67 DECEMBER 2021 WorldWide Drilling Resource® RENEW - SUBSCRIBE NOW! Tracking Past Earthquakes on the Ring of Fire Adapted from Information by the International Ocean Discovery Program and European Consortium for Ocean Research Drilling Japan is located on the Pacific Ring of Fire which is a region of special interest in earthquake research since most of the planet’s earthquakes occur along this ring. It is here that oceanic plates bend and move below overriding plates, while accumulating long-term global plate tectonic strain. This energy is released catastrophically during so-called subduction zone or megathrust earthquakes. The range of possible earthquake magnitudes, and specifically the processes driving giant earthquakes - i.e. earthquakes with a moment magnitude (Mw) of nine or more - and how often they occur, however, is not fully understood. High-impact earthquakes and associated tsunamis - for example the Sumatra Earthquake in 2004 and the Tohoku-oki Earthquake in 2011 - are major geological events with devastating implications for society and infrastructure, and represent the greatest hazard for coastal societies in the Pacific region. Giant earthquakes such as these are likely to happen again, but instrumental and historic records of these events are currently insufficient to adequately assess seismic and tsunami hazards in areas located close to subduction plate boundaries. An improved perspective of earthquake maximum magnitude and recurrence requires looking back into the geological record. This was the aim of the International Ocean Discovery Program (IODP) Expedition 386 “Japan Trench Paleoseismology”, involving a team of scientists who left the port of Yokusuka onboard the research vessel Kaimei. While on this research voyage, IODP Expedition 386 reached two new depth records in scientific ocean drilling and coring. Rewind to 43 years ago when the legendary drilling vessel Glomar Challenger set the record for the deepest coring site in 50 years of scientific ocean drilling by recovering two cores - one roughly 50 feet long and the other 67 feet long - below waters over 23,000 feet deep in the Mariana Trench. That record remained until 2021 when Captain Naoto Kimura positioned the Kaimei vessel at IODP Expedition 386 Site M0081 where the water depth is over 26,000 feet. The crew prepared the 131-foot-long giant piston corer (GPC), and ran it into the water. The GPC is basically a long tube with a large weight at the top. The GPC is deployed over the side of the ship, initially rotating it from a horizontal orientation on deck to vertical just outboard. A transponder is then attached to the winch and a trigger arm is locked in place. This provides depth and location information. Once this process is complete, the assembly is lowered to the seabed. The GPC is lowered relatively slowly, but as it passes through the water column, the core barrel rotates slightly. To improve sample collection, the winch is stopped roughly 328 feet from the seabed and allowed to stabilize before being lowered toward the seabed again. At this point, the trigger core is engaged. This core hangs lower in the water than the GPC, and when it touches the seabed, the weight on the trigger arm comes off releasing the excess winch wire holding the GPC allowing it to free fall into the sediment. The “piston” in the name is initially located at the bottom of the corer, but when it penetrates the seabed it moves up as the sediment enters and finally comes to rest at the maximum penetration point forming an airtight cap at the top of the barrel. When the GPC is pulled out of the seabed, the piston remains stationary ensuring the core remains in the barrel. The GPC is then hauled back up to the ship where the core tube is extracted and cut into lengths of three feet. The process of lowering the GPC assembly to just above the seafloor on Expedition 386 took around two hours and 40 minutes. It led to the new record for the deepest water site drilled and cored in scientific ocean drilling history. Over 123 feet of sediment core was curated, setting another scientific drilling depth record for the deepest subsea level sample at over 26,440 feet below sea level. Cochief Scientist Professor Michael Strasser of IODP Expedition 386 stated, “To study giant earthquake history and processes along subduction plate boundaries, we have to access the sedimentary records from these ultra-deepwater environments. With the new giant piston corer system onboard the [research vessel] Kaimei, it has now become possible to efficiently and safely sample the deeper subsurface in these environments.” The offshore phase of this research expedition lasted 50 days. “It was a really tough expedition,” said Cochief Scientist Dr. Ken Ikehara. “Many atmospheric low pressure systems and unexpectedly strong Kuroshio Current got in our way. However, we obtained giant piston cores from 15 sites along [the] entire Japan Trench.” Following the offshore phase of research, the international science team will meet for an onshore phase which will involve splitting, describing, analyzing, and sampling the cores, and combining the resulting data with data collected during the offshore phase to compile a comprehensive scientific report. The curated cores and samples will become a focus for further state-ofthe-art analyses by the wider international science community for years to come. The Giant Piston Corer during recovery. Courtesy of ECORD/IODP/JAMSTEC. ENV Research Vessel Kaimei. Courtesy of IODP. Cutting the core into lengths of three feet. Courtesy of IODP.
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