The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multiexpedition, multistage project focused on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults. The focus of this project is the Nankai Trough region off the Kii Peninsula off Japan (Fig. 1). This region has suffered some devastating earthquakes and tsunami events in modern times (1944 and 1946), and has a historical record of these events for the last 1300 years.
The drilling program includes a coordinated effort to drill (i.e. collect downhole logging data and core samples) and instrument (with sub-seafloor observatories) the plate boundary system at several locations offshore the Kii Peninsula off Japan’s main island. The main objectives are to understand:
- The mechanisms and processes controlling the updip aseismic–seismic transition of the megathrust fault system, including fault processes associated with very low frequency (VLF) earthquakes and tremor;
- Processes of earthquake and tsunami generation;
- Mechanics of strain accumulation and release;
- The absolute mechanical strength of the plate boundary fault; and
- The potential role of a major upper plate fault system (termed the “megasplay” fault) in seismogenesis and tsunamigenesis.
Subduction zones account for 90% of the global seismic moment release and generate damaging earthquakes and tsunamis with potentially disastrous effects on heavily populated coastal areas, e.g. the Sumatra 2004 and Tohoku 2011 earthquakes and tsunami. Understanding the processes that govern the strength, nature and distribution of slip along these plate boundary fault systems is a crucial step toward evaluating earthquake and tsunami hazards. More generally, characterizing fault slip behavior and mechanical state at all plate boundary types through direct sampling, near-field geophysical observations, measurement of in situ conditions, and shore-based laboratory experiments is a fundamental and societally relevant goal of modern earth science. To this end, several recent and ongoing drilling programs have targeted portions of active plate boundary faults that have either slipped coseismically during large earthquakes or nucleated smaller events. These efforts include the San Andreas Fault Observatory at Depth (SAFOD), the Taiwan-Chelungpu Drilling Project, IODP NanTroSEIZE drilling, and the Japan Trench Fast Drilling Project (J-FAST).