Shin'ichi Kuramoto

Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation

Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

A low-velocity zone with weak reflectivity along the Nankai subduction zone

Three-dimensional seismic reflection data reveal the presence of a low seismic velocity zone (LVZ) with weak reflectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ∼2 km thick, ∼15 km wide, and ∼120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fluid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fluid-rich LVZ supplies a significant amount of the fluid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault.

The Chikyu and Ocean Drilling Science

The construction of a Deep Sea Drilling Vessel, the Chikyu began in April 2001 and was completed in the summer of 2005 [Taira, 2005]. The Chkyu is a 210 meter-long vessel, with a gross tonnage of 57,087 tons, and a drilling derrick standing 121 m above sea level. The Chikyu is a state-of-the-art drilling platform, with a highly automated drill floor system capable of being run efficiently and safely by only a small number of operators, and provides a fully integrated riser and blow-out preventer (BOP) drilling system. The Chikyu enables operations in geological environments and at depths previously inaccessible to scientific drilling. The plan for building a new scientific deep-sea drilling vessel in Japan started more than ten years ago. JAMSTEC took leadership for this project, and construction started at the Mitsui Tamano Shipyard and then rigging for drilling system took place at the Mitsubishi Nagasaki Shipyard. The Chikyu is designed to drill deeper than ever before beneath the deep-sea floor. The target was set to drill to 7000m, in water depths initially up to 2500m. The ship is required to remain stationary for a long time at sea against wind, waves, and currents. This stability is achieved by six powerful computer-controlled thrusters with 360-degree, screw-axis rotation capability (azimuth thrusters).

Challenge to the unexplored deep earth-Integrated Ocean Drilling Program.

A brand new international scientific ocean drilling will start from October 1st, 2003, instead of the ODP (Ocean Drilling Program). The new program calls “IODP: Integrated Ocean Drilling Program” that integrates multi-platforms for scientific ocean drilling. The IODP has scientific initiatives for the future scientific activities that will be archived through the platforms. Japan has a responsibility to provide a riser drilling capability for the IODP, and constructing a state of the art drilling ship. Her name is “CHIKYU” (means the earth in Japanese). The CDEX (Center for Deep Earth Exploration) was established in the JAMSTEC and play as an implementation organization for the “CHIKYU” under the IODP umbrella. The vessel will be a core capability in the IODP and bring new scientific breakthroughs.