A swarm of long-period (LP) seismic events was recorded in December 2001 before heightened eruptive activity of Tungurahua Volcano, an andesitic stratovolcano in Ecuador. The LP events showed simple decaying harmonic oscillations with frequencies ranging from 2 to 3.5 Hz and quality factors (Q) significantly larger than 100. Our spectral analysis of the LP events identified systematic temporal variations in both frequency and Q of the LP events. Assuming a crack resonator at the source of LP events, the temporal variations can be consistently explained by increasing the ash content within the resonator, which may be caused by repetitive injections of an ash-laden gas into a pre-existing crack in the conduit as a preparatory process for eruptions.