The 2008 explosion that started a new eruption at the summit of Kīlauea Volcano, Hawai'i, was not preceded by a dramatic increase in earthquakes nor inflation, but was associated with increases in SO2 emissions and seismic tremor. Here we perform shear wave splitting analysis on local earthquakes spanning the onset of the eruption. Shear wave splitting measures seismic anisotropy and is traditionally used to infer changes in crustal stress over time. We show that shear wave splitting may also vary due to changes in volcanic degassing. The orientation of fast shear waves at Kīlauea is usually controlled by structure, but in 2008 showed changes with increased SO2 emissions preceding the start of the summit eruption. This interpretation for changing anisotropy is supported by corresponding decreases in Vp/Vs ratio. Our result demonstrates a novel method for detecting changes in gas flux using seismic observations and provides a new tool for monitoring under-instrumented volcanoes.