Demian Saffer leads the first riser drilling expedition for the seismogenic zone experiment

Professor Demian Saffer served as co-chief of Expedition 319 of the Integrated Ocean Drilling Program (IODP) that conducted drilling operations at three drill sites in the Nankai Trough offshore Japan. At the first site, located directly above the seismogenic zone where great earthquakes occur, scientists conducted the first riser drilling in the IODP history and successfully drilled down to a depth of 1,603.7 meters beneath the sea floor (click to continue reading).

Riser based drilling allowed the scientists to conduct several scientific operations unprecedented in IODP, including 12 successful measurements of stress and pore fluid pressure in the subsurface using the dynamic formation testing tool, a two ship seismic experiment using a dense seismic array in the borehole, real-time mud gas analysis, and laboratory analyses of drill cuttings that are generated as the drill bit penetrates through the formation.  In addition, 57.87 meters of core sample (a cylindrical geological sample) were obtained from depths between 1,510 and 1,593.9 meters below the seafloor. The scientific party developed several new techniques for analyzing these materials, which will be essential for future riser based drilling.

The stress and pore pressure measurements are critical to understanding the mechanics of active tectonic fault zones, but have previously been unavailable in the scientific ocean drilling. Successful deployment of the test tool to measure these quantities deeper within the upper plate and near major fault zones in future riser holes will constitute a major breakthrough in understanding subduction zone fault earthquakes.

Also a walk-away Vertical Seismic Profiling (VSP) involving the CHIKYU and JAMSTEC’s Research Vessel KAIREI was conducted to characterize the structure of the seismogenic plate boundary below the borehole by utilizing an array of seismic sensors temporarily clamped inside the borehole. Air guns towed by the KAIREI generated seismic waves, and reflected seismic waves from the fault system were clearly observed by the borehole sensors. Experience from the VSP experiment will open the way to in-depth study of seismogenic faults that are beyond the reach of drilling.

At a second borehole (Site C0010), drilling crossed one of the major faults in the plate boundary, known as the mega-splay fault, at a depth of about 400 meters below the seafloor. This fault is a prime candidate for tsunami generation, and may have slipped in historical great earthquakes. During the drilling operation, scientists documented rock physical properties and gained information about stresses in the formation. The borehole was then cased and utilized for observatory operations for future long-term borehole monitoring. These included lowering of test instruments, as well as emplacement of a temporary sensor package that will monitor conditions in the fault zone in the next few years.

The new data from Expedition 319 indicate that the stresses in the upper plate reflect the forces acting on the earthquake generating fault zones below. The direction of the maximum stresses follows the direction of tectonic plate motion in most of the region, but rotates drastically in a very narrow region above the mega-splay fault. In addition, the rock units, and in particular the ages of the rocks obtained by examining microfossils and the sediment types observed in the drill cuttings, provide new constraints on the geologic history of the major fault zone and its activity level.

The CHIKYU is now berthed at the Port of Yokkaichi for the rotation of science parties and loading of materials. It is scheduled to set sail on September 4th to begin the following IODP Expedition 322, which will conduct coring operation by riser-less drilling at the Site C0011, the sediment before being subducted to the seismogenic zone until around October 10th. The Expedition 322 is to be led by Co-Chief Scientists, Prof. Michael Underwood, the University of Missouri, and Dr. Saneatsu Saito, the Institute for Research on Earth Evolution of JAMSTEC, joined by twenty-seven scientists from nine countries. Operations will include core sampling and logging for all layers in the formation, with an aim to better understand the initial state of geological input materials before they are entering the seismogenic zone.