Earthquake archive in the geological record off Japan

Our perspective of earthquake maximum magnitude and recurrence is limited by short historical and even shorter instrumental records, which are inadequate to fully characterize Earths complex and multi-scale seismic behavior. Motivated by the mission to fill the gap in knowledge about the long- term history of great earthquakes, we investigate the traces, which major earthquakes leave behind on the ocean floor to calibrate the marine sediment record for studying past earthquake in geological archives. As documented after the devastating Magnitude 9 Japan Earthquake and Tsunami in 2011, continuous records of comparable past natural impacts are preserved in the geological archive as characteristic event deposits formed by soft marine sediments remobilized by turbidity currents, that were induced due to instabilities of the ocean floor upon strong earthquake shaking. The EAGER-Japan projects aims at characterizing and precise dating of such extreme event deposits from the marine geological record of the Japan Trench by applying novel analytical techniques applied on samples recovered from the deep sea during sea-going research expeditions. In particular, we aim at identifying the source and transport dynamics of materials remobilized during past extreme events by analyzing composition and texture of individual sand grains, and to measure radioactive decay of individual organic compounds to constrain the timing when such remobilization occurred in the past. The results of these analyses will reveal a dense spatio-temporal data set elucidating when and where strong earthquake shaking has impacted the ocean floor offshore NE Japan in the past. While contributing to unravel a potentially fascinating earthquake history for Japan beyond any current knowledge from historical archive, anticipated conceptual advances in our understanding of causes and consequences of earthquakes and related geological processes in offshore environments are expected to be transformative to other regions worldwide, and may even open up new perspectives for deciphering the rock-record from outcrops on land.

Our perspective of earthquake maximum magnitude and recurrence is limited by short historical and even shorter instrumental records, which are inadequate to fully chara cterize Earths complex and multi-scale seismic behavior. Motivated by the mission to fill the gap in knowledge about long-term earthquake histories, this project investigated the traces, which earthquakes offshore Japan left behind on the ocean floor, aiming at developing the methodology to decipher marine sedimentary records for investigating past earthquakes. By initially learning from what sedimentary deposits were generated from the 2011 giant (magnitude 9+) earthquake in the Japan Trench (by analyzing geophysical data and sediment samples recovered during sea-going expeditions), we further investigated these data and samples for older event deposits to compare occurrence and distribution to known historical earthquakes. Surveying reveals that several insolated ultra-deep water basins (7-8km water depth) along the Japan Trench plate boundary comprise the terminal sinks of earthquake- triggered sediment and organic carbon remobilization. For the 2011 earthquake, we mapped and documented the entire spatial distribution and detailed characteristic of the resulting event deposits and constrained the dominant mechanism of how earthquakes remobilize sediments: Strong seismic shaking remobilizes the uppermost few centimeters of surficial slope sediment that also contain high amount of fresh organic carbon which gets transported by sediment gravity flows to the terminal sinks. We quantified the total amount of organic carbon that was transported to the trench by the 2011 giant earthquake to be more than 1 million tons, and show that giant earthquakes can have important influences on the deep -sea carbon cycle. Furthermore, our results reveal that the continuous sedimentary successions of trench basins also comprise several older event deposits. Their spatio-temporal distribution and sedimentologic characteristics (that we could analyze in available cores from the upper 10m of the geological archive covering the past ~2000 years) correlate well to major past earthquakes and tsunamis reported in Japanese History that also must have induced surficial sediment remobilization. Thus, its as if the sedimentary archive of individual basins were ultra- deep-water seismographs that have been recording major earthquake events in the past. The project succeeded in calibrating these deep-marine paleoseismometers and in developing the novel multi-proxy methodology needed to decipher its long-term earthquake history. The project also significantly contributed to the development of the International Ocean Discovery Program (IODP) Expedition 386 Japan Trench Paleoseismology, that just recently cored the deeper subsurface. This expedition recovered the paleoseismometers from the deep sea to be analyzed in follow-up projects. This will unravel the earthquake history over time periods extending several tens of thousands of years into the past, constrain earthquake maximum magnitude and recurrence, and help to better assess earthquake hazards in the future.