抄録
The Median Tectonic Line active fault system (MTL) is one of the most active and longest active faults on land in Japan, and has a potential source of destructive large earthquakes. However, paleoseismological data along the MTL are sparse in Shikoku. The history of surface-rupturing earthquakes, particularly the timing of the latest events, on each segments is key data for examining the interaction of neighboring faults and evaluating the long-term seismic risk.
To date the latest surface-faulting events on the MTL active fault system in Shikoku, we excavated mini-trenches on the Iyo fault and extracted vertical thin sections of unconsolidated soil layers by the Geoslicer on the Okamura and Ikeda faults. In this paper, we discuss the characteristics of active faulting along the MTL based on the results at the three sites and previously published data.
The latest event occurred after the 14th century at Ichiba on the Iyo fault. The slip associated with the event was 2.1-2.3m based on a series of right-laterally offset rice paddy dikes. Geoslicer studies at Kishinoshita on the Okamura fault and at Iyo-Mishima on the Ikeda fault suggest that the latest event occured after the 16th century and after the 13th century, respectively.
The youngest age of the latest event is not well defined by the geological studies. Since historical documents during the 18th century do not record any activity of the MTL on land, the timing of the most recent events on the fault is confined to between the 13th and 17th centuries. Therefore, timing of the latest events in Shikoku is concentrated within the past several hundred years. This suggests a temporal clustering of active faulting on the MTL, similar to that observed from other major fault worldwide.
