Microseismic event relocation based on PageRank linkage at the Newberry Volcano geothermal site

We adapt the PageRank method to define signal-correlation topology for microearthquakes at the Newberry Volcano enhanced geothermal systems (EGS) demonstration in central Oregon. The Newberry EGS was stimulated in 2012 and 2014, producing hundreds of microearthquakes. Event locations based on P-wave and S-wave picks and a double-difference method resulted in diffuse clouds of seismicity for both stimulations (Foulger and Julian, 2013b; Cladouhos et al., 2016). We expand on the seismological application of PageRank (Aguiar and Beroza, 2014) to define event families based on signal-correlation topology and analyze each event family by measuring differential arrival times of P and S waves. We relocate each family using the Bayesloc method (Myers et al., 2007, 2009) and find that, after relocation, event families are tightly clustered spatially, indicating that the rock volume affected by stimulation was more than an order of magnitude smaller than would be inferred from double-difference analyses. Relocation also reveals that most events occurred at the contacts between lithologic units, suggesting preferential fluid flow and hydroshearing along pre-existing weaknesses, as well as microseismic activity progressing away from the well with time.