Journal of Geography The Ise Bay Fault off the Chita Peninsula, Central Japan Atsumasa OKADA*, Isamu TOYOKURA**, Takeshi MAKINOUCHI***, Yafue FUJIWARA**and Takashi ITO**** Abstract Construction of the Chubu International Airport has started off Tokoname City (Chita Peninsula) in Ise Bay, Central Japan. Various surveys such as bathymetry, seafloor drilling, sonic prospecting, and geologic examinations of recovered borehole core samples have been performed to reveal soil engineering characteristics, submarine topography, submarine geology, and the precise locality and mode of movement of the Ise Bay Fault at the bay area (Chubu Kukou Chousakai=The Chubu International Airport Research Foundation, 1994). Many fruitful results on the Ise Bay Fault have been gained, as follows: (1) The bay area is underlain by the A (Holocene), B (upper Pleistocene), C1-C2 (upper to middle Pleistocene), and T (basement) Formations, in descending order. The A, B, C1, and C2 Formations are correlated with the Nan'yo, Nohbi-First Gravel-Toriimatsu Gravel, Atsuta (lower part), and Ama Formations below the Nohbi Plain, respectively. The basement T Formation corresponds to the Mio-Pliocene Tokoname Group. (2) The Ise Bay Fault extends several kilometers off the west coast of Chita Peninsula, parallel with the coast line, downthrown on the Ise Bay (west) side. (3) Mode of faulting is divided into three types, namely, a) flexure type in the northern part. b) steeply westward dipping type with small-scaled thrusts downthrown Chita Peninsula (east) side in the middle part. c) high-angle faulting type in the southern part. (4) Mean vertical slip of the Cl Formation is measured 46, 42, and 35 meters in the southern, middle and northern part, respectively. Vertical slip of the B Formation measures 2-11 and 2-5 meters in the southern and middle part, respectively, and is not clearly detected *Geophysical Institute, Graduate School of Science, Kyoto University **Tokyo Division, Dia Consultants Co., Ltd. ***Department of Earth Science, Faculty of Science and Engineering, Meijo University ****Nagoya Branch Office, Dia Consultants Co., Ltd.
in the northern part because of horizontal bedding. Accordingly, the fault is inferred to be more active in the southern and middle parts. Based on the above mentioned results, the Ise Bay Fault is evaluated as follows, (5) The average slip rate of vertical displacement ranges from 0.23 to 0.45 meters per 1000 years, which ranks Class B (lower) in the degree of activity. Key words: Chubu International Airport, Chita Peninsula, Ise Bay Fault, active fault, Quaternary
Fig. 1 Active faults in and around Ise Bay. Data are followed after the Hydrographic Department, M. G. A., Japan (1995), Aichi Pref. (1996), The Disaster Prevention Board, Aichi Pref. (1997), Ito et al. (1998), etc.
Fig. 2 Bathymetric chart and submarine topogra phy off Tokoname City. (modified from The Chubu International Airport Research Foundation, 1994). Trace of the Ise Bay Fault is also super imposed.
Table 1 Stratigraphy and correlation of strata in the bay area off Tokoname City and the surroundings. (The Chubu International Airport Research Foundation, 1994)
Fig. 3 Typical E-W geologic profiles of the bay area off Tokoname City. (The Chubu International Airport Research Foundation, 1994)
Fig. 4 Geologic profiles based on the sonic prospecting with borehole logs. (The Chubu International Airport Research Foundation, 1994) Profiling lines are refered to Fig. 5.
Fig. 5 Structural map of the bay area off Tokoname City. (The Chubu International Airport Research Foundation, 1994)
Fig. 7 Outline of the Ise Bay Fault. (The Chubu Fig. 6 Sonic prospecting profiles of the Ise Bay Fault. (The Chubu International Airport Research Foundation, 1994) Profiling lines are referred to Figs. 5 and 7. International Airport Research Foundation, 1994)
Model Example A THR;Throw A (Reference plane; Bottom of the C1 Formation) B TDF; Throw due to flexure B (Reference plane; Bottom of the C1 Formation) C IDF; Summatioi of throws C (Reference plane ; Bottom of the C1 Formation) Fig. 8 Terminology and measurement manner on displacement and deformation along the faults. (The Chubu International Airport Research Foundation, 1994)
Table 2 Mode of faulting, displacement, and deformation along the Ise Bay Fault. (The Chubu International Airport Research Foundation, 1994) TT; Total TDF (=Throw+Throw due to flexure). The value of throw is in meters (m). (); tentative. TOP, MID, and BOT show the top, middle, and bottom horizons of the respective formations and members. F; faulting displacement, D; faulting deformation, NF; no displacement, ND; no deformation. UN; unknown, D?; probable deformation. ab; abut. W; west upheaval. l; lower, m; middle, u; upper. Note;*1: Multiple reflection within C1, probable sedimentary structure within B2 and B1.*2: Probable sedimentary structure of B2.*3: Total throw of the east upheaval in C1, and probably WF in C2and C1.*4: Total throw of the east upheaval in C1.*5: BOT of B1 may be the sedimentary structures.*6:total throw of the east upheaval in C1. *7: Total throw of the east upheaval in C1.*8: Sedimentary structures or deformation in A3 and A2 is ambiguous. *9: Sedimentary structures or deformation in A3 and A2 is ambiguous.*10: Probable sedimentary structures in A3. *11: A3 may be deformed.*12: C2 is deformed and abut.*13: A3 and A2 are probably displaced.*14: Disconformity in A2.*15: Disconformity in A2.*16: Probably, the upper part of C1 covers unconformably the fault.
Table 3 Throw, throw due to flexure, and total throw in respective profiling lines. (The Chubu Internatioal Airport Research Foundation, 1994) Total TDF=THR+TDF, TDF; Summation of throws. Mean of Total TDF of the the southern part; 5.44. B Form. in Mean of Total TDF of the A Form. in the southern part; 2.62. BOT, MID, and TOP are the bottom, middle, and top horizons of the respective formation and members.
Table 4 Throw, throw due to flexure, and total throw in the southern, middle, and northern parts. (The Chubu International Airport Research Foundation, 1994) Total TDF=THR+TDF Upper line; Maximum value, Lower line; Mean value. ); Value of the west block upheaval. (- The value in () is smaller than the younger member because of an apparent value.
Fig. 9 Value distribution of vertical slip along the Ise Bay Fault. (The Chubu International Airport Research Foundation, 1994)
Table 5 Average rates of slip of the Ise Bay Fault in respective profiling lines. (The Chubu International Airport Research Foundation, 1994) Total TDF=THR+TDF, ƒ TDF; Summation of throws.
Table 6 Average rates of slip of the Ise Bay Fault in the southern, middle, and northern parts. (The Chubu International Airport Research Foundation, 1994) Total TDF=THR+TDF Upper line; Maximum value, Lower line; Mean value. (-): Value of the west block upheaval.
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