Abstract:
The SmartPlug is the first borehole observatory in the IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). It was installed at Site C0010 where it penetrates one of the shallow branches of the Megasplay fault to obtain pressure and temperature data from the fault and from a hydrostatic reference section. Here, a 15 months-lasting pore pressure and temperature record collected by the SmartPlug was evaluated. The main objective was to clarify the origin of transients in the data and its possible relationship to natural processes such as earthquakes, tectonic deformation or splay fault activity, as well as storms or low-pressure weather systems. After pressure and temperature data were processed properly, comparisons were made using seismic data from the Japanese F-Net and Hi-Net, theoretical travel time calculations provided by the USGS as well as earthquake lists from the ISC. Additionally, meteorological data provided by the JMA and the U.S. COAPS as well as theoretical travel time calculations for tsunamis from the U.S. NGDC were used. It can be shown that pulse-like pressure transients are related to regional/teleseismic earthquakes, originating mainly from the "Pacific Ring of Fire", from various depths and with diverse focal mechanisms. Approaching seismic waves of at least one regional earthquake led to a significant drop in the formation pressure, which is interpreted as a seismic wave-induced increase in permeability. The arrival of Rayleigh waves caused amplification of the borehole pressure, probably due to induced fluid flow. Tremor-like pressure transients are interpreted to be microseism, which is, based on pressure transient characteristics, triggered by storms or low-pressure weather systems on the open ocean. Approaching tsunamis look similar but caused longer period oscillations in the pressure record. Mainly in the seafloor pressure data distinct peaks are visible, some of which look similar to distinct peaks in the temperature data suggesting the same, so far unresolved triggering mechanism. The reason for the other distinct peaks remains unclear at this point. Regarding the long-term and medium-term transients in the pressure and temperature data, respectively, no satisfying explanations were found. For the pressure transients, due to the similar characteristics, related mechanisms can be suggested. The results obtained so far exclude any seismogenic event at the Megasplay Fault or within the accretionary prism to be responsible for the observed pore pressure and temperature variations. After the 15-months deployment, the SmartPlug temporary borehole observatory got replaced with an extended GeniusPlug, which also represents an instrumented bridge plug in the cased borehole. In addition to monitoring pressure and temperature, the GeniusPlug contains a 30 cm-long unit hosting an osmotically driven geochemical fluid sampler as well as chambers for microbiological experiments under in situ conditions. The instrument is designed to give a full 2-year record of pressure and temperature data and material for geochemical and biological analysis. Recovery with D/V Chikyu is envisaged for 2012.