TIME-LAPSE OBSERVATIONS OF INJECTION-INDUCED EARTHQUAKE SHEAR-WAVE SPLITTING

Shear-wave splitting is the physical phenomena of seismic birefringence where a shear-wave splits into two components, a fast wave polarized in the direction of the dominant anisotropy, and a slow wave approximately perpendicular to the fast wave. Shear-wave splitting has previously been proven to be sensitive to changes in pore fluid pressure. Here, we study injection-induced earthquakes in Sumner County, to look for changes in shear-wave anisotropy which could relate to changes in in-situ pore fluid pressure. These earthquakes are processed using automated shear-wave splitting methods, eliminating human influence and drastically increasing the speed at which shear-wave parameters can be calculated. We utilize clusters of earthquakes, which occur on presumably the same fault. Separating clusters limits variance in azimuth and distance of the raypath, two major factors in shear-wave splitting. Using these clusters is the most direct way of sampling the same raypath, changing only temporal factors, such as pore fluid pressure. Shear-wave splitting shows a temporal change in individual clusters of earthquakes over the study period of 2015-2017. This change correlates to a change in pore fluid pressure over the same time period in south-central Kansas. Monitoring for changes in shear-wave splitting shown here has the potential to identify changes in pore fluid pressure and help mitigate the risk of induced seismicity.