NEPTUNE Project: Exploring Fluid-triggered Slip Mechanisms through Scientific Drilling in the Noto Peninsula, Japan

The NEPTUNE (Noto Peninsula Earthquake Drilling Project for Understanding Fluid Triggered Slip Events) initiative aims to elucidate the mechanisms underlying the 2024 Noto Peninsula Earthquake (January 1st, 2024: Mw 7.6), a major seismic event characterized by a complex rupture sequence across multiple fault segments. This earthquake began with a slow initial rupture that evolved into a dynamic rupture extending over 150 km, highlighting the critical need to understand the interactions between fault behavior and pre-seismic crustal processes.

A central focus of the project is the influence of elevated pore fluid pressure, which promotes fault slip by lowering effective normal stress. The migration and accumulation of fluids—likely derived from the mantle—have been identified as key factors that triggered the preceding earthquake swarms. Geochemical signatures, including high ³He/⁴He ratios, support this interpretation. The event further demonstrated that rupture propagation was facilitated by both segmented fault structures and fluid-induced weakening.

The project plans to drill from the coastal region of the Noto Peninsula, targeting the fault plane of the 2024 earthquake. Core objectives include retrieving fluid, gas, and rock samples to investigate fluid sources, chemical interactions, and fault zone microstructures. Long-term monitoring of fluid and gas behavior near the fault zone is also planned to track post-seismic evolution and enhance preparedness for future seismic events.

Three primary research areas are emphasized:

• Observing fluid migration and pressure fluctuations through direct sampling, numerical simulations, and seismic analysis.
• Characterizing the origins of fault zone rocks and fluids to evaluate their role in earthquake generation.
• Assessing mineralogical and geochemical transformations within the fault zone to understand their impact on fault strength and slip behavior.

The outcomes of NEPTUNE are expected to deepen our understanding of earthquake nucleation, particularly the transition from swarm activity to rapid fault rupture. Aligned with the geohazard priorities of the ICDP Science Plan 2020–2030, the project aims to improve forecasting capabilities for intraplate seismic hazards. In addition, the project includes a complementary proposal for Land-to-Sea (L2S) drilling, aiming to access and study the tsunami-generating fault system from an onshore platform, to be submitted to IODP.