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doi:10.2204/iodp.sp.343.2012

Science objectives

The science objectives are closely aligned with the overall goals of IODP. In the IODP Initial Science Plan, research concerning solid earth cycles and geodynamics highlights the seismogenic zone initiative, which 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. Deformation microstructures and physical rock properties at in situ conditions, along with observatory monitoring of temperature, pore pressure, and stress, are also emphasized in the plan. These are all key components of the Japan Trench Fast Earthquake Drilling Project (JFAST). Furthermore, JFAST directly addresses Challenge 12 of the IODP Science Plan for 2013–2023: “What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami?”

The prioritization of science objectives reflects the unique possibilities provided by rapid response drilling into a slipped fault following an earthquake. The shallow distribution of large slip for the Tohoku earthquake provides an unprecedented ability 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.

Specifically, the science objectives and strategies for achieving them are as follows:

1. What was the stress state on the fault that controls rupture during the earthquake and was the stress completely released?

• Dynamic friction during the rupture: potentially the most significant result of this project will be a value for the dynamic frictional stress. Time decaying temperature measurements 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 friction.

• Rupture to the toe of the accretionary 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 constrain the healing of the fault.

2. What are the characteristics of large earthquakes in the fault zone, and how can we distinguish present and past events in fault zone cores?

• Core analyses: detailed analyses of textures and small-scale structures of core samples of the fault zone will be used to infer the role of fluids and pressurization during rupture. We will look for evidence of melting and other processes that contribute to dynamic strength reduction. Trace elements 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 document (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 massive amounts of hydrogen may greatly stimulate microbiological activity; thus, samples of the fault may contain records of biogeochemical and microbiological processes.

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