SUMMARY

Geomorphological investigations have been significantly developed in recent earth science, therefore civil engineers are forced to consider the engineering meanings 'of such "active faults".

1.1 The author considers that so called "long term" geomorphological active faults during the quaternary period are not always related to the "realistic short term" historical earthquakes and geodetic crustal strains during the latest hundreds years. The relations among geomorphological, seismological and geodetic informations are macroscopically discussed about all districts in Japan. These relations should be further studied in more detail based on the local conditions.

1.2 According to the above mentioned systematic comparisons, the author has a engineering concept that the reliable engineering judgement might be led mainly by the geodetic approaches if it would become more systematic around the youngest active faults, and by young active faults during the latest 5,000–6,000 years after the latest stabilized sea level, and also by another younger active faults for the latest 10.000 15,000 years after the beginning of raising of seal level at the end of the latest Wuerm Glacial Period.

If necessary from the specific social problem in active faults area, two kinds of criteria might be proposed respectively for the Design Basis Earthquake and the Maximum Credible Earthquake for the most important or developing structure in order to assess the structure. This proposal might be based on the technical feasibility of dating of fault activity and realistic knowledge of crustal movement. However, the author emphasizes that this assessment shall be limited to the fewest and selected cases.

1,4 Such extraodinary developing structure like as nuclear Power facilities should be considered more conservatively than the proposed criteria for the above mentioned engineering structure.

GEOMORPHOLOGICAL ACTIYE FAULTS IN JAPAN ISLANDS

Distinguished geologists and geomorphologists have studied and published "Active Faults in Japan" in 1980.16) The author interpreted it from the engineering points of veiw.

2.1 Most realistic active faults in hitorical age A1 are merely found by 12, more realistic alluvium active faults A2 are 32 and even the latest Diluvium active faults A3 are limited to 110 among total references 1479, as shown in Table 1.

(Table in full paper)

2.2 Average slip rates of active faults are classified in A, B, C and D in Table 2. The most active faults in A class are only 9% even if the experienced earthquake faults are included.

2.3 Engineers are interested in longer faults than 10km from the reason which will be discussed in later paragraph. These are limited to about 25% (Table 3).

TECTONIC STRAIN

Geodetic shear strain vector has been analysed by Harada T and Kasai A3), based on the difference between older trigonometric survey in 1883–1909 and later one in 1948–1968 under the national project. Although this result involves the disturbances caused by experienced earthquake faulting during the pried, the author tried to illustrate contour map of maximum shear strains (Fig. 2), taking into account of grid system of Fig. 1.

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