Improbable Earthquakes: Mixing Water and Faults in the Midcontinent

The midcontinent of the United States has been experiencing a remarkable increase in the rate of M≥3 earthquakes since 2009. In Oklahoma alone, the rate of M≥3 earthquakes in 2014 exceeds the long-term background rate by a factor of 50 or more. This unprecedented increase in seismicity far from any active plate boundary raises many questions.  Why are they occurring?  How large might they be?  Do they shake the ground as strongly as earthquakes elsewhere?  Do they represent a significant hazard?  Can the hazard be reduced or managed?  To address these questions in a region with very sparse seismological coverage the USGS in partnership with universities and state geological surveys has undertaken a series of targeted investigations of specific earthquakes and areas to gain insight into their origin, mechanics and ground motions.  The nature of the space/time distribution of the increased seismicity strongly suggests that the increase is of anthropogenic origin.  Disposal by underground injection in deep disposal wells of formation water co-produced with oil and natural gas from shales and other tight formations has been identified as the causative agent in numerous published case studies.  Injection raises the pressure of the pore fluid that in turn can weaken faults through the well-established effective stress mechanism.  The process of preparing those formations for production by hydraulic fracturing, however, does not appear to be a major contributor to the increased seismicity or seismic hazard.  Rather, the hazard is an unintended consequences of long-term injection of waste water at relatively few of the tens of thousands of UIC Class II wells in the U.S. Earthquakes with M > 5 in Oklahoma and Colorado have been linked to nearby injection wells, and damage to vulnerable structures has occurred in M 4 earthquakes in several states. Although instrumental recordings of ground motion are few in number, several M 4-5 earthquakes shook the ground near their epicenters with comparable strength to tectonic earthquakes in the western U.S.   This indicates that earthquakes as small as M 4 have the potential to damage seismically vulnerable structures in their immediate epicentral vicinity.