Wastewater injected in an underground reservoir layer crossed by a fault triggers an earthquake. The earthquake rupture grows larger than the zone pressurized by water injection.
Earth scientists develop a model to estimate the largest possible quake in a given location that could be caused by the disposal of water used in hydraulic fracturing. In work that offers insight into the magnitude of the hazards posed by earthquake faults in general, seismologists have developed a model to determine the size of an earthquake that could be triggered by the underground injection of fluids produced as a by-product of hydraulic fracturing. Hydraulic fracturing, or "fracking," is a petroleum-extraction procedure in which millions of gallons of water (as well as sand and chemicals) are injected deep into underground shale beds to crack the rock and release natural gas and oil. According to the United States Geological Survey, fracking itself does not typically trigger earthquakes. Instead, the increased risk for seismicity is more strongly linked with the subsequent injection of the wastewater from fracking and other oil-extraction processes into massive disposal wells that are thousands of feet underground. Previous attempts to model the relationship between injection of wastewater and the triggering of earthquakes suggested that the maximum magnitude of the seismic activity induced in this way would be proportional to the volume of the fluids injected. However, this interpretation fails to account for the fact that earthquakes can grow beyond the area impacted by fluid pressure, says Jean Paul Ampuero , professor of seismology at Caltech and co-author of a new study on the topic that appears in the journal Science Advances on December 20.
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