Acoustic properties of a crack containing magmatic or hydrothermal fluids

We estimate the acoustic properties of a crack containing magmatic or hydro thermal fluids to quantify the source properties of long-period (LP) events observed in volcanic areas assuming that a crack-like str structure is the source of LP events. The tails of synthetic waveforms obtained from a mode l of a fluid-driven crack are analyzed by the Sompi method to determine the : complex frequencies of one of the modes of crack resonance over a wide ra nge of the model parameters alpha /a and rho (f)/rho (s) where alpha is the P wave velocity of the rock matrix, a is the sound speed of the fluid, and rho (f) and rho (s) are the densities of the fluid and rock matrix, respec tively. The quality factor due to radiation loss (Q(r)) for the selected mo de almost monotonically increases with increasing alpha /a, while the dimen sionless frequency (nu) of the mode decreases with increasing alpha /a and rho (f)/rho (s) These results are used to estimate Q and nu for a crack con taining various types of fluids (gas-gas mixtures, liquid-gas mixtures, and dusty and misty gases) for values of a, rho (f), and quality factor due to intrinsic losses (Q(i)) appropriate for these types of fluids, in which Q is given by Q(-1) = Q(r)(-1) + Q(i)(-1). For a crack containing such fluids , we obtain Q ranging front almost unity to several hundred, which consiste ntly explains the wide variety of quality factors measured in LP events obs erved at various volcanoes. We underscore the importance of dusty and misty gases containing small-size particles with radii around 1 mum to explain l ong-lasting oscillations with Q significantly larger than 100. Our results may provide a basis for the interpretation of spatial and temporal variatio ns in the observed complex frequencies of LP events in terms of fluid compo sitions beneath volcanoes.