Behavioral Segmentation of the Surface Ruptures Associated with the 1891 Nobi Earthquake, Central Japan, Based on Paleoseimological Studies

The 1891 Nobi earthquake is the largest earthquake on land in Japan during the modern history of this country. The surface ruptures associated with this earthquake were extended over 80 km long along the Nukumi, Neodani, Umehara and some adjoining active faults. Therefore, segmentation of these ruptures is one of the most important subjects for the scaling laws of the fault dimensions in Japan. We excavated a series of trenches at Nukumi on the Nukumi fault, Kadowaki on the Neodani fault and Kamibora on the Umehara fault to reveal the paleoseismic activities of these faults. On the trench walls at Nukumi, several strands of high-angle fault planes were observed. Our precise observation of the trench walls made clear that four faulting events including the 1891 Nobi earthquake occurred after the deposition of the K-Ah tephra layer exploded in about 7,200 cal yBP. The radiocarbon dates indicate that the average recurrence interval of the Nukumi fault is estimated to be about 2,300 to 2,400 years. The Kadowaki trench, which excavated on the Neodani fault, revealed that only three faulting events including the 1891 Nobi earthquake occurred after the deposition of the K-Ah tephra layer. And the recurrence interval of this fault was about 2,700 years during the Holocene Epoch. These results show that the Nukumi and Neodani faults have almost the same recurrence interval, however they may not always move simultaneously. In contrast to these two faults, according to the Kamibora trench, the penultimate faulting event of the Umehara fault was older than 12,000 cal yBP. Consequently, the results of these trenching studies show that each of the Nukumi, Neodani and Umehara faults is defined as a single behavioral segment, and the 1891 Nobi earthquake was a multiple-segments earthquake.