A simplistic triggering mechanism, pore pressure increase from injection, has been the focus of injection‐induced seismicity studies for decades. Research into other possible mechanisms, like poroelastic stress changes, is ongoing, but there has been relatively little focus on earthquake interaction. While studies have looked at how moderate‐magnitude events (M ≥ 3.0) may trigger larger magnitude‐induced seismicity, research into the cumulative effect of the hundreds to thousands of small‐magnitude (M ≤ 3.0) events is lacking. Here we use generic models to compare the possible stress changes from pore pressure increase and from earthquake interactions of small‐magnitude events. We find that the area of increased pore pressure is much larger than that of positive Coulomb static stress transfer; however, maximum Coulomb static stress change is larger than maximum pore pressure increase. We argue that, yes, small earthquakes do matter, and their interaction may be an important triggering mechanism to consider.

Causal mechanisms for fluid injection-induced earthquakes remain a challenge to identify. Past studies largely established spatiotemporal correlations. Here, we propose a multiprocess causal mechanism for injection-induced earthquakes through a case study of the 2017 M w 5.5 induced earthquake near Pohang Enhanced Geothermal System, Korea, where detailed hydraulic stimulation and on-site seismicity monitoring data provide an unprecedented opportunity. Pore pressure modeling reveals that pore pressure changes initiate seismicity on critically stressed faults and Coulomb static stress transfer modeling reveals that earthquake interactions promote continued seismicity, leading to larger events. On the basis of these results, we propose the following causal mechanism for induced seismicity: pore pressure increase and earthquake interactions lead to fault weakening and ultimately triggering larger earthquakes later in the process. We suggest that it is prudent that pore pressure change, initial seismicity locations, and Coulomb static stress transfer from seismicity earlier in the sequence are assessed in real-time.

Mitigation of injection‐induced seismicity in Greeley, Colorado, is based largely on proximity of wastewater disposal wells to seismicity and consists of cementation of the bottom of wells to eliminate connection between the disposal interval and crystalline basement. Brief injection rate reductions followed felt events, but injection rates returned to high levels, >250,000 barrels/month, within 6 months. While brief rate reduction reduces seismicity in the short term, overall seismicity is not reduced. We examine contributions to pore pressure change by injection from 22 wells within 30 km of the center of seismicity. The combined injection rate of seven disposal wells within 15 km of the seismicity (Greeley Wells) is correlated with the seismicity rate. We find that injection from NGL‐C4A, the well previously suspected as the likely cause of the induced seismicity, is responsible for ~28% of pore pressure increase. The other six Greeley Wells contribute ~28% of pore pressure increase, and the 15 Far‐field Wells between 15 and 30 km from the seismicity contribute ~44% of pore pressure increase. Modeling results show that NGL‐C4A plays the largest role in increased pore pressure but shows that the six other Greeley Wells have approximately the same influence as NGL‐C4A. Furthermore, the 15 Far‐field Wells have significant influence on pore pressure near the seismicity. Since the main mitigation action of cementing the bottom of wells has not decreased seismicity, mitigation based on reduced injection rates and spacing wells farther apart would likely have a higher potential for success.

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