In the past few years, Oklahoma has experienced thousands of earthquakes thought to be related to wastewater injection, or induced seismicity. Few of these seismic events seemed to occur on mapped faults, thus making seismic hazards difficult to estimate. A study by the US Geological Survey (USGS) and the Oklahoma Geological Survey (OGS) has provided new insight into the geology of state and seismic activity.
Low-altitude airplane flights were made last August through October to map magnetic images miles beneath 18 Oklahoma counties with high groupings of seismic activity. The data revealed contacts of different rock types, some in linear patterns similar to faults. Many of these contacts were aligned with earthquake sequences, suggesting they represent ancient faults not previously identified that had been reactivated due to fluid injection. Some of these linear features indicate the deep rocks have a dominant preferred orientation or ‘grain direction’ that is favorably oriented for fault slip under natural background stresses within the earth, such as wastewater injection.
Previous geological maps have shown relatively young faults in shallow rocks. The recent magnetic data identified information on deeper, older rocks. Differences in the fault directions between these rock types are likely due to the different histories of ancient tectonic and magmatic events that shaped the rocks.
The study results were recently published in Geophysical Research Letters last month. Lead author for the study, USGS scientist Anji Shah stated, “We are hoping the results will be used to guide more detailed studies at local scales to assess potential earthquake hazards.”
Dr. Jeremy Boak, OGS Director, emphasized the fact. “The ideal is we’d be able to point to an area and say here’s an area that’s at risk. That’s what you’re really targeting.”
Additional information may be found on the USGS website.