Great megathrust earthquakes and the tsumanis they generate are among are the greatest threats to populated coastlines worldwide, such as Chile, Sumatra, Alaska, and Cascadia (the regions extending from southern British Columbia into northern California). The risk of catastrophic consequences and the acute need for improved understanding of these events are underscored by the devastation caused by the 11 March 2011 Tohoku-Oki earthquake off northern Japan. While the destruction from these events is acute, they also offer important opportunities to gain new insights into the processes that spawn them. In the wake of the earthquake in Japan, the proponents of this project have been invited to collaborate with Japanese scientists at JAMSTEC (Japan Agency for Marine Earth-science and Technology) to investigate and document the effects of the earthquake offshore. The broader impacts of this program include enhanced international collaboration, and a contribution to instruction at the investigators? respective institutions, but by far the greatest broader impact of this activity is the very high societal relevance of studying these catastrophic events.
The Japan Trench marks the boundary where the Pacific Plate starts subducting beneath Honshu Island. The 2011 Tohoku earthquake occurred along that plate boundary and was the largest earthquake to occur? since the 2004 Indian Ocean earthquake. The seafloor displacement caused by this earthquake generated a large tsunami that was responsible for killing more than 16,000 people and devastating the NE coast of Honshu. Irreparable damage to nuclear reactors at the Fukushima power plant is having long-term national and international consequences. The objective of this project was to initiate collaboration with Japanese scientists to investigate the offshore impacts of that great earthquake, to determine the geological factors that control tsunamigenic earthquakes, and to assess the approximate recurrence intervals of these great earthquakes. The evaluation of the seismic hazards associated with any fault critically depends on understanding its seismogenic evolution through time. Major earthquakes along a fault typically recur every few centuries to several millennia, and historical records rarely extend long enough to contribute sufficient information. Earthquake geology, or paleoseismology, is the investigation of individual earthquakes from their geologic signatures. It applies forensic analysis to the crustal deformation and sedimentary features that resulted from prior earthquakes. Paleoseismology has become a primary tool for seismic hazard evaluation on land, but few paleoseismologic studies are attempted on submarine fault systems. The investigators on this project have collaborated for years on developing a methodology for submarine paleoseismology, investigating fault systems beneath the Marmara Sea (Turkey) and offshore Haiti. In the wake of the devastating 2004 Indian Ocean earthquake and tsunami, they also initiated collaboration with Japanese scientists at the Japan Agency for Marine Earth-science and Technology (JAMSTEC) to study the causes and effects of that great earthquake. Hence, investigating the 2011 Japan earthquake with the same Japanese team came as a natural extension of their effort. Interestingly too, because the Tohoku earthquake occurred in an area that had been well surveyed by Japanese scientists prior to that earthquake, post-earthquake investigations are offering the first-ever opportunity to precisely document and quantify the morphological, sedimentological, and structural changes that occur along a submerged plate boundary following an earthquake. In 2011-2012, the investigators participated in scientific meetings in both the US and Japan and met with JAMSTEC scientists to discuss a joint investigation of the Japan Trench. While most deep-submergence vehicles are typically limited to surveying in waters shallower than 6,000-6,500 m, the new American remotely operated vehicle (ROV) NEREUS is well adapted for the investigation of the Japan Trench, down to 7,500 m. Drs. Kodaira, Fujiwara, and Kanamatsu from JAMSTEC provided the investigators with geophysical data, including detailed bathymetric data, as well as recently acquired seismic reflection and chirp data, a type of data that image subsurface geological structures along ship tracks. In parallel, Dr. Kanamatsu invited one of the investigators (McHugh) to participate in two research cruises that took place in 2013, to collect sediment cores across the Japan Trench. 31 piston cores averaging 5 m in length were recovered in a range of water depths (1000-6000 m) along the Japan Trench. The three investigators are now interpreting and analyzing the combined sediment and geophysical dataset and preparing a joint publication with their Japanese colleagues. Preliminary results are as follows. Surficial sediments in some of the new cores present some high concentration of the short-lived radioisotopes released by the Fukushima nuclear plant disaster, and also show evidence of having been reworked by mass movement. This is tentatively interpreted as an indication that the 2011 earthquake triggered a mass-wasting event within the sampling area. Sediment cores also exhibit variations in sedimentary characteristics that may be linked to variations in the morphology of the trench. North of 39°N, there exists a series of small, closed-contour basins in water depths ranging from 4,500 to 6,000 m; these basins appear to collect any sediments shed by ground shaking and marine landslides. This is evidenced by a succession of sedimentary units characteristic of earthquake events – known as "turbidite-homogenite". Assuming sedimentation rates of ~1 mm/year, simple calculations suggest that large earthquake may recur on average every 800-900 years in that area. Similar slope basins are not as well-defined south of 39°N, but some of the cores in this southern area show evidence of sediment fluidization. The initial interpretation is that these fluidization structures result from strong shaking during earthquakes The investigators have submitted a new proposal to NSF for a detailed, near-bottom investigation of the Japan Trench using deep-submergence technology, in direct collaboration with Japanese scientists. The proposed very high-resolution geophysical survey and sediment sampling program is expected to provide new insights on tsunamigenic great earthquakes. Results from this project are expected to provide much needed information for the assessment of seismic and tsunami hazards in Japan.
