Results of analyses of unusual long-period earthquakes, recorded between July and September 2004 at Mt. Spurr, Alaska, are presented. The waveforms of these events are characterized by quasi-sinusoidal signatures of long duration (up to 40 sec) with slowly decaying amplitudes; bandwidths of 0.5--4.0 Hz are typical; amplitude spectra are marked by strong and sharp peaks, reflecting the quasi-monochromatic nature of the signal. The temporal variations of the complex frequencies are investigated by use of the Sompi method; the dominant mode is resolved and its Q factor estimated for each available event. Dominant frequencies are found in the band 0.8--2.2 Hz with Q varying between 25 and 100. The variations of the complex frequencies show an overall decline with time. The dynamic response of a shallow fracture filled with bubbly water to the flux of hot gases from depth, is proposed as a possible mechanism for the generation of the observed waveforms.