After a quiescence of 12 years, the Izu-Ooshima volcano, about 30 km east of the Izu-Peninsula in central Japan, erupted on November 15, 1986. On August 27, 1985, about 1 year before the eruption, a peculiar earthquake of magnitude 2.7 took place near the Moho boundary just beneath the Izu-Ooshima volcano, where seismic activity is otherwise absent. Analysis of digital data obtained by the telemetered seismic network of the National Research Center for Disaster Prevention reveals the following distinctive features of the seismic waves:(1) The main part of the wave train is composed of S waves, and a small P phase precedes it. (2) The spectral density has a sharp peak at 1.0 Hz, which is independent of the epicentral distance, azimuth, and time, indicating a monochromatic spectrum at the source. (3) Gound motion for the initial part of the S wave is polarized in the N--S direction at almost all the stations. The amplitude ratios of S to P waves and the polarization pattern of S waves suggest that a traction force caused by magma flow is a more probable source than a double-couple mechanism, free oscillation of the magma resevoir or opening of a tensile crack. We propose a traction-force model for this earthquake, in which magma flow through a conduit produces unidirectional viscous force on the conduit walls. The observations imply a N-S force direction and a maximum force of about 2.8¿1010 N. The pressure difference between the ends of the conduit is estimated as 6.1¿107 Pa (610 bar), under the assumption of conduit dimensions of 2 km¿1 km. The monochromatic nature of the source spectrum is difficult to account for by the free oscillation of a magma reservoir. We tentatively suggest the intermittent opening of a barrier in the conduit as a plausible mechanism of the periodic oscillation. ¿ American Geophysical Union 1987