A state of emergency was declared in Pawnee, Oklahoma on Saturday, September 3, as a powerful M5.6 earthquake shook the state. Felt as far as Chicago around 1,110 kilometers (690 miles) away, this tremor was the state’s most powerful since November 2011, when another 5.6M quake occurred in Lincoln County.
Fortunately, only one person was injured, and just a handful of buildings were damaged. “I'm glad to hear no one was seriously hurt in today's earthquake and damage appears to be limited,” Governor Mary Fallin told reporters shortly after the event took place, as reported by NBC News.
However, there is something rather mysterious about this tremor. Most occur along major fault lines, particularly those along tectonic boundaries. Within the US, the San Andreas Fault running through California is easily the most well-known, and will one day be responsible for a truly catastrophic, highly damaging, death-dealing quake.
Oklahoma is far from any such boundaries, so what exactly caused this recent rumbling? Some evidence points towards an uptick in regional fracking and wastewater disposal, but first, let's look at what released all that energy on Saturday.
According to the United States Geological Survey (USGS), this quake took place in the core of the North America tectonic plate. Although there aren’t any grinding, grating, colossal tectonic plates here – either sliding past each other or over and under one another, to generate stress – smaller faults still exist, and it appears two segments within the plate itself suddenly slid past each other after getting stuck.
After the main event took place, additional stop-start sliding continued, generating several aftershocks. As the primary earthquake happened right within the shallow crust at a depth of just 4.5 kilometers (2.8 miles), the shaking intensity was extremely high. If the epicenter was located right on or next to the built-up Oklahoma City, the damage would have been far worse than it turned out to be.
This mechanism is known as “intraplate” faulting, and it is a poorly understood phenomenon. Either generated by minor faults or hidden “mantle scars” left over from the beginning of plate tectonic movement billions of years ago, quakes in these areas are incredibly difficult to predict, as there is no major tectonic fault movement to track.
The star marks the epicenter of the quake, with the yellow and green encircled areas feeling the majority of the shaking. USGS