The great Tohoku earthquake that struck northeastern Japan in 2011 was one of the largest earthquakes ever recorded on the Earth. The 9.1 magnitude earthquake resulted in a devastatingly shaking tsunami that not only produced large death tolls and structural damages but also revealed that our understanding of how the large subduction zone faults behave needs to improve in order to better predict and mitigate future earthquakes. Integrated Ocean Drilling Program Expedition 343 drilled down to the slip interface near the earthquake epicenter in offshore Japan subduction zone with the aim of recovering both geophysical data and material samples around the fault surface (Figure 2). Geoscientists used these data to form a detailed understanding of what is happening to the earth material around the fault when large ruptures occur, incorporating many parameters like states of temperature and stress, sediment type, pre-existing fault structure, and fluid flow (Chester et al., 2013)
Megathrusts along subduction zones produce the largest earthquakes on the Earth due to the long fault length and relatively wide slip surfaces. The fault rupture associated with the 2011 earthquake reached over 62 m in slip along a 40 km long segment of the Japan Trench (Sun et al., 2017). The slip was considered unusually large for an event that took place at shallow depths in the Earth near the subduction trench, where the fault surface was generally believed to be stable (Figure 1, Yao et al., 2013). While the occurrence is less frequent, shallow rupture events of this magnitude need to be monitored because the slip can breach and significantly displace the seafloor, which will cause tsunamis.
The deepest points in the Earth’s ocean are subduction trenches, where the downgoing oceanic plate bends and sinks beneath the overriding continental plate (Figure 1). The JFAST project reached one of the deepest water depths amongst all of the expeditions in the International Ocean Discovery Program (IODP Expedition Statistics) in order to tap into the shallowest portion of the subduction megathrust. In total, five boreholes were attempted at Site C0019 (Figure 2), three of which successfully reached the target fault surface (Chester et al., 2013). Multiple types of data were carefully collected over the course of a month. In addition to successfully recovering the cores, the expedition also performed real time well logging while drilling (LWD) to retrieve in situ conditions of the sediment and materials surrounding the fault surface. High-resolution electrical images also illuminated the structures and lithological properties in the borehole and enabled the identification of plate boundary fault zones (Figure 3, top). In particular, the patterns of the beddings and fractures inferred from the borehole images showed multiple faults that comprise the fault zone and drastic change in sediment structures as they are sheared by plate motion (Figure 3, bottom). Finally, temperature sensors were directly deployed along the fault interface in Hole C0019D during expedition 343T to measure the remaining frictional heat produced from the slip event 16 months prior (Figure 4).
Expedition 343/343T Scientists (2013). Site C0019. In Chester, F.M., Mori, J., Eguchi, N., Toczko, S., and the Expedition 343/343T Scientists, Proc. IODP, 343/343T: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.343343T.103.2013
Sun, T., Wang, K., Fujiwara, T., Kodaira, S., & He, J. (2017). Large fault slip peaking at trench in the 2011 Tohoku-oki earthquake. Nature communications, 8(1), 14044.
Yao, H., Shearer, P. M., & Gerstoft, P. (2013). Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures. Proceedings of the National Academy of Sciences, 110(12), 4512-4517.
Japan Agency for Marine-Earth Science and Technology (JAMSTEC). (n.d.). Expedition 343 – Shimokita Coalbed Sampling. Photo gallery. Retrieved March 26, 2023, from https://www.jamstec.go.jp/chikyu/e/exp343/gallery.html#photo
Expedition 343/343T Scientists (2013). Expedition 343/343T summary. Chester, F.M., Mori, J., Eguchi, N., Toczko, S., and the Expedition 343/343T Scientists, Proc. IODP, 343/343T: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.343343T.101.2013
IODP. (n.d.). Expedition statistics. Retrieved March 26, 2023, from https://www.iodp.org/expeditions/expedition-statistics