Trends in long-period seismicity related to magmatic fluid compositions

Sound speeds and densities are calculated for three different types of flui ds: gas-gas mixture; ash-gas mixture; and bubbly liquid. These fluid proper ties are used to calculate the impedance contrast (Z) and crack stiffness ( C) in the fluid-driven crack model (Chouet: J. Geophys. Res., 91 (1986) 13, 967; 101 (1988) 4375; A seismic model for the source of long-period events and harmonic tremor. In: Gasparini, P., Scarpa, R., Aki, K. (Eds.), Volcani c Seismology, IAVCEI Proceedings in Volcanology, Springer, Berlin, 3133). T he fluid-driven crack model describes the far-field spectra of long-period (LP) events as modes of resonance of the crack. Results from our calculatio ns demonstrate that ash-laden gas mixtures have fluid to solid density rati os comparable to, and fluid to solid velocity ratios lower than bubbly liqu ids (gas-volume fractions < 10%). This difference results in synthetic far- field spectra with higher impedance contrasts and narrower spectral bandwid ths for ash-laden gas mixture than spectra for bubbly liquids. Spectral characteristics are described in terms of the quality factor Q(-1) .Q(-1) is measured by the ratio of the frequency of the dominant spectral p eak to the bandwidth of the peak measured at one half of its amplitude. Thi s factor expresses the losses of energy due to elastic radiation Q(r)(-1) a nd other dissipative mechanisms Q(i)(-1) at the source, Q(-1) = Q(r)(-1) Q(i)(-1). Spectra for LP events recorded at active volcanoes such as Galera s in Colombia and Kilauea in Hawaii, have Q-1 factors in the range of 0.1-0 .002. The Q(r)(-1) factors due to radiation loss calculated for a sphere fi lled with a H2O-CO2 or H2O-SO2 gas mixture, vary between 0.0015 and 0.0040 with a change in wt% H2O at 800-1600 K and 10-50 MPa. For gas-rich mixtures , Q(r)(-1) has a strong dependence on resonator geometry (spherical versus rectangular). The spectra from a resonating sphere filled with gas-rich mix ture yields values of Q(r)(-1) an order of magnitude smaller than those fro m a rectangular crack. For a resonating crack filled with an ash-gas mixtur e (or pseudogas), Q(r)(-1) varies parabolically from similar to0.006 for an ash-rich mixture, to 0.0015 or 0.0023 for a H2O-rich Or CO2-rich mixture a t 800 K and 25 MPa. For low (< 20%) gas-volume fraction fluids (foams, bubb ly fluids and ash-rich pseudogases), the magnitudes for Q(r)(-1) are indepe ndent of crack geometry. Spectra associated with a foam (gas-volume fractions 10-90%) or bubbly basa lt (gas-volume fractions < 10%) may have a dominant spectral peak with valu es of Q(r)(-1) on the order of 0.01 and 0.1, respectively. The spectra from a resonating sphere filled with a foam containing > 20% gas-volume fractio n yields values of Q(r)(-1) similar to those for a rectangular crack. As wi th gas-gas and ash-gas mixtures, an increase in mass fraction narrows the b andwidth of the dominant mode and shifts the spectra to lower frequencies. Including energy losses due to dissipative processes in a bubbly liquid inc reases attenuation. Attenuation may also be higher in ash-gas mixtures and foams if the effects of momentum and mass transfer between the phases were considered in the calculations. (C) 2001 Elsevier Science B.V. All rights r eserved.