Recently, Oklahoma has been experiencing an increase in seismic activity. In September 2016, a 5.8 magnitude earthquake struck Oklahoma—the largest recorded in the state to date. In response to the devastating quake, the governor declared a state of emergency for Pawnee County, and ordered 37 wastewater disposal wells near the epicenter to shut down.
The U.S. Geological Survey (USGS) was able to confirm that many earthquakes that have occurred in Oklahoma lately have been triggered by wastewater injection. During the drilling process, a mixture of water and chemicals comes to the surface with the oil and gas. Instead of recycling this wastewater, the oil and gas producers remove it by pushing it deep underground, which in turn affects naturally occurring faults that have the potential to slip.
More Earthquakes, More Questions
I'm currently working as a structural engineer for two different projects that require special considerations due to this increase in seismic activity. On one of those projects, a building owner came to me to see if we should design the facility to higher seismic load requirements. I currently serve as the president of the Oklahoma Structural Engineering Association, and this topic has been widely discussed among our group of professionals. The consensus seems to be that no one is currently designing structures to withstand higher earthquake loads. The USGS reports have identified that mostly older structures are experiencing issues, and that even though we're seeing an increase in earthquake activity in this area, we're not enduring magnitudes high enough to cause safety concerns or damage to newer structures. Additionally, the two standards that generally mandate the loads required for building structures, the International Building Code (IBC) and Minimum Design Loads for Building Structures (ASCE 7), aren't changing their load requirements because the increased seismic activity is industry based and could fluctuate from year to year. So, after reading through the various USGS reports and conferring with other professionals, I recommended to the owner that it wasn't necessary to design the building's structure for anything higher than what the code requires, but that it was ultimately their decision if they desired a more stout structure.
Designing my First Seismic Design Category "D" Facility in Oklahoma
We've also been designing the Lawton Public Safety Facility, a comprehensive municipal building in Lawton, Oklahoma, that will house the city's police headquarters, courts, jail, and central fire station. This project is particularly interesting because of the seismic category it's in that we are required to design for. The average building in Oklahoma is designed for the Seismic Design Category "B," but this project requires it be designed to a Category "D." There are two main reasons for this, first because of its role as an emergency response facility, which makes it necessary for the facility to remain operational after any major event, but also because of its location in the city of Lawton—which is currently the highest seismic region in the state. Because these two criteria don't often coincide for structures in Oklahoma, this has become the first building I've designed to Seismic Design Category "D" requirements in the state.
Situated in the highest seismic region in Oklahoma, the facility is designed to withstand major earthquakes and other natural events.
Since there have been no updates to the IBC and ASCE 7 standards, it's up to engineers to use their best judgment when it comes to designing structures in higher seismic zones. In the future, we may see these codes and standards change, but for now we'll rely on the seismic maps and actively checking the data USGS distributes to help our clients make the best and safest decisions.