The seismological data of JMA (Japan Meteorological Agency) make epochs at 1961 and 1976. The detectability of seismic events improved from each epoch, and especially it rose remarkably from the latter epoch. Moreover, the result of reexamination of the seismological data for the period 1926-1960 by JMA, shows that the yearly frequency of seismic events in the period rose almost equal to the yearly frequency found during the period 1961-1975. The yearly frequency of events of
M≥3.5 after 1976 is almost equal to that of events for the period 1961-1975, too. Therefore, if the events of
M<3.5 after 1976 are eliminated from the data, it is possible to consider as the seismological data are almost homogeneous throughout the period 1926-1982. This means that it is possible to analyze seismicity quantitatively. The new seismic data are available for the statistical test of time-spatial change of seismicity around the dam areas effected by impoundment.
The present paper is a revision of the previous paper [TERASHIMA (1983)] surveyed qualitatively the seismicity change between pre- and post- impoundment for the 43 dam areas, and the revision was done by applying a hypothesis test for the new seismic data of the same dam areas surveyed in the previous paper.
The hypothesis test is done for the significant difference between the yearly frequency of seismic events from 1926 to the start of impoundment for each dam and the seismicity index that is defined here by the yearly mean frequency of seismic events for 5 years after the start of impoundment for each dam.
The significant difference is detected in 20 dam areaes. In the following 6 dam areas of them, that is, Kurobe, Nagawado, Kusaki, Hitotsuse, Kuzuryu and Midono, it is estimated that the significant differences are caused by the seismicity induced by the impoundment, taking a time series of seismicity around the each dam into consideration.
It is concluded that the seismicity of Kusaki dam area became quiescent after the impoundment and it of the other 5 dam areas did active after the impoundment.
In the remaining 23 dam areas, no significant difference is recognized by the statistical test. They show also no change between the actual seismicities before and after impoundment.
There are some differences in the results between the present paper and the previous one. The causes come from:
(i) use of the new seismic data for the period 1926-1982 in the present paper,
(ii) use of the seismicity in epicentral distance Δ≤30km centering around each dam in the present paper, but Δ≤50km in the previous one,
(iii) application of the statistical test for the standards of objective and quantitative judgment in the present paper, but only qualitative in the previous one, and
(iv) use of the started date of impoundment in the present paper instead of the date of completion of dams in the previous paper, for the date to divide the seismicity into two groups, before and after the impoundment (There are usually the difference of 1 to 2 years between the started date of impoundment and the date of completion of dams).
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