[M-GI28] Drilling Earth Science

[MGI28-09] Noto Peninsula Earthquake Drilling Project for Understanding Fluid Trigger Slip Events (NEPTUNE)

*Makoto Otsubo¹, NEPTUNE Proponents (1.Geological Survey of Japan, Research Institute of Earthquake and Volcano Geology)

Keywords:2024 Noto Peninsula Earthquake, Fluid, Fault Slip, Crustal Deformation, ICDP

The NEPTUNE (Noto Peninsula Earthquake Drilling Project for Understanding Fluid Trigger Slip Events) initiative aims to investigate the mechanisms behind the 2024 Noto Peninsula Earthquake (Mw 7.6), a significant seismic event that exhibited a complex rupture sequence involving multiple fault segments. The earthquake's onset, marked by slow initial rupture and subsequent propagation over 150 km, emphasizes the need to understand the interplay between fault dynamics and pre-seismic crustal activity. A critical focus of the project is the role of high pore fluid pressure, which is known to facilitate fault slip by reducing effective normal stress. Fluid migration and pressure buildup, attributed to mantle-derived fluids, were identified as primary triggers for preceding earthquake swarms in the region. Geochemical evidence, including elevated ³He/⁴He ratios, supports this hypothesis. The event also showcased how segmented fault networks and fluid-induced weakening contributed to rupture propagation. The project proposes to drill in the coastal region of the Noto Peninsula, targeting the fault plane of the 2024 earthquake. Objectives include recovering fluid, gas, and rock samples to analyze fluid origins, chemical reactions, and fault zone microstructures. Long-term monitoring of fluid and gas movement near the fault is planned to assess post-seismic changes and prepare for future seismic activity. Three key research areas are prioritized: (1) Observing fluid migration and pressure variations through direct sampling, numerical modeling, and seismic data analysis, (2) Investigating the origins of host rocks and fault zone fluids to understand their contribution to seismicity, and (3) Analyzing mineralogical and chemical changes in the fault zone to evaluate their impact on fault friction and earthquake dynamics. The project's findings are expected to enhance understanding of earthquake generation mechanisms, particularly the transition from seismic swarms to fast-slip earthquakes. By addressing geohazard themes outlined in the ICDP 2020–2030 Science Plan, NEPTUNE aims to advance predictive capabilities for seismic hazards in intraplate settings. Amd, drilling from onshore locations to access and study the fault responsible for earthquakes that triggered tsunamis (L2S: Land-to-Sea drilling). It is a related proposal be submitted to IODP.