MULTI-DISCIPLINARY SOLID EARTH SCIENCE BELOW THE WATERLINE: THE NANKAI TROUGH SEISMOGENIC ZONE BOREHOLE OBSERVATORY AS AN EARTHSCOPE ANALOG

Tectonic research in the marine setting has long required a high level of coordination and collaboration among geologists, geochemists, and geophysicists, especially when it involves borehole drilling. Recently, dozens of scientists from many countries have participated in two ODP drilling legs and associated geophysical studies focusing on the plate interface decollement at the Nankai Trough off southwest Japan. Results of this project include detailed fault zone architecture, evidence for near-lithostatic fluid pressure in the vicinity of the fault, and seismic reflection signature of fault zone physical properties, at locations well up-dip of the seismogenic zone.

One limitation of these studies has been that present technology does not permit drilling into the seismogenic zone. Globally, most seismic energy is released in great subduction earthquakes, and much of the hazard comes from these large events and associated tsunami, and the Nankai Trough is the world’s best-studied great-earthquake-generating subduction zone. Hence, an international team has now proposed an ultra-deep borehole observatory be emplaced into the Nankai plate boundary seismogenic zone (NanTroSEIZE) using a new drillship under construction in Japan. This project is similar in scale to the SAFOD component of Earthscope. Elements of this project will include a suite of ~1 km deep boreholes and geophysical studies, leading up to drilling of a 4-6 km deep borehole across the plate interface near the up-dip limit of seismogenesis. Core sampling, geophysical logging, and a comprehensive long-term monitoring program will test a range of hypotheses on the mechanics and dynamics of seismic vs. aseismic fault slip, tsunamigenesis, and the nature of fault asperities. These recent and planned projects exemplify the multi-disciplinary approach to “big science” that will be essential to the success of Earthscope. The ocean drilling communities’ collective experience with fostering tight integration of subdisciplines across the earth sciences will be explored.