The central United States has undergone a dramatic increase in seismicity over the past few years. Many new studies indicate that the majority of the increase in seismicity is induced by the deep disposal of fluids produced by oil and gas production (wastewater disposal). Injection into deep wells is a common way, in the U.S. and elsewhere, to dispose fluids produced by the oil and gas industry.

Oklahoma’s recent earthquakes and saltwater disposal (Weblink)
F. Rall Walsh III and Mark D. Zoback

High-rate injection is associated with the increase in U.S. mid-continent seismicity (Weblink)
M. Weingarten, S. Ge, J. W. Godt, B. A. Bekins, and J. L. Rubinstein

Hydraulic fracturing and earthquakes

Large-scale disposal into deep wells should not be confused with pumping of fluids into the underground by enhanced oil recovery or hydraulic fracturing operations. The United States Geological Service (USGS) published an article in June that explains very well how induced seismicity relates to the different types of operations. The authors conclude: "Although enhanced oil recovery and hydraulic fracturing have been implicated in some recent seismicity, studies indicate that the majority of the increase in seismicity is induced by the deep disposal of fluids produced by oil and gas production (wastewater disposal). Hydraulic fracturing does not play a key role in the increase in that (1) hydraulic fracturing does not typically induce felt earthquakes; (2) in Oklahoma, the location of the largest increase in seismicity, spent hydraulic fracturing fluid does not represent a large percentage of the fluids comprising disposed wastewater; and (3) oil produced from many fields with large volumes of produced water did not involve any hydraulic fracturing. Similarly, enhanced oil recovery does not play a major role in the increase in seismicity, likely because operators attempt to keep fluid pressures in the reservoir balanced with the fluid pressure prior to production."

Myths and Facts on Wastewater Injection, Hydraulic Fracturing, Enhanced Oil Recovery, and Induced Seismicity (Weblink)
Justin L. Rubinstein and Alireza Babaie Mahani

Weblink to general information on the USGS website and links to recent publications.

Ways forward

A group of scientists from Stanford University has developed a risk-assessment workflow meant to provide a framework for which oil and gas operators and regulators can build upon in order to reduce the risk of earthquakes triggered by fluid disposal. From the summary: "To date, there are many different guidelines, regulations, and studies that have been published or put into practice that focus on triggered earthquake risk. Many of these are ad hoc, prescriptive, and reactionary. We present here a framework for risk assessment for triggered seismicity associated with saltwater disposal and hydraulic fracturing and offer systematic recommendations for factors to be considered. This framework includes an assessment of the site characteristics, seismic hazard, operational factors, exposure, and tolerance for risk. The process is intended to be site specific, adaptable, and updated as new information becomes available. We describe factors that are not currently included in standard earthquake hazard and risk-assessment procedures, including considering the necessary anthropogenic factors that are inherent in fluid-injection operations. We use risk-tolerance matrices as a means for including all aspects that influence the tolerance risk regulators, operators, stakeholders, and the public have for triggered earthquakes. The hazard and risk-assessment workflow includes the use of a traffic-light system that focuses on geologic and geophysical observations, rather than only earthquake magnitudes or ground motions, as the determining factors for whether a particular site needs to consider enhanced monitoring and decreased injection practices or possible injection-well abandonment." 

Characterizing and Responding to Seismic Risk Associated with Earthquakes Potentially Triggered by Fluid Disposal and Hydraulic Fracturing (Weblink to original article and submitted version)
Randi Jean Walters, Mark D. Zoback, Jack W. Baker, and Gregory C. Beroza

Scientific Principles Affecting Protocols for Site-characterization and Risk Assessment Related to the Potential for Seismicity Triggered by Saltwater Disposal and Hydraulic Fracturing, Spring 2015 (Weblink) 
Randi Jean Walters, Mark D. Zoback, Jack W. Baker, Greg C. Beroza