We observed volcanic tremor associated with the Asama volcano eruption on February 6, 2003. The seismic activity of the Asama volcano began to increase at about 3:00, and the eruption occurred at 12:00. The volcanic tremor was preceded by one earthquake having large amplitude, and was followed by another. The source of the tremor, which is estimated by using amplitude decay with distance, is below the vent. Moreover, the tremor contained harmonic oscillation having a dominant frequency of 1.4 Hz. Amplitude of the harmonic oscillation increased just after the first large quake, and decreased just before the second one. Particle motions of the harmonic tremor point to the vent. We interpret the source mechanism of the harmonic tremor as a resonance of gas in the conduit.

Key words:
Asama volcano, eruption on February 6, 2003, volcanic tremor, Helmholtz resonance