active fault system in eastern Shikoku, marks a southern range front of the Sanuki Range with well-defined geomorphic features related to its late Quaternary faulting. We have excavated multiple sites along the Chichio fault to date Holocene surface-rupturing earthquakes. A 4-m-deep trench excavated into floodplain sediments of the Higaidani River at Kamigirai, Ichiba Town, Tokushima Prefecture, contains stratigraphic evidence for the most recent and penultimate earthquakes on the Chichio fault. The fault trace which ruptured during the most recent earthquake offsets all the sediments except for an artificially modified zone immediately below the ground surface.
A V-shaped silty deposit near the top of the fault trace is interpreted as cultivated soil for rice farming which filled a coseismic ground fissure associated with the most recent earthquake. From this deposit, we obtained a pottery fragment which was identified as the mould of a Buddhist artifact used around the 16th century A.D. This age constrains the time of the most recent faulting on the Chichio fault to be during or after the 16th century A.D. This earthquake may be correlated to the 1596 Keicho-Kinki earthquake as was proposed by Ishibashi (1989). However, we cannot preclude the possibility that the Chichio fault ruptured during an earthquake separated from the 1596 event.
Another fault trace farther south which does not propagate upsection as high as the fault trace during the most recent event suggests the penultimate earthquake. A radiocarbon age and pottery fragments of Yayoi age near the top of the fault trace suggest the time of the penultimate surface-rupturing earthquake at around 2, 000 yr B.P.
On the eastern projection of the faults exposed in the trench at Kamigirai, a series of rice paddy dikes are offset right-laterally approximately 7 meters. A contact between gravels and sands exposed on a plan view during an archeological excavation prior to our trenching was also offset right-laterally approximately 6 meters across the fault. These observations suggest that the amount of right-slip and vertical displacements associated with the most recent earthquake on the Chichio fault was 6-7 meters and about 1 meters, respectively.

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