Induced Earthquakes Throughout the United States

Fig. 1: Largest earthquakes in the United States thought to be induced. (Image courtesy the USGS.)

Induced seismicity -- earthquakes that are induced or triggered by human activity -- is on the rise all across America. The biggest seismic news is coming out of Oklahoma, but Ohio, Arkansas, Texas, Colorado, New Mexico and West Virginia have also experienced an increase in significantly sized earthquakes due to human activity.

There is a clear correlation between high-volume fluid injection and recent increases in earthquake activity in Oklahoma and other states. The process of fluid injection activates old basement faults that already exist. The introduction of such high volumes of fluid increases pressure in the pores of the rock deep underground, which in turn, can weaken faults and trigger earthquakes. It's important to note that the fluids do not create new faults: they simply activate old faults that already exist. Wastewater disposal wells associated with hydrofracking are the leading cause of induced earthquakes today, but waterflooding wells associated with enhanced oil recovery may also be part of the problem.

Keep up to date with the latest news about induced seismicity here.

Fig. 2: Earthquakes associated with reservoirs migrate from the source (reference coming soon).

For nearly a century, seismologists have known about the connection between water and seismicity. As far back as the 1930s, seismologists were reporting increased earthquake activity after reservoirs were built. In 1967, a magnitude 6.3 earthquake struck near the Koyna Dam in India, killing 177 people and injuring over 2000, making it the largest and most devastating earthquake suspected to be induced by a reservoir. About a decade later, in South Carolina, researchers began a 10-year project to track earthquake activity after two new reservoirs were added to the state. The researchers found that as water levels increased at the reservoir, pore pressures increased at depth, and the pressure then migrated away from the reservoir, triggering nearby faults. Figure 2 shows that not only did earthquakes occur near the new reservoir, but they also migrated farther away with time.

But reservoirs aren't the only cause of induced earthquakes. In the 1960s, the Rocky Mountain Arsenal near Denver began injecting wastewater into the ground to dispose of toxic chemicals. After an earthquake shook the area, researchers began tracking the volume of water injected into the ground and the resulting seismicity. As seen in figure 3, the correlation they found was strong, to say the least. This pivotal research confirmed that injecting wastewater into the ground can directly trigger ancient faults to slip and cause earthquakes. The seismologists also found that earthquakes in this particular study could occur as much as 5-7 miles away from the well. The distance to which the increased pressures can migrate and cause earthquakes in other areas is still uncertain. Another uncertainty is timing between when the water is injected and when the earthquakes begin. In figure 3, there appears to be about a month's lag time between when the water is injected and when earthquakes occur. Whether or not that lag time is consistent everywhere is unclear.

Increase in OK Earthquakes

Fig. 4: Oklahoma earthquakes (red) and disposal wells (black)

Oklahoma, like so many states in the country, has historically had a few small earthquakes per year, with the occasional larger earthquake occurring every 50 to 100 years. Various faults crisscross the basement rock of the state, and even though they're not very active, they do sometimes slip, causing a small event. Or at least that was the case until 2010.

Fig. 5: The number of Oklahoma earthquakes increases dramatically after 2009 (data only through May, 2014)

Figure 5 shows the number of intsrumentally detected and located earthquakes in Oklahoma. In 2010, the number of earthquakes dramatically increased, and they've been increasing ever since. The earlier events on the chart were mostly too small to be felt, but after 2010, the number of larger earthquakes also began increasing. Most are still smaller than magnitude 3, but there have been many earthquakes large enough to be felt in recent years, including the large M5.7 earthquake that shook Oklahoma City in 2011 (note: this data is only through May of 2014, and there have been thousands of earthquakes since).

Migrating Earthquakes

No one is debating that Oklahoma's seismicity is unusual, but the question of what's causing it has triggered a major debate. The magnitude 5.7 earthquake that occurred in November of 2011 near Oklahoma city was directly associated with a nearby disposal well by research performed by USGS and other researchers. However, many people still question whether or not all of the earthquakes in the region are induced or triggered. If all the events occurred right underneath disposal wells immediately after water was injected, the correlation would be easy to make. This is not what happens in Oklahoma though. Figure 4 shows earthquakes (red) and disposal wells (black), and many of the earthquakes are located farther away from disposal wells. That can cause confusion, but it doesn't mean the earthquakes aren't connected to the wells. As seen with the Denver Arsenal research and the reservoir research, water pressure can migrate away from its source and trigger earthquakes farther away, both in distance and time.

This video shows earthquakes occuring over time in Oklahoma, and as each event occurs, its time stamp appears above the video. The time period is from January of 2000 to May of 2014, but the first 10 years of earthquakes only represent the first 3 seconds of the video. It's still a work in progress, but it does show how the earthquakes cluster in a few locations and then migrate farther away. Also note the earthquakes that begin to occur in northern Oklahoma in late 2013 and 2014. (This should work in Chrome and Firefox, but it may not work in other browsers yet.)

Oklahoma More Active Than San Francisco

Fig. 6: In early 2014, Oklahoma became more seismically active than a similarly sized area surrounding San Francisco, CA

Focal Mechanisms

Fig. 9: Focal mechanisms for largest Oklahoma earthquakes in the first half of 2014