Long-period seismic waves, which were excited by two eruptions of Izu-Oshima volcano in November 1987 and recorded at an Abbreviated Seismic Research Observatory station and at several volumetric strain meter stations, are analyzed to determine kinematic seismic source models associated with the eruptions. The seismic waves have unusual features in the radiation pattern and waveforms of Rayleigh waves which cannot be explained in terms of a shear dislocation model. Waveform inversion is applied to obtain a kinematic source model which is parameterized by a single force and a seismic moment tensor. Many geophysical and field observations make clear that the two eruptions occurred in a lava drain-back stage of the volcanic activity. The source model of the seismic event associated with the eruption on November 16, 1987, is mainly composed of a vertical single force with a magnitude of 4¿1011 N, and suggests a rapid collapse of overloading lava into the deeper vent. The long-period seismic waves excited by the eruption on November 17, 1987, can be explained by a force system composed of two vector dipoles with nearly vertical orientation which are separated in time by 81 s. The sum of the two dipoles is 1.7¿1017 N m. This source model suggests that tension crack openings, or volume expansions of horizontally extended magma reservoirs or of aquifers occurred because of lava drain back. ¿ American Geophysical Union 1990