2006 Volume 2006 Issue 26 Pages 137-150
We intend to calculate the vertical slip rate of the Uozu fault zone for affording the basical data on earthquake disaster prevention at Central Japan. Vertical slip rate of the Uozu fault zone has been estimated to be over lm/ky by previous reports. But the slip rate has been calculated only by the offsets on fluvial terrace surfaces because the offsets due to the frontal fault on buried terrace surfaces have not yet been clarified. We obtained the displacement of the frontal fault on a buried terrace surface by arrayed boring and seismic reflection survey.
Fluvial terrace surfaces are divided into higher (H1-H4), middle (M1) and lower (LH1, LL1, lowers) surfaces. Ages of the surfaces are estimated from optical and chemical analyses of tephra particles in loess deposits. Loess deposits on fluvial terrace surfaces include cryptotephras such as K-Tz (Kikai-Tozurahara: 90-95 ka), Aso-4 (85-90 ka), DKP (Daisen-Kurayoshi: 55-60 ka) and AT (Aira-Tn: 25-30 ka) LH1 terrace surface formed about 60 ka because the surface is covered by the 55-60 ka DKP tephra. Buried LH1 terrace surface is recognized by arrayed borings across the frontal fault. Buried LH1 terrace surface recognized in four boring cores is covered by buried loess deposits (including DKP and AT) and recent alluvium in ascending order. H2, H3 and M1 terrace surfaces are older than the 90-95 ka K-Tz tephra because the surfaces are covered by loess deposits (including K-Tz, DKP and AT). H2, H3 and M1 terrace surfaces are estimated to be 280-310 ka,240-260 ka and 155-165 ka respectively as long as the accumulation rate of loess deposits above each surface is constant.
The frontal fault is detected in the seismic reflection profile as an east-dipping reverse fault. Vertical slip rate of the frontal fault is calculated as ca.0.1 m/ky from the offset of LH1 buried terrace surface. Vertical slip rates of other faults are calculated by using terrace surfaces as 0.2-0.4 m/ky. Vertical slip rate of the Uozu fault zone is summed to 0.3-0.5 m/ky and the value is half or one-third less than the rate previously reported.
