Volcanic tremor at Volcán de Colima, México recorded during May 2002 and its interactions with the seismic signals produced by low-energy explosive activity and rockfalls

• The longest episode of volcanic tremor during 1998–2015 eruption at Volcán de Colima was recorded during 2002–2003 activity.
• The May 2002 activity was characterized as transition between two effusive stages.
• The May 2002 volcanic tremor was recorded simultaneously with the seismic signals of low-energy explosions and rockfalls.
• Volcanic tremor served as a modulating signal for the seismic signals of low-energy explosions and rockfalls.
• The existence of two fundamental frequencies of harmonic tremor may indicate a two-vent magmatic conduit.

The May 2002 eruption episode at Volcán de Colima, México represented the transition period between two stages of effusive activity which were characterized by the formation of lava flows. The short-period seismic signals, recorded during May 2002 at a distance of 1.6 km from the crater, were represented by volcanic tremor and the signals produced by low-energy explosions and rockfalls. Two types of volcanic tremor were recognized: harmonic with two fundamental spectral frequencies of 1.2–1.4 Hz and 1.6–1.7 Hz and non-harmonic. The existence of two fundamental frequencies of volcanic tremor may indicate a two-vent magmatic conduit. No clear relationship between the variations in the spectral content of tremor and occurrence of explosions was observed. The waveforms of the signals, produced by low-energy explosions and rockfalls and recorded on the background of volcanic tremor, were strongly modulated by the low-frequency harmonic tremor signals, forming, in this manner, pseudo-long period events. Fourier analysis of the seismic signals associated with low-energy explosions and rockfalls but recorded on the background of regular seismic noise indicated their high-frequency sources characterized by dominant frequencies within 2–3 Hz and 3–4.5 Hz, respectively.