This study clarifies the recurrence interval of earthquakes in the southern part of the Western Boundary Fault Zone of the Yamagata Basin by the formative age of fluvial terraces based on drilling survey on the terrace to obtain overlying loamy soil including wide-spread volcanic ashes, such as K-Tz (95 ka) and AT (26-29 ka). The late Quaternary fluvial terraces in this area are divided into Terraces I to V in descending order. K-Tz is contained at the lowest part of loamy soil on Terrace I and found in the fluvial deposits of Terrace II, AT is found at the middle part of loamy soil on Terrace III. From the ages and the vertical displacements of the fluvial terraces, the average vertical slip rates of the southern part o the Western Boundary Fault Zone of the Yamagata Basin are 0.22-0.28 mm/yr, and recurrence interval of the earthquakes is estimated to be 7100-13800 years.
