In April, the United States Geological Survey released a report highlighting earthquakes triggered by human activity such as wastewater disposal or hydraulic fracturing. Robert Bauer, an engineering geologist with the Illinois State Geological Survey, part of the Prairie Research Institute at the University of Illinois, discussed the causes and risks of such earthquakes with News Bureau physical sciences editor Liz Ahlberg.
Can human activity really cause earthquakes? What kinds of operations can trigger an earthquake?
Since the 1960s, we have been aware that the injection of fluids deep underground associated with wastewater disposal from oil or gas recovery methods may cause an earthquake. In this case, a pre-existing fault has to be oriented in the proper direction. The two sides of a fault are touching each other, with friction stopping them from sliding. If the forces in the earth exceed the level of friction, the sides will slide past each other very rapidly, releasing the energy of an earthquake. The injection of fluid, under pressure, near such a fault increases the water pressure in the fault plane, pushing the two sides apart, decreasing the friction between the two sides. This may occur with hydraulic fracturing and much more often with wastewater disposal wells.
What has been in the news recently are earthquakes triggered by deep salt water disposal wells in Oklahoma and Kansas. According to the Oklahoma Geological Survey staff, most of this salt water comes out of the ground along with the oil. The problem does not stem from the disposal of hydraulic fracturing fluids, such as the cases in Arkansas and Texas several years ago. In Illinois, most of this produced salt water is re-injected back into the oil formations.
Rarely, earthquakes can be triggered on pre-existing faults by filling large, deep water reservoirs, removing a very large amount of rock in surface quarries, or removing large quantities of oil from an underground oil formation without returning the salt water also extracted back into that rock formation. All of these require a pre-existing fault below these operations, angled in the proper direction so that when these substantial loads are applied or removed, the fault slips and causes an earthquake.
How do you know that the earthquakes are caused by these operations and not just coincidentally clustered where those activities take place?
One way is by knowing the background of natural earthquakes in an area and their rate of occurrence. Second, with injection wells, changing the rates of injection has changed the rate of earthquake events, indicating a relationship along with the proximity of the earthquakes to the well locations.
Earthquakes strong enough to be felt from hydraulic fracturing are very rare. There have been three documented cases of felt earthquakes in the world from the estimated 2.5 million sections of horizontal wells hydraulically fractured. Several felt earthquakes have been documented from hydraulically fracturing vertical wells in Oklahoma and Canada over the past decades.
How severe are these human-induced quakes in relation to “natural” earthquakes?
Earlier it was thought that some of the largest magnitude earthquakes from wastewater injection would be in the magnitude-4 range. Earthquakes typically aren’t causing damage until the magnitude approaches 5, and most of the damage is called cosmetic, where bricks are knocked off walls or chimneys. However, Oklahoma has experienced some of this type of damage with triggered earthquakes as low as magnitude 3.4. The largest triggered event near Prague, Oklahoma, in 2011 was a magnitude 5.6, which caused costly damage to some brick structures.
What areas are at highest risk?
Heavily faulted areas, such as some places in Oklahoma, are at the highest risk, but not just any fault is at risk. Faults have to be oriented such that the high horizontal pressures in the earth are trying to slide the two sides past each other. Deep disposal wells are triggering earthquakes in the top of the crystalline basement rocks where the existence and direction of any faults are usually not known.
Are there methods for wastewater disposal that are less likely to induce earthquakes? How can we mitigate the risks?
All these wells are regulated by states and a permitting process. Many states are producing more detailed regulations that fit the monitoring capabilities in the state, limiting flow rates and pressures, limiting injection near known faults, and limiting which rock formations underground can be used for wastewater disposal. In addition, the states are producing detailed operating procedures to follow if felt or detected earthquakes start to occur with injection. This is called a “traffic light” system. Illinois has recently instituted such a system in the oil and gas wastewater disposal regulations.