MeBoCORK: Development of borehole observatories for MeBo (MARUM Meeresboden- Bohrgerät): Application to the seismogenic Nankai Trough subduction zone, Japan

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Approximately 90% of the seismic moment on Earth is released in often devastating subduction zone earthquakes (EQs). In the Integrated Ocean Drilling Program (lODP), the study of seismogenesis along an active convergent margin plays a key role in the form of a multi-expedition effort named "NanTroSEIZE" (Nankai Trough Seismogenic Zone Experiment). To date, eight expeditions (lODP Expeditions 314-316, 319, 322, 326, 332-333) drilled 12 sites along a transect from the incoming plate across the frontal accretionary prism to the forearc basin. In the near future, the deepest hole will tap into the seismogenic portion of the subduction megathrust using Riser drilling. The proponent participated in three of the NanTroSEIZE expeditions, most recently as co-chief scientist overseeing the successful recovery of a mini-CORK and the installation of a mini-CORK and a full CORK observatory (Exp 332). This proposal aims lo reach two major scientific milestones:(i) In addition to the German component of the lODP borehole observatory network (i.e. mini- CORKs during Legs 319 and 332 plus some contribution to the full CORK), we want to extend the long-term monitoring efforts to active mud volcanoes some 30 km landward of Site C0009. These features carry gas hydrates and deep-seated fluids, most likely tapping into the seismogenic zone at a depth of several km below seafloor, and are also overlying an area of high strains and a locked plate boundary-thrust.(ii) The second goal is to develop a smart MeBo (MARUM Meeresboden-Bohrgerät) CORK observatory to be installed in two of these mud volcanoes. These observatories will be self-contained and monitor pore pressure and temperature as proxies for strain and fluid flow near terminal depth of the borehole. Their hydraulic lines can further be used for geochemical (osmo-sampler) and microbiological (FLOCS units) monitoring and sampling.The underlying reason for the engineering effon of smart observatories for MeBo is founded in the new lODP Science Plan where specialist rigs such as MeBo are discussed as ECORD's contribution from 2013-2023. So far, CORKs or other sophisticated long-term devices were restricted to DVs Joides Resolution and Chikyu, however, we here present an affordable solution for versatile, multi-disciplinary MeBo borehole instruments. The proposed system is to be installed by MeBo alone and data can be retrieved by ships of opportunity. A seafloor extension, to be deployed and recovered by ROVs (during and/or after the anticipated lODP expedition) can add capability and payload, and hence represent the means for more than decade of monitoring. Data recovery will be from ships of opportunity and hence frequently possible.Funding for MeBo drilling and ROV installations in the Nankai mud volcano field has already been granted (RV Sonne cruise SO222 in summer 2012) and is brought into the project as in kind contribution.