ELECTROKINETIC SIGNATURE OF THE NANKAI TROUGH ACCRETIONARY COMPLEX - PRELIMINARY MODELING FOR THE KAIKO-TOKAI PROGRAM

Fluid flow in the Nankai Trough accretionary complex has been observed during the previous phases of the Kaiko project to be of order 100 m year(-1) in small-scale high-permeability conduits, and 10 m year(-1) where shallow detachment faults intersect the seafloor. The rate of fl ow was also variable, increasing by up to 30% over a month. Monitoring the fluid flow will be one of the principal objectives of the next ph ase of the Kaiko program, known as Kaiko-Tokai. One method is to measu re the electric and magnetic fields generated by the electrokinetic ph enomena (or streaming potential), resulting from fluid flow in the por ous sediments. This paper describes modelling of fluid flow in the acc retionary prism and the resulting electrokinetic signals which may be observed from sensors on the seafloor or in shallow boreholes. A two-d imensional finite-difference model is used to study the hydrogeology o f a realistic section of the Nankai Trough, with constraints from seis mic reflection data and Ocean Drilling Program boreholes. Fluid source s owing to porosity reduction in the prism and of distant external ori gin are modelled. We find that porosity reduction alone is insufficien t to reproduce the observational estimates of excess pore pressure and fluid flux. Additional fluid source terms are therefore required alon g a low-angle decollement, or detachment fault, which separates the de formed prism sediments from the undeformed underthrust sediments benea th. The decollement must be significantly more permeable than the adja cent sediments (our models suggest by approximately four orders of mag nitude), and is probably anisotropic parallel to the main structural t rends. From the hydrogeological model, the electric potential and elec tric and magnetic fields within the ocean and sediments were calculate d from the electrokinetic phenomena. Seafloor electric potentials are found to be of the order of a few millivolts, with seafloor horizontal and vertical electric fields of the order of 0.5 mu V m(-1) close to the deformation front. As the electric potential rises significantly w ith depth through the sediments, borehole measurements may be particul arly useful. We conclude with some comments about noise sources and ex perimental logistics for the Kaiko-Tokai program.