Seismicity During the Initial Stages of the Guy-Greenbrier, Arkansas, Earthquake Sequence

We analyze the background seismicity, initiation, and earliest stages of the Guy-Greenbrier, Arkansas, earthquake sequence, which was potentially induced by wastewater injection starting in July 2010, during the 3month time period 1 June to 1 September 2010. High-resolution observations of low-magnitude seismicity, and the high-quality Arkansas public well database, facilitate detailed analysis of spatial and temporal correlations between earthquakes, wastewater injection, and hydraulic fracturing. We detected 14,604 earthquakes, with magnitudes -1.5M(L)2.9, using two sensitive, waveform similarity-based event detection methods in parallel: Fingerprint And Similarity Thresholding, and template matching. We located the 1,740 largest earthquakes that form 16 spatially compact clusters, using P and S phases from 3 stations with the double-difference relocation algorithm and an improved velocity model constrained by the location of quarry blasts. We enhanced the temporal resolution of these event clusters by assigning smaller unlocated events to a cluster based on waveform similarity. Most clustered earthquakes during this time were both spatially and temporally correlated with hydraulic fracturing stimulation at several production wells. For one cluster, microseismicity was correlated with individual stages of stimulation. Many other wells had no detectable nearby seismicity during stimulation. We found a smaller number of events located on the Guy-Greenbrier Fault that were likely induced by wastewater injection. The concurrent presence of seismicity induced by hydraulic fracturing and wastewater injection presents a challenge for attribution and seismic hazard characterization, but the combination of precision seismology and high-quality well information allows us to disentangle the effects of these two processes. Plain Language Summary A magnitude 4.7 earthquake occurred in Arkansas in 2011, after several months of smaller earthquakes that started in July 2010. Many scientists think that pumping wastewater (from oil and gas production) deep underground caused these earthquakes. However, hydraulic fracturing, where high-pressure fluid is pumped underground to break rocks containing oil and gas, is known to cause small earthquakes in other areas (Ohio, Canada). This area in Arkansas has both hydraulic fracturing and natural earthquakes, so did these earthquakes happen for other reasons? We found that hydraulic fracturing caused most of these earthquakes, while a smaller fraction of earthquakes were caused by wastewater disposal. We reached this conclusion by identifying as many earthquakes as possibleeven the tiniest ones recorded on instruments but are too small for humans to feel. Depending on why the earthquakes are happening, well operators at oil companies need to take different actions. These earthquakes were too small to cause damage, as the largest earthquake was magnitude 2.9, but it is important to monitor small earthquakes early and often. If they are caused by human activity, we advocate taking early action before a larger, possibly damaging earthquake can happen.