The year was 1944. The place: Tonankai, near the south coast of western Honshu. The event: A historic 8.1 magnitude earthquake that killed at least 1,200 people and destroyed more than 73,000 homes. This catastrophic event spurred scientists to research why part of the seafloor near the southwest coast of Japan is particularly prone to generating devastating tsunamis.
The Nankai Trough, located south of Honshu, Japan, is in a subduction zone also known as an area where two tectonic plates are colliding, pushing one plate down below the other. The grinding of one plate over the other in subduction zone leads to some of the world’s largest earthquakes. To date, the Nankai Trough subduction zone may be the most studied subduction zone in the world.
At UH Mānoa, leading the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is Marine Geophysics Professor Gregory Moore. NanTroSEIZE is a large international effort using many kinds of oceanographic studies to understand the region within the Nankai Trough that has a 2,000 year recorded history of very large earthquakes and tsunamis. The behavior of the Nankai Trough is very similar to regions off Sumatra and northeast Honshu where recent devastating earthquakes have occurred, and is also similar to the region offshore of Oregon and Washington where scientists expect a very large earthquake to occur in the future.
The project has characterized the region and local structure of the rocks and sediments with this zone. Thus far, the international group has used a new Japanese drilling ship to drill several holes and obtain samples from the upper mile below the seafloor. The current phase involves drilling a deep hole into the region to a depth of about 4.5 miles below the seafloor, which is 1.5 miles deep (so the total depth below the sea surface is about 6 miles).
“We will characterize the physical properties of the rocks within the zone that generates earthquakes and will leave a system of instruments that will continuously monitor the temperature, pressure and stress around the borehole to try to understand how the rocks are being deformed during the time leading up to the next large earthquake,” noted Moore.
Moore has been involved since 1987 in characterizing the features of this submarine region, including leading a project with a petroleum industry ship that collected 3-dimensional seismic reflection data around their current drilling area in 2006 (3D seismic data is a very large CAT scan).
He joined the first drilling expedition in 2007 with D/V Chikyu in the NanTroSEIZE area, and has been involved in preparation for the subsequent drilling expeditions and in analyzing the data from those expeditions. Stage 2 of NanTroSEIZE took place during June-October 2009, and Stage 3 began in 2010 and is scheduled to continue in 2012. Moore plans to participate in one or two of the upcoming deep drilling expeditions.
The work offshore Japan is part of a regional early-warning system that gives the Japanese a few minutes warning of approaching earthquake waves and tsunamis. According to Moore, such instrumentation has not yet been deployed around the United States, but their work will help this development.
Further related UH Mānoa research projects include a Tohoku University colleague arriving from Japan to study correlating earthquake locations with features observed on seismic reflection records for six weeks and two of Moore’s graduate students participating in a Japanese research expedition to help make images of the region of the March 2011 earthquake off Tohoku.
The findings from the NanTroSEIZE experiment will help to explain what causes these types of earthquakes, how to better effectively plan for their occurrence, and to help scientists assess the risk of giant tsunamis in other regions of the world. In essence, it’s earth-shaking research.
Top photo: Petroleum industry ship used to collect 3D seismic data.