The U.S. Geological Survey (USGS) says the number of earthquakes east of the Rocky Mountains has increased "dramatically" since 2009.

More earthquakes in these areas have coincided with the increase in oil and natural gas production from shale formations. Seismic events caused by human activity—also known as induced seismicity—are most often caused by the underground injection of wastewater produced during the oil and natural gas extraction process.

Chart from EIA shows increased number of earthquakes in Oklahoma.Chart from EIA shows increased number of earthquakes in Oklahoma.The Energy Department's Energy Information Administration (EIA) says that most induced earthquakes are small, measuring in the 3-4 magnitude range on the moment magnitude scale. These earthquakes are large enough to be felt by most people, but they do not often cause damage to structures.

In Oklahoma, where production is in areas with high water-to-hydrocarbon ratios (meaning there is more produced wastewater that must be disposed), the number of earthquakes has increased significantly since 2009.

Before 2009, Oklahoma might have experienced one to two low-magnitude earthquakes per year. Since 2014, Oklahoma has experienced one to two low-magnitude earthquakes per day, with a few instances of higher magnitude (between magnitude 5 and 6) earthquakes that caused some damage, EIA says.

In addition to the increased use of wastewater injection related to oil and natural gas production in the region, the geologic conditions in central Oklahoma are conducive to triggering seismic activity. The rock underlying the formations where disposal water is being injected in the region has existing faults that are susceptible to the changing stresses caused by fluid injection.

Without these geologic conditions, induced seismicity would be much less common. For example, induced seismicity in the Bakken region of North Dakota and Montana is relatively rare, EIA says.

The USGS and the Federal Emergency Management Agency (FEMA) issued an updated seismic hazard forecast for the central and eastern United States. This forecast attempts to estimate the chance of damage caused by earthquakes in the region of interest. The 2017 forecasted seismic rates are still significantly elevated compared with pre-2009 levels but lower than their peak in 2015.

The USGS-FEMA report says that recent earthquakes show a pattern of steadily increasing damages and losses that are primarily due to three key factors: (1) significant growth in earthquake-prone urban areas, (2) vulnerability of the older building stock, including poorly engineered non-ductile concrete buildings, and (3) an increased interdependency in terms of supply and demand for the businesses that operate among different parts of the world.

In the U.S., earthquake risk continues to grow with increased exposure of population and development even though the earthquake hazard has remained relatively stable except for the regions of induced seismic activity.

(Click to enlarge.) Map of potential earthquake damage for the U.S. (Click to enlarge.) Map of potential earthquake damage for the U.S. The USGS and FEMA report says the total estimated economic exposure (building stock as well as content) for the nation is approximately $59 trillion, of which over 30% comes from California, Texas, New York, and Florida.

The 10 states with highest populations exposed to very strong ground shaking levels are California, Washington, Utah, Tennessee, Oregon, South Carolina, Nevada, Arkansas, Missouri, and Illinois. Together, these states account for over 26% of the nation’s total economic exposure.

The report says that although such a level of shaking is estimated to occur relatively infrequently, it could cause significant damage and causalities. Within the central and eastern United States, the New Madrid seismic zone (NMSZ) and the Charleston South Carolina earthquake zone poses "significant" earthquake threat.

The NMSZ covers parts of eight states: Illinois, Indiana, Missouri, Arkansas, Kentucky, Tennessee, Oklahoma, and Mississippi, and together they amount to approximately 15% of the total national exposure.

The USGS report indicates that the recent decline may be related to decreased wastewater injection, because production in the region has decreased since the 2014 drop in oil prices. Actions by authorities in various states to regulate wastewater injection practices and restrict injection into the most sensitive areas may also be helping to reduce both the number and intensity of small earthquakes.

Induced seismicity is one of the many topics that will be addressed at the 2017 EIA Energy Conference. The panelists will discuss the evolution of this seismicity, the geology, economics, and regulatory frameworks associated with wastewater injection, as well as the challenges and opportunities related to managing wastewater produced from oil and natural gas production.