Title: What Causes Seismic Anisotropy at Volcanoes and Geothermal Areas, and can it be used for Monitoring?
Seismic anisotropy may reveal the state of stress in the crust, and its temporal changes have been attributed to deformation, seismicity, magmatic activity and geothermal extraction. We review crustal anisotropy in volcanic and geothermal regions from articles published through the end of 2019. We provide a database of anisotropy measurements to test hypotheses about the origin of anisotropy and about its utility for monitoring magmatic unrest or geothermal production. The majority of the articles (~100) examined shear-wave splitting, and were about evenly divided between causes related entirely to regional stress, local stress or structure. Delay times (a measure of anisotropy strength) increased with period and with depth in the two sets, but with much scatter.
Time variations in shear wave splitting were examined in 29 studies, but few of these presented statistical tests. Changes were reported in delay times and fast azimuths, and were mostly reported to vary with the occurrence of eruptions or intrusions, seismicity or tremor rate changes, or deformation changes such as GNSS, tilt or strain measurements There is a clear need for studies that examine statistical relationships between anisotropy and other parameters to test monitoring capabilities.