The complex frequencies of long-period seismic events as probes of fluid composition beneath volcanoes

Long-period (LP) events have been widely observed in relation to magmatic a nd hydrothermal activities in volcanic areas. LP waveforms characterized by their harmonic signature have been interpreted as oscillations of a fluid- filled resonator, and mixtures of liquid and gas in the form of bubbly liqu ids have been mainly assumed for the fluid. To investigate the characterist ic properties of the resonator system, we analyse waveforms of LP events ob served at four different volcanoes in Hawaii, Alaska, Colombia and Japan us ing a newly developed spectral method. This method allows an estimation of the complex frequencies of decaying sinusoids based on an autoregressive mo del. The results of our analysis show a wide variety of Q factors, ranging from tens to several hundred. We compare these complex frequencies with tho se predicted by the fluid-filled crack model for various mixtures of liquid , gas and ash. Although the oscillations of LP events with Q smaller than 5 0 can be explained by various combinations of liquids and gases, we find th at ash-laden gases are required to explain longlasting oscillations with Q larger than 100. The complex frequencies of LP events yield useful informat ion on the types of fluids. Temporal and spatial variations of the complex frequencies can be used as probes of fluid composition beneath volcanoes.