Seismic properties of a shallow magma reservoir in Kilauea Iki by active and passive experiments

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Abstract
The use of multiple methods is essential for determination of seismic properties of a complex structure like a partially frozen lava pond. In our experiment with Kilauea Iki during March 1976 (1) the spatial distribution of seismic events originating from the crust best defined the lateral location of the magma lens, (2) the S waves transmitted through the magma lens and dispersion of Love waves generated by explosive sources in the Iki floor constrained the S wave velocity structure, and (3) P waves from explosions revealed an extremely low P velocity zone below the crust. From Love and S wave data we infer a rather thin (less than 10 m) magma lens, which, in response to our weak seismic signal, behaves like a viscous liquid with apparent viscosity of about 107 P and apparent shear velocity of about 0.2 km/s. Apparent high viscosity at low stress level was reported by Shaw et al. (1968), who made an in situ measurement of viscosity at Makaopuhi, Hawaii, and attributed its possible source to the presence of vesicles. Liquid containing vesicles apparently behaves like a Bingham body: a solid below and liquid above a threshold stress. The presence of vesicles of a few volume percent in the melt can also reduce the apparent bulk modulus to as low as the apparent rigidity inferred from Love and S wave data. The P velocity of about 0.3 km/s is possible in the melt with 5% vesicles. The observed refraction data require the P velocity in the lower crust to be as low as 0.9 km/s. This low velocity may be attributed to dry cracks unfilled with liquid magma.