Microseismic events associated with the Oroville Dam spillway

On 14 February 2017, two small (equivalent M_D 0.8 and 1.0) seismic events occurred in proximity to the Oroville Dam in the Sierra Nevada foothills, California. To examine possible causal relationships between these events and reservoir operations, including the spillway failure starting prior to these events, we applied a new optimized template matching approach to seismic data between May 1993 - April 2018. We identified more than 19,000 smaller-magnitude events that were similar in character to the February 14 events. These events are located in proximity to the Oroville spillway and occurred in tight temporal clusters that strongly correlate with periods of spillway discharge. Seismic source inversion is inconclusive, but we suggest that these events might be induced by rapid changes in pore pressure along a fracture (or fractures) near the spillway. Cavitation cannot be ruled out, but it is unlikely to be the primary cause of the signals observed because these events are intermittent, impulsive and of short duration. The inferred repetitive opening and closing of the fracture(s) occurred long before any damage to the spillway and is thus probably not directly associated with spillway failure in February 2017. These events were not related to the 1975 M_L 5.7 earthquake sequence that may have been induced by the filling of the Oroville reservoir.