Proceedings of the General Meeting of the Association of Japanese Geographers

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Using 10m-DEMs, we got differential measures at 1,867,044,257 points in the eastern part of main Island (Honshu), Japan (east of 135E excluding Hokkaido). The DEMs derived from contour maps of 1:25,000 was provided by Hokkaido Map Co. Ltd. The norm (gradient) and the Laplacian of slope are treated as fundamental measures oflandform characteristics, because the former determines transportation of materials on the slope, and the latter does concavity or convexity of slope, that is, convergence or divergence of material flow respectively. The gradient and the Laplacian play role for integral terms of the diffusion equation of landform development models.

Then, we summarizes them statistically for 250m-grids, equal to spatial resolution of geological data. The most frequent gradients among 625 (25x25) sampling points was chosen as an indicator of slope for the grid, the standard deviation of 625 Laplacians as one of texture or roughness of surface, the 5-percentile value of Laplacians as one of valley sharpness, the 95-percentile value as one of ridge sharpness. Finally we got 2D matrices (2560 x 4160) of these 4 statistics.

Using above-mentioned 4 variables, we analyzed the 2,986,995 samples (land grids), and got the "mountainousity" index as the first principal component (Eigen value: 3.59, contribution: 89.8%) and the "grandness" index as the second (Eigen value: 0.344, contribution: 8.6%). The mountainousity increases its value in the mountains with steep slopes, sharp ridges and valleys and rugged forms, the grandness does with steep slopes but smooth surfaces.

Cross-checking the variables with the geological data, we could summarize conclusions as follows.

1) Landforms on sedimentary rocks change with the age of formation to be mountainous.

2) Landforms on volcanic rocks change with the age of volcano formation to be normal mountains.

3) Landforms on the early Miocene (15 Ma) and the older sedimentary rocks, and on the Early Pleistoce volcanos (Ma.0.7) has reached to an equilibrium state in Japanese Islands where receive more than 2000mm/yr precipitation.

4) Landforms on the older sedimentary rocks, older volcanic rocks, accretion complexes, metamophic and plutonic rocks depends largely on these categories.

5) The residuals may be explain by past climatic conditions and regional uplift velocities. The latters range averagely 0.1 to 1.0 mm/yr of which values were obtained from displacement of marine and fluvial terraces in Japanese Islands. In the Glacial Times, the northernmost Island, Hokkaido and the mountainous regions of the Honshu Island had belonged to the peri-glacial climate.

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The historical records of tsunamis caused by the Hoei and the Ansei Nankai earthquakes in the Edo period have been left in Yonozu Village in the form of some village mayors' diaries. The pulse height of the tsunami by the Hoei Nankai earthquake (1707) was the greatest in the Urashiroura district located in the northern part of the village. The Urashiroura district faces the open sea directly, and its drowned valley formed last glacial stage stretches almost straight from the Pacific Ocean. It is recorded that the second step in the stone steps of Yofukuji Temple survived this tsunami which reached a height of 11.5m above sea level and therefore it is presumed that the entire Urashiroura district was influenced by this tsunami. In fact, 18 people were killed there.
It is thought that liquefaction phenomena took place in the rice fields of the Iroriura district, and the tsunami infiltrated up to the height of approximately 10m above sea level. It is said that tsunami retraced approximately 700 m along the Irori River in this district.
In Miyanoura district located in the southern part of Yonozu Village, the tsunami advanced to the third step, which measures 5.7m above sea level, of the stone steps of Koshoan Temple. This district is presumed to have lied in the place in which the pulse height of the tsunami was lower than in other districts.
The tsunami caused by the Ansei Nankai earthquake (1854) was smaller than that by the Hoei Nankai earthquake, and pulse height in the Iroriura district is thought to have been approximately 3 - 4 m above sea level. The tsunami retraced from the mouth of the Irori River approximately 400 - 500 m at this time. Liquefaction phenomena took place in the vicinity of the coast. In the Urashiroura district one old woman was killed in this tsunami with over 2.5 m in height.
The Ansei tsunami in the Koura district infiltrated to the higher part of the village, which is about 7m above sea level, and it is thought that this Tsunami was higher than that of the Iroriura district. There was a considerably high pulse in the Urashiroura district, taking into account the pulse height in the Koura district.

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