Scientists in Oklahoma have unraveled different types of earthquakes that can be hard to see in the squiggles of seismograms that record shaking.
By Meghomita Das, Palomar Fellow (@meghomita)
Citation: Das, M., 2022, On the lookout for unique quakes in Oklahoma, Temblor, http://doi.org/10.32858/temblor.269
Oklahoma, a centrally situated U.S. state on the North American tectonic plate, unexpectedly shakes from occasional earthquakes. In fact, in recent years, this state has had more than its fair share of earthquake activity that primarily results from injection of wastewater into the ground and during hydraulic fracturing, or fracking, to produce oil and gas. Oklahoma’s rapid increase in induced earthquakes has led to ongoing monitoring to understand the behavior, energy and potentially devastating impacts of these events.
In a recent study led by a team of seismologists at the University of Oklahoma, researchers extracted two new types of earthquakes whose seismic signatures don’t look like typical temblors. By distinguishing these unique and difficult-to-detect signals, the scientists can better understand the degree of seismic-related damage that might be expected for Oklahoma — a state that may lack earthquake-resistant structures due to historically low seismic activity.
Quake hunting
On a seismogram — the wavy lines recorded at seismic stations that monitor Earth’s movement — a typical earthquake stands out to seismologists with a distinct pressure or a primary wave (p-wave) followed by an equally sharp shear or a secondary wave (s-wave). However, the newly identified earthquakes’ seismograms contain more complicated signals.
So-called “overlapping earthquakes,” the first type of signal identified by the researchers, happen when many small earthquakes (often less than magnitude-3) occur so closely in time and space that they appear to create one big signal. The combined signal obscures information from the individual events and could mislead researchers into thinking that they’re seeing a single, continuous earthquake.
The second newly identified earthquake type, which the researchers called “multiphase arrival earthquakes,” also had similarly small magnitudes. In this case, waves from a single small earthquake bounce along the boundary between contrasting rock layers, giving the illusion of multiple earthquakes occurring simultaneously. Both types of earthquakes could skew estimates of ground shaking in seismic hazard assessments, necessitating proper detection and classification of these events.
Unsurprisingly, finding these signals can be challenging. “You want a dense and well-distributed network of seismic stations that can detect these small earthquakes,” says Paul Ogwari, a seismologist at the University of Oklahoma and the lead author of this study, published in Seismological Research Letters. “Proper identification of these events is important to understand how likely the fault is to produce an earthquake. If you count the overlapping earthquakes as one, then you are underestimating the count of total earthquakes,” he explains. On the other hand, by counting a multiphased earthquake as multiple events, “then you are overestimating [the count],” he says.
Making a (unique) earthquake
The team found that some of the overlapping earthquakes occurred on faults that previously ruptured in some of Oklahoma’s largest magnitude events. Moreover, the occurrence of the overlapping earthquakes matched closely with the areas undergoing intensive hydraulic fracturing, which is an oil extraction and production method that involves fracturing the bedrock with high pressure water jets. This process, commonly used in Oklahoma, could cause changes in stress that might trigger overlapping earthquakes in the area.
The mechanism for the multiphase arrival earthquakes, however, is a bit different, likely relating to contrasting rock properties found in Oklahoma’s sedimentary basins. In such locations, layers of loose sand and other sediment sit on top of rigid bedrock. These contrasting rock types create a density contrast between the layers such that when an earthquake’s seismic waves try to pass from the loose sediment layers to bedrock, some of the energy gets reflected back into the sediment. These reflected waves can then bounce along the boundary between the layers, as though they are trapped in the basin. A bouncing wave can seem like multiple earthquakes whose seismic waves are arriving at the same time, when, in fact, these signals come from a single event.
Consequences of atypical tiny temblors
To cause damage, earthquakes must shake the ground, which tiny temblors tend not to do. Earthquakes induced by fracking or injecting wastewater into the ground for disposal purposes can cause damage if they are big enough though, says Honn Kao, a seismologist at Natural Resources Canada who was not associated with this study. These two new types of earthquakes tend to be associated with wastewater injection and fracking. Thus, if one of these earthquakes did cause shaking that resulted in damage, Kao says, “the prolonged duration can make the damage … worse.”
This extended shaking could be especially hazardous in areas with loose, water-rich soils, common throughout the Oklahoma basin that is replete with loose sediment left by both ancient and active rivers. Any prolonged shaking could cause these sediments to liquefy, where they behave like a mobile slurry of soil and water. This liquefaction effect can increase the damage inflicted by otherwise small quakes.
Because the multiphase arrival earthquakes occur where loose sediment sits on top of rigid rock, detecting the earthquake’s correct location can help determine the depth at which these events occur across the basin, with an eye toward a more realistic hazard map. Also, Ogwari says, the team plans to extend their analysis to other parts of the basin to hunt for additional events hidden in the seismic record. “We plan to automate our methods of detecting these events using artificial intelligence,” he says, “so that the process of finding these events happens faster and can be expanded to other regions as well.”
Meghomita Das is Temblor’s Palomar Fellow. She is a Ph.D. candidate at McGill University in Montreal, Canada, where she studies the signals of ancient earthquakes and slow slip events (www.meghomita.com). Palomar Holdings is sponsoring a science writing fellow to cover important earthquake news across the U.S.