The Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President) aboard the Deep-sea Scientific Drilling Vessel Chikyu has been conducting IODP Expedition 358, “NanTroSEIZE Plate Boundary Deep Riser 4: Nankai seismogenic/slow slip megathrust” since October 7, 2018 (reported September 27, 2018). The original plan of this expedition was to deepen the existing cased borehole at Site C0002 in the Kumano Basin, and to reach the plate boundary fault lying below a high-velocity zone by using logging while drilling (LWD) technology and drill cuttings analysis, in addition to limited coring intervals. Having ended drilling at Site C0002, we have decided to carry out contingency plans for riserless drilling at other sites in the Nankai Trough.

Site C0002 has a complex geological structure, called an accretionary prism, which was formed by intensive deformation (folding) and fractures (faults) of the sediments coming off a subducting oceanic tectonic plate. The plan for this expedition, drawn up through repeated and detailed discussions with researchers and engineers in Japan and abroad, called for the use of the best approach available at present to drilling, that is, to use sidetracking to drill deeper from the deepest section of the previously cased borehole. The geological structure at this site, however, made further drilling more difficult than anticipated. Drilling at this site thus ended at a shallower depth than the intended plate boundary fault target.

On December 7, 2018, this expedition reached 3,262.5 meters below sea floor (1,939 m water depth) a new depth record in scientific ocean drilling. Furthermore, core samples of 2.5 meters in total length were recovered from 2,836.5 to 2,848.5 meters below sea floor, also a new depth record in scientific ocean coring. Research using the data and samples collected by this expedition will provide new insights into seismogenesis in the Nankai Trough area.

As of this writing, JAMSTEC has sealed the hole and is proceeding with recovery of the blow out preventer (BOP) and riser pipes to end drilling operations at this site. Once completed, the plan is to move to riserless drilling at the contingency sites in the toe region of Nankai Trough and in the Kumano Basin, as described in the appendix. These operations target such phenomena as slow slip events near the trough axis, and their relationship with mega-earthquakes having fast slip along the plate boundary. This is a phenomenon that was only recently observed here by several long-term observatories previously installed during earlier Chikyu IODP expeditions. The contingency plan aims to complement and reinforce scientific results from previous NanTroSEIZE project expeditions.

Appendix

International Ocean Discovery Program (IODP)
Expedition 358 contingency plan

1. Schedule

2. Objectives and operation plan

(1)

Plan 1: The relationship between great earthquakes (fast slip) and slow earthquakes
One recent focus of observation and research is on slow earthquakes, related to the crustal deformation surrounding the source region of great earthquakes. Slightly landward of the Nankai Trough, we have evidence of fast slip from recovered core at Site C0007 during IODP Expedition 316. In addition, the Long-Term Borehole Monitoring Systems installed during IODP Expeditions 332, 365, and 380 are sending real-time tectonic activity data to shore stations.

Based on recent progress in research on slow earthquakes, we plan to drill and investigate the geological and geophysical properties and condition at and above the frontal portion of the plate boundary fault at Site NT1-03C. Logging while drilling and coring are planned (up to 1,000 meters below sea floor). We anticipate that the data and samples collected by this plan will clarify behavior at the very tip of the plate subduction zone (such as the properties where both fast slips and slow earthquakes can occur). We believe that this research (at the shallow portion) will help expand our understanding of the mechanisms behind the great earthquakes and tsunamis that occur at the deep portion of plate boundary faults.

(2)

Plan 2: Background of geological evolution for the onset of seismogenic zone of the Nankai Trough
One of the key scientific implications from the NanTroSEIZE project is that the subduction at the Nankai Trough and the formation of the accretionary prism probably restarted ca. 6 million years ago. In this expedition, new coring at Site KB-01C (up to 600 meters below sea floor) at the northern margin of the Kumano Basin and into the underlying accretionary prism will document the precise age of the onset of the Nankai forearc and accretionary prism.

One of the primary properties of the host rock of the seismogenic fault is the elastic property to store strain energy. The accretionary prism, composed of sediments, starts from an unconsolidated state. The soft sediments evolve to hard rock (sedimentary and metamorphic rocks) with enhanced elastic properties through physical processes of compaction, deformation, and grain breakage along with chemical processes of fluid rock interaction under the appropriate thermal and pressure conditions.
If the present Nankai accretionary prism is ~6 million years old, it suggests an unusually fast transformation from sediments to rock and rapid development of the high-velocity zone which is considered to store strain energy in the hanging wall above the seismogenic megathrust. Documenting such an evolving background, by drilling, is a key to understanding the physical-chemical evolution of the Nankai seismogenic zone.

3. Science team