Hydraulic structure of the Tohoku plate boundary fault zone: insights and evidence from direct drilling (IODP expedition 405)

In 2011, the Tohoku subduction zone endured a Mw 9.0 large earthquake and a large tsunami with co-seismic displacement exceeding 40 meters at the trench. Such an unexpected tsunami earthquake is expected to have created intense damage across the plate boundary fault zone (PBFZ).

We propose to characterize the hydraulic damage pattern across the PBFZ of the Tohoku earthquake. To achieve this, we will use the rich dataset collected during IODP Expedition 405, during which the PBFZ was drilled directly several times between September and December 2024. In particular, we focus on drilling and logging data to estimate the flow entering the borehole. By carefully modelling the mud pressure, we can evaluate the hydraulic inflows and outflows to the borehole as drilling advances. The flow profile provides valuable insights into permeable and/or overpressurized intervals.

Using the Logging-While-Drilling data collected in Hole C0019H, we obtain a high-resolution and continuous fluid flow profile along the borehole.  The results show that above the plate boundary (~815 mbsf), we observe the incoming fluid flow, with strong flow pulses at 775 mbsf and 805 mbsf. These two pulses are associated with variations in mud temperature and some clear fracture zones identified by the electrical images. This suggests the hydraulic structure of the PBFZ has two components highlighted by the two types of flow: (1) A background damage, increasing progressively in the hanging wall when approaching the PBFZ, with a potential for fluid flow across the fault zone. (2) A fracture-supported flow, related to the major subfaults which sustain a more longitudinal flow.