Seismic biomarkers in Japan Trench fault zone reveal history of large earthquakes
In the aftermath of the devastating Tohoku-Oki earthquake that struck off the coast of Japan in March 2011, scientists were stunned by the unprecedented 50 meters of displacement along the fault, which ruptured all the way to the surface of the seafloor. This extreme slip at shallow depths exacerbated the massive tsunami that, together with the magnitude 9.1 earthquake, caused extensive damage and loss of life in Japan.
In a recent study published in Nature Communications, researchers used a new technique to study the faults in the Japan Trench, the subduction zone where the Tohoku-Oki earthquake struck. Their findings reveal a long history of large earthquakes in this fault zone, where they found multiple faults with evidence of more than 10 meters of slip during large earthquakes.
鈥淲e found evidence of many large earthquakes that have ruptured to the seafloor and could have generated tsunamis like the one that struck in 2011,鈥� said coauthor Pratigya Polissar, Associate Professor of Ocean Sciences at UC Santa Cruz.
Heather Savage, an Associate Professor of Earth and Planetary Sciences at UC Santa Cruz, and Polissar have developed a technique for assessing the history of earthquake slip on a fault by analyzing organic molecules trapped in sedimentary rocks. Originally synthesized by marine algae and other organisms, these 鈥渂iomarkers鈥� are altered or destroyed by heat, including the frictional heating that occurs when a fault slips during an earthquake. Through extensive laboratory testing over the past decade, Savage and Polissar have developed methods for quantifying the thermal evolution of these biomarkers and using them to reconstruct the temperature history of a fault.
The Japan Trench Fast Drilling Project (JFAST) drilled into the fault zone in 2012, extracting cores and installing a temperature observatory. UCSC seismologist Emily Brodsky helped organize JFAST, which yielded the first direct measurement of the frictional heat produced by the fault slip during an earthquake. This heat dissipates after the earthquake, however, so the signal is small and transient.
鈥淭he biomarkers give us a way to detect permanent changes in the rock that preserve a record of heating on the fault,鈥� Savage explained.
For the new study, the researchers examined the JFAST cores, which extended through the fault zone into the subducting plate below. 鈥淚t鈥檚 a complex fault zone, and there were a lot of faults throughout the core. We were able to say which faults had evidence of large earthquakes in the past,鈥� Savage added.
One of their goals was to understand whether some rock types in the fault zone were more prone to large slip in an earthquake than other rocks. The cores passed through layers of mudstones and clays with different frictional strengths. But the biomarker analysis showed evidence of large seismic slip on faults in all different rock types and that differences in frictional properties do not necessarily determine the likelihood of large shallow slip or seismic hazard.
Jamie Kirkpatrick and Christie Rowe, professors in 平特五不中鈥檚 Department of Earth & Planetary Sciences and co-authors on the new study, were part of the drilling expedition that recovered the sediment cores from the seafloor.
鈥淲hen the drilling samples were brought onto the ship, we were tasked with identifying cores that seemed to have interesting faults in them,鈥� said Rowe. 鈥淲hen the analyses for the paper were finished, the work showed that there were some major faults that we totally missed during our careful description on board. This is a great example of how powerful a multidisciplinary approach can be. By combining our geological observations with the analyses of the organic biomarkers, we have a much more complete picture of the subduction thrust system that generated the M9 earthquake in 2011.鈥�
鈥淲e鈥檝e tested this technique in different rocks with different ages and heating histories, and we can now say yes, there was an earthquake on this fault, and we can tell if there was a large one or many small ones,鈥� Savage explained. 鈥淲e can now take this technique to other faults to learn more about their histories.鈥�
About this study
鈥溾�� was published by H. S. Rabinowitz, H. M. Savage, P. J. Polissar, C. D. Rowe and J. D. Kirkpatrick was published in Nature Communications.
This work was funded by the National Science Foundation.
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