By Laura Fattaruso, M.S., Ph.D. Candidate, UMass Amherst (@labtalk_laura)
A new study uses organic biomarkers to detect heat produced during large earthquakes. Scientists found several faults off the coast of Japan that have hosted earthquakes of magnitude 8 or greater.
Citation: Fattaruso, Laura (2020), A new way to find old earthquakes uses organic molecules, Temblor, http://doi.org/10.32858/temblor.073
The 2011 magnitude-9.1 Tohoku-Oki earthquake and tsunami in Japan motivated widescale scientific efforts to understand how such large magnitude earthquakes occur. Now, nine years later, the history of major earthquakes in the region has been further illuminated with a new method that uses organic molecules to identify faults that have produced large earthquakes. By taking a detailed look at a drill core collected off the coast of Japan, researchers identified several faults that have produced earthquakes of magnitude 8 or greater, including the possibility of many Tohoku-Oki sized temblors.
The recent study published in Nature Communications combines methods from several fields of geology to figure out which faults have hosted large earthquakes and which ones haven’t.
“If we can understand where earthquakes have happened, can that tell us why that earthquake propagated here?” asks Heather Savage, a structural geologist at the University of California, Santa Cruz and a co-author of the study. The goals of the study, she says, were to figure that out. “Is it something about the structure or the properties of the rock? Or something else?”
Developing the method
This new method relies on organic biomarkers—any molecules left behind by living organisms. In this case, they used molecules from plant waxes and algae. The molecules accumulate in the sediment record as these organisms die and are more commonly used in other applications—like reconstructing climate history or searching for petroleum. When the organic biomarkers are exposed to high temperatures, they degrade and change in measurable ways. The amount they degrade is a function of the amount of heat they are exposed to, similar to your morning toast—the more heat applied to the toast, the darker the toast. When a fault ruptures with enough displacement to produce a large earthquake, heat is generated by the friction and fast movement between the two opposing sides of the fault.
In earlier studies, the authors investigated how these molecules change when exposed to different amounts of heat over different timescales. By comparing biomarkers near these fault surfaces to the those from sediments collected far away from the fault, researchers can estimate how much heat the biomarkers have experienced. And from that, they can estimate the size of the earthquakes that have occurred on those faults.
A new way to find old earthquakes
Faults can experience both seismic and aseismic slip—meaning that sometimes a fault may have hosted large displacements, but not generated earthquakes. During aseismic slip—often called creep—movement across the fault is slower, so there is not a rapid pulse of heat generated like there is during seismic slip, when an earthquake occurs.
Figuring out which faults have generated earthquakes is important for understanding how these quakes happen, but also poses a real challenge, since they may be visually indistinguishable from faults that have hosted no earthquakes. In cases of extreme heating, melted rocks can be left behind on the fault, a type of rock called pseudotachylyte. This method provides a new way to estimate the history of earthquakes on faults even when there is no melt, says Savage.
“We know from the geologic record that big fault movements generate heat, like we see with pseudotachylytes,” says Ben van der Pluijm, a geologist at University of Michigan who was not involved with the study. “They are showing that there’s another type of memory in natural rocks that we can use,” he says.
Piecing together the puzzle
Hannah Rabinowitz, a AAAS Science and Technology Policy Fellow at the Department of Energy and the lead author of the study, has previously published work that connected the sediments drilled from a fault zone to sediments that are far away and unaltered. This provided a baseline for figuring out how much heat the biomarkers in the faulted samples have experienced, says Pratigya Polissar, an organic geochemist at the University of California Santa Cruz, and co-author of the study.
In the new study, Rabinowitz and colleagues found that even faults that could barely be seen in the core had hosted large amounts of seismic slip—at least 10 meters in many cases.
One idea they tested was whether earthquakes would only propagate through weaker units like clays. Weaker rocks are more likely to break under stress than those that are stronger, but the researchers observed significant seismic heating signatures in all the units sampled, despite the contrasts in their strength and frictional properties. They found that faults in every type of rock sampled could have hosted Tohoku-Oki sized earthquakes.
Into the future
Many of the researchers involved in this study are already using organic biomarkers to understand faults and earthquakes in other places around the world. New regions of interest that have already been sampled include the Marin headlands in Northern California, the San Andreas Fault near Parkfield, Calif., and the Hikurangi Subduction Zone off the coast of New Zealand, which could host large tsunamigenic earthquakes—earthquakes capable of generating tsunamis—like those experienced in Japan.
“It’s an incredibly versatile technique,” says Christie Rowe, a structural geologist at McGill University and co-author. “Anywhere that you have sedimentary rocks, it could be applied.”
Further Reading
Rabinowitz, H. S., Savage, H. M., Plank, T., Polissar, P. J., Kirkpatrick, J. D., & Rowe, C. D. (2015). Multiple major faults at the Japan Trench: Chemostratigraphy of the plate boundary at IODP Exp. 343: JFAST. Earth and Planetary Science Letters, 423, 57-66.
Rabinowitz, H. S., Polissar, P. J., & Savage, H. M. (2017). Reaction kinetics of alkenone and n‐alkane thermal alteration at seismic timescales. Geochemistry, Geophysics, Geosystems, 18(1), 204-219.
Rabinowitz, H. S., Savage, H. M., Polissar, P. J., Rowe, C. D., & Kirkpatrick, J. D. (2020). Earthquake slip surfaces identified by biomarker thermal maturity within the 2011 Tohoku-Oki earthquake fault zone. Nature Communications, 11(1), 1-9.
