Short Note Shaking from Injection-Induced Earthquakes in the Central and Eastern United States

In this study, I consider the ground motions generated by 11 moderate (Mw 4.0–5.6) earthquakes in the central and eastern United States that are thought or suspected to be induced by fluid injection. Using spatially rich intensity data from the U.S. Geological Survey “Did You Feel It?” system, I show the distance decay of intensities for all events is consistent with that observed for tectonic earthquakes in the region, but for all of the events, intensities are lower than the values predicted from an intensity prediction equation that successfully characterizes intensities for regional tectonic events. I introduce an effective intensity magnitude MIE, defined as the magnitude that on average would generate a given intensity distribution. For all 11 events, MIE is lower than the event magnitude by 0.4–1.3 magnitude units, with an average difference of 0.82 units. This suggests stress drops of injection-induced earthquakes are systematically lower than tectonic earthquakes by an estimated factor of 2–10. However, relatively limited data suggest intensities for epicentral distances less than 10 km are more commensurate with expectations for the event magnitude, which can be reasonably explained by the shallow focal depth of the events. The results suggest damage from injection-induced earthquakes will be especially concentrated in the immediate epicentral region.