PREHISTORIC LARGE EARTHQUAKES DEDUCED FROM FAULT ACTIVITIES ALONG THE ATERA FAULT, CENTRAL JAPAN

Abstract

Prehistoric fault activities along the Atera fault, one of the most prominent active strike slip faults in central Japan, have been revealed from studies on faulted humic soil layers and river terraces. Many organic samples for radiocarbon datings were collected from the outcrop on the Atera fault at Rgo, Kashimo Village, Gifu Prefecture, in order to determine the periods of prehistoric earthquakes. Displacements of river terraces around Sakashita Town on the right bank of the Kiso river, have been measured in order to deduce the amounts of coseisimic displacements. Prehistoric earthquakes deduced from fault activities have been repeatedly taken place along the fault at the periods of before 10, 300 y. B. P., 7, 900 y. B. P., 5, 500 y. B. P, and 1, 850 y. B. P., Recurrence intervals between succeeding two earthquakes are not equal but differ from each other judging from the amounts of displacements associated with each earthquake. The average vertical slip rate has been calculated as 0.71 m/103 yrs. from the amounts of coseismic displacements obtained from faulted river terraces and the receurrence intervals of earthquakes. The average lateral slip rate is calculated as 3.55 m/103 yrs., since it is known that the left lateral displacement is about 5 times larger than the vertical one (Sugimura and Matsuda, 1965). Along the Atera fault a great earthquake is presumed to occur within a thousand years in future, based on “Time-predictable recurrence model for large earthquakes” (Shimazaki and Nakata 1980).