The Earth's crust is dynamic. Constantly shifting tectonic plates interact, building and storing energy. When the stress or pressure becomes great enough, the crust shifts along faults, releasing energy. The natural release of stored energy causes earthquakes.
Many more earthquakes are observed now than in the past due to the increased number of seismic stations throughout the world and improved instrumentation accuracy and reliability. According to the United States Geological Survey (USGS), millions of earthquakes occur worldwide annually, but most go unnoticed because their strength is extraordinarily small. The energy released by earthquakes ranges from very low levels that are undetectable at the Earth's surface to very high levels that can cause widespread and catastrophic damage.
The Richter Scale Explained
The amount of energy released from an earthquake is measured on a logarithmic scale known as the Richter scale. Each whole-number increase on the Richter scale represents 10 times more ground motion and 32 times more energy released.
According to the USGS, the smallest earthquakes that can be felt at the surface are about magnitude 3 on the Richter scale, which is equivalent to the vibrations felt from a passing truck.
While almost all earthquakes are caused by naturally occurring releases of energy in the Earth’s crust, some human activities can trigger seismic events. These include geothermal development, construction of large dams, the impounding of water, mining activity and underground fluid injection.
Hydraulic Fracturing: Not Felt at the Surface
By design, hydraulic fracturing treatments release energy in the deep subsurface to fracture the rock within the targeted hydrocarbon formation, creating extremely low-level seismic events. Energy released underground during hydraulic fracturing is very small and poses no increased risk to the public.
Given that the Earth's crust is constantly in motion, it would be expected that naturally occurring tremors overlap with regions where our industry operates. It would be extremely rare for hydraulic fracturing to trigger a seismic event that could be felt at the Earth's surface. Hydraulic fracturing stimulation typically releases energy that would equate to a magnitude minus 2 or less on the Richter scale. This is well below a magnitude 3 earthquake, which is the smallest seismic event usually reported to be felt at the Earth’s surface.
More than two million wells have been hydraulically fractured worldwide. Of these, only a handful of situations occurred where seismic activity was felt at the surface, and no damage or injuries resulted. The evidence is largely inconclusive as to whether hydraulic fracturing caused these events
Since the 1930s, injection wells have been safely and reliably used in the U.S. to dispose of produced and flowback water from oil and natural gas operations. These disposal wells are highly regulated in accordance with the Safe Drinking Water Act Underground Injection Control program.
The oil and natural gas sector is one of many industries using injection wells to safely dispose of waste fluids. The chemical, manufacturing, agriculture and steel industries also rely on subsurface injection wells as a safe method to dispose of waste fluids.
It is possible for the injection of wastewater into deep underground disposal wells to trigger small seismic events in the subsurface. Of the more than 144,000 Class II disposal wells in operation in the U.S., only a few have been associated with triggered seismic events that could be felt at the surface. These were typically less than a magnitude 3 on the Richter scale, so they were too small to be felt at the surface or cause damage. A 2011 report conducted by New York’s Department of Environmental Conservation concluded that “no significant adverse impacts are identified with regard to the disposal of liquid wastes.”
ConocoPhillips supports continued research by government agencies to better understand how injection of wastewater deep underground may trigger seismic activity.