An earthquake of magnitude 6.1 occurred in Shizukuishi town, Iwate prefecture, northeastern Japan, on September 3,1998. The focal area of this event is located near Iwate volcano, which has been active since February,1998. The focal mechanism of the main shock is a reverse fault, whose pressure axis is nearly horizontal and almost east-west-a typical focal mechanism for shallow earthquakes beneath the northeastern Japan arc. Precise hypocenter locations of the main shock and its aftershocks delineate a fault plane that strikes northeast and dips northwest in the depth range 3-8km. The rupture probably began about 7km deep at the northern edge of the fault plane, then propagated southward along a deep part of the Nishine active fault.