International Ocean Drilling Programme (IODP³) D/V Chikyu Completes Expedition 502E: Extended Monitoring and Resurveying of Japan Trench Borehole Observatories

The Deep-Sea Scientific Drilling Vessel Chikyu^*1, operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Hiroyuki Yamato, President) has completed International Ocean Drilling Programme (IODP³)^*2 Expedition 502E: Extended Monitoring and Resurveying of Japan Trench Borehole Observatories, the first Chikyu IODP³ expedition on 30 October (Thu) 2025, having achieved its objectives.

1. Operation Summary

This expedition started on 18 October 2025, recovering and redeploying temperature sensor strings (hereinafter referred to as “devices”) at two drill holes in the Japan Trench (Figure 1), the location of the 2011 Tohoku-Pacific Ocean Earthquake, using the deep-sea drilling vessel Chikyu (previously reported on October 20, 2025).
The devices, originally installed in borehole C0019D during IODP Expedition 343 (JFAST) in 2012 and in borehole C0019Q during JTRACK^*3, were retrieved aboard the vessel. Following data download from the sensors, battery replacement, reprogramming for extended-duration observations, After confirming the condition of the ropes connecting the sensors and performing necessary maintenance, the deployment of the devices into the two boreholes (C0019D and C0019Q) was completed. Subsequently, Chikyu entered Sendai Port (Takamatsu Second Pier), Miyagi Prefecture, on 29 October, concluding this expedition.

2. Overview of Results and Future Plans

On this Expedition, we deployed the long-term temperature observatories installed in two observatory holes (C0019D, C0019Q) near the Japan Trench fault zone where the Tohoku-Pacific Ocean Earthquake occurred. After downloading the temperature data (continuously recorded since deployed during JTRACK), we reinstalled the devices in the same boreholes. Going forward, we aim to analyze the obtained temperature measurement data and conduct borehole investigations to capture the detailed hydrogeological structure around the fault zone over time and clarify its relationship with seismic activity.

3. Expedition website

A dedicated website for the expedition has been launched. This website provides an overview of the expedition, introduces the participating researchers and will be updated with the progress of the expedition.
https://www.jamstec.go.jp/chikyu/e/exp405/

Fig.1 Survey Area

Fig 2. Drilling Plan

1. The drill string re-engaged the wellhead of Hole C0019D, and the retrieving tool passed through the string to recover the observatory.
2. Repeat this process at Hole C0019Q. Once onboard, the sensors were refurbished, checked, data downloaded, and prepared for reinstallation.
3. The drill string reconnected to the wellhead of Hole C0019D and the observatory was redeployed.
4. The observatory for Hole C0019Q was redeployed in the same way.

Photo1　Underwater TV camera image of the reengagement to Hole C0019Q on October 21.