Discriminating between natural and anthropogenic earthquakes: insights from the Emilia Romagna (Italy) 2012 seismic sequence.

The potential for oilfield activities to trigger earthquakes in seismogenic areas has been hotly debated. Our model compares the stress changes from remote water injection and a natural earthquake, both of which occurred in northern Italy in recent years, and their potential effects on a nearby Mw 5.9 earthquake that occurred in 2012. First, we calculate the Coulomb stress from 20 years of fluid injection in a nearby oilfield by using a poroelastic model. Then, we compute the stress changes for a 2011 Mw 4.5 earthquake that occurred close to the area of the 2012 mainshock. We found that anthropogenic activities produced an effect that was less than 10% of that generated by the Mw 4.5 earthquake. Therefore, the 2012 earthquake was likely associated with a natural stress increase. The probability of triggering depends on the magnitude of recent earthquakes, the amount of injected water, the distance from an event, and the proximity to the failure of the activated fault. Determining changes that are associated with seismic hazards requires poroelastic area-specific models that include both tectonic and anthropogenic activities. This comprehensive approach is particularly important when assessing the risk of triggered seismicity near densely populated areas.