FAULT ZONE PROPERTIES AND PROCESSES IN THE NANKAI TROUGH SUBDUCTION ZONE: A SYNTHESIS

The Nankai Trough is the most intensively studied modern accretionary subduction complex on Earth. Its fault zone geometry and architecture have been imaged in detail with 3D seismic reflection surveys, and have been directly sampled with cores, borehole logging and long-term monitoring systems along two different corridors known respectively as the Muroto and Kumano transects. Kumano is the setting for the NanTroSEIZE project, a long-term MARGINS/IODP effort to sample and instrument key faults in several locations spanning the up-dip end of the seismogenic zone and what has been hypothesized to be the aseismic shallow region. Synthesizing these studies yields a new understanding of complex and temporally-varying fault conditions and mechanics.

A wealth of disparate information on the in situ properties of these active, sediment-hosted fault zones has been collected. Low-angle, high displacement rate thrust faults are hosted in semi-lithified, high porosity sediments. 10-30 m thick damage zones composed of dense networks of open fractures and scaly fabric zones have been observed, in some cases hosting concentrated fault-core zones of finely-comminuted ‘gouge’ form apparent mm- to cm-scale principal slip zones. In contrast to earlier studies at other convergent margins, no fault-hosted porewater chemical anomalies suggesting localized fluid flow regimes along the fault zones have been observed at Nankai. Despite the shallow, cold, and clay-rich setting, multiple lines of evidence suggest localized slip and perhaps substantial frictional heating indicating seismic slip velocity right to the toe of the accretionary wedge at < 500 m depth below the wedge surface. Related earthquake seismology has shown that so-called VLF earthquakes take place within the “aseismic” zone. 3D seismic interpretation coupled with borehole-based stress orientations suggest stress is controlled by far-field plate motions but coupled with time-varying stress magnitudes, and that even in the “simple” fold-and-thrust wedge geometry, considerable structural complexity exists across many scales. A variety of lines of indirect evidence suggest basal faults are in a state of high pore fluid pressure modulating effective stress, but direct observations of near-lithostatic pore pressure have been elusive to date.