Time-varying seismic anisotropy on Japanese volcanoes

Speaker: Martha Savage
Seismic anisotropy, the directional dependence of wave speeds, is caused by stress-oriented cracks and can be used to monitor stress changes from magmatic movement. Shear wave splitting fast polarisations (Φ) align with cracks and hence with the compressive stress field. Delay times (dt) measure the density of cracks. Time variations in both Φ and dt on volcanoes have been reported by ourselves and other workers. Here we report results from an objective automatic technique, applied to Asama, Aso, Unzen and Sakurajima volcanoes. Thousands of measurements made on each volcano allow us to determine correlations with other volcano monitoring techniques. Common features at all volcanoes are that stations closest to the craters yield the fewest good measurements, and even those tend to give varying results at closely spaced stations. Scattering from the volcanic edifice may be making the S waves difficult to pick, and the local stresses may be varied. Stations on the volcanic flanks give many good measurements. Some stations yield variations in Φ and dt that depend upon the earthquake location. But at most volcanoes, some stations show changes that are better explained by variations in time than in space. Where GPS measurements are available, the variations in dt sometimes but not always correlate with previously-modeled inflation or deflation events and Φ usually matches well with the stress field modelled from GPS-derived locations of magmatic sources. The temporal variations in Φ are large, ranging from 30ο at some stations to 90ο at other stations.