Collaborative Research: Developing Argon Techniques to Elucidate Earthquake Chronologies (DATEEQ)

DESCRIPTION: Our understanding of how earthquakes start, propagate, and stop is hindered by the fact that they occur deep underground where we cannot directly observe faults before, during, and after the quake. To better study what faults do at depth, one can drill through them or look at faults that used to be buried deep in the Earth but have been exposed at the surface. In this study, the researchers look at rock samples from drill cores through faults to identify areas of the fault that have experienced earthquake slip. Because individual earthquakes can slip along zones with small thicknesses - just millimeters compared to the total fault thickness of 10s-100s of meters) - it is unlikely that the whole fault will have experienced earthquakes. The goal of this study is to develop a new technique to determine not only where earthquakes have occurred in a fault, but when. This is important because longer earthquake records (on the million-year timescale) allow to better understand earthquake cycles and develop better earthquake forecasts. Here, by measuring the ratio of radioactive potassium to its decay product argon in the fault rocks, the team determine how long it has been since the rock was heated by earthquakes. This is a novel application of one of the best-established geological dating techniques. The study takes advantage of the fact that earthquakes generate a lot of heat from friction; the generated heat can reset the potassium-argon clock, thereby recording the time of the earthquake in the rock record. The project supports graduate and undergraduate students at University of California – Santa Cruz (UCSC, a Hispanic serving Institution) and Columbia University. It broadens participation in Earth Sciences by involving students from underrepresented group at UCSC and from the City University of New York. It also fosters outreach to local schools and the public.

To characterize past earthquake occurrences, the researchers first run a series of experiments to determine how fast the argon is liberated from its host mineral when heated by an earthquake. They run these experiments on several different rocks at different times and temperatures (up to several hundred degrees in just a few seconds). The experiments allow quantifying how fast argon diffusion occurs. Next, the team measure potassium and argon in drill core rocks from major faults including the subduction zone off of northern Japan (which hosted the 2011 Mw 9.1 earthquake) and the Hikurangi subduction zone off of the North Island of New Zealand. Previous work on these cores using a geochemical method for measuring rock temperature has demonstrated that hot earthquakes have occurred on these faults. However, this previous method, using the change in molecular structure of organic matter when heated, only provides a temperature estimate with no indication of when the earthquake occurred. By combining dating and geochemical methods, the researchers create dated records of earthquake slip over millions of years.