In-situ Estimation of Poisson’s Ratio in Volcanoes and Active Fault Zones

Seismologically determined Poisson’s ratio is a powerful tool to complement the petrological and geochemical studies in the investigation of the composition of the Earth. We estimate the ratio of compressional- to shear-wave velocity (Vp/Vs), which is directly related to Poisson’s ratio, in Long Valley Caldera and the Loma Prieta rupture zone, California, using a high-resolution Vp/Vs estimation technique by Lin and Shearer (2007) from waveform cross-correlation data within similar event clusters. We obtain seismic data, both first-arrival times and waveforms, from the California Seismic Networks and perform waveform cross-correlation and similar event cluster analysis for high-resolution Vp/Vs calculations. This approach provides highly precise results because cross-correlation can measure differential times to within a few milliseconds and can achieve a precision of 0.001 in the estimated Vp/Vs ratio, corresponding to about 0.0004 in Poisson’s ratio. We also apply the double-difference tomography inversions in our study areas to obtain additional constraints of absolute velocities. The spatial and temporal variations in the resulting Vp/Vs ratios will help to track movement of magma and volatiles within a volcano and to estimate spatial variations in fracturing and fluid content in active fault zones.