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doi:10.2204/iodp.pr.343343T.2012

Scientific objectives and operational strategy

The main goal of Expedition 343 is to understand the stress conditions and physical characteristics of the fault that allow very large fault slip to occur near the trench. The following specific science objectives reflect the unique possibilities provided by rapid response drilling into a fault following a large earthquake. The shallow distribution of large slip for the Tohoku earthquake provides an unprecedented opportunity to directly access a fault that has recently moved tens of meters. As outlined in the report from the International Continental Scientific Drilling Program/Southern California Earthquake Center international workshop on rapid response drilling (Brodsky et al., 2009), fundamental questions regarding stress, faulting-related fluid flow, and the structural and mechanical characteristics of the earthquake rupture zone can be addressed uniquely through rapid response drilling.

The science questions and strategies for addressing them are as follows:

1. What was the stress state on the fault that controls rupture during the earthquake, and was the shear stress on the fault completely released?
• Dynamic friction during the rupture: potentially the most significant result of this project will be determining a value for the dynamic frictional stress. Measurements of the time-decaying temperature will be used to estimate the frictional heat produced at the time of the earthquake, which can be used to infer the level of dynamic frictional stress.
• Rupture to the toe of the sedimentary wedge: past thinking was that sediments in this region are weak and rate strengthening, so earthquake instability should not nucleate or easily propagate through this region. Measurements of current stress and stress during the earthquake can be used to explore different models to explain how dynamic slip occurred. Hydrogeological measurements can also help constrain the healing process of the fault.
2. What are the characteristics of large earthquakes in the fault zone, and how can we distinguish recent and geologic-past events in fault zone?
• Core analyses: detailed studies of textures and small-scale structures of core samples from the fault zone will be used to infer the role of fluids and pressurization during rupture. We will look for evidence of melting or other processes that contribute to dynamic strength reduction. Trace element chemistry and other physical and geochemical anomalies will be used to estimate the thermal history of the recent and past events.
• Laboratory experiments: high-speed friction and petrophysical experiments on fault material can be used to characterize the frictional behavior of the fault.

Secondary science objectives include carrying out other geological, geochemical, and microbiological observations to the greatest extent possible during drilling in accordance with the IODP Measurements guidelines (www.iodp.org/program-policies/). As a specific example, there is some evidence that great amounts of hydrogen may be released at the time of large faulting (e.g., Kita et al., 1982). The production of hydrogen may stimulate microbiological activity; thus, samples of the fault may contain records of biogeochemical and microbiological processes.

The primary science objectives are closely aligned with the Initiative: Seismogenic Zone of the IODP Initial Science Plan. This initiative advocates subduction zone studies that include investigating the behavior of rocks and sediments to better understand the fault zone and integration with studies of earthquake mechanics. Furthermore, this project directly addresses Challenge 12 of the IODP Science Plan for 2013–2023: “What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami?”

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