Oregon Researchers Discover New Ways to Date Historic Earthquakes

  Researchers at Portland State University and the University of Oregon developed a technique that may result in more accurate dating of historic earthquakes.  

The work was published in the Bulletin of the Seismological Society of America, and offers a possible solution to a frequent problem that happens when using radiocarbon dating to determine the past fault activity.  

The research abstract states, “Age constraints developed from detrital charcoal are compared with absolute dates from dendrochronology. We develop a new method for estimating inbuilt layer ages that makes use of more than just the youngest radiocarbon dates in a given layer. The improved age model relates likely layer deposition dates.”  

“This is what people really want to know: how often do these things happen? And we’re dealing with really messy data to try to give clean numbers on earthquake timing and earthquake frequency,” lead author and Portland State University Professor Ashley Streig told OPB.   

Radiocarbon (Carbon-14) dating is a primary method that paleoseismologists like Streig use to estimate the age of earthquakes. Large earthquakes sometimes rupture the ground’s surface resulting in cracks or ridges. Over time, new sediment covers the ruptures.   

Digging down into the earth at these fault lines allows researchers to see a cross-section of sediment, including the layers that were shifted during an earthquake. The top of those layers would have been exposed when the earthquake happened and the organic material there can be dated with radiocarbon. Charcoal from wildfires is in some ways ideal because it stays preserved. In theory, the presence of charcoal indicates the top layer for dating purposes, but sometimes there are inconsistencies.   

The new method involves using tree rings to help calibrate the radiocarbon dating.   

“Our hope is that we can take sites where we have a lot of charcoal that’s been dated, and we can try to use the approach that’s presented in this paper to reduce the uncertainty on the age of these earthquakes,” said Streig. She is using the technique to figure out when the Gales Creek Fault and Mount Hood Fault were last active.  

By Samantha Sied