Seismic Reflector Polarity and Attributes Along the Megasplay Fault, NantroSEIZE Transect: Implications for Fault Zone Properties

As part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project, a 12 x 56 km 3-D seismic survey was shot off the Kii peninsula of Honshu Island, Japan that images the accretionary complex and up-dip end of the interplate earthquake seismogenic zone. Here, we focus on imaging of the upper ~2 km of the megasplay fault zone located seaward of the Kumano Basin in the region of recent IODP drilling. The quality of the pre-stack depth migrated seismic volume allows us to closely examine the character of the 3-D fault reflector throughout the survey area. Characterized by branching thrust splays, this generally high-amplitude set of reflectors extends from the decollement to the seafloor. The nature of the shallow portion of the reflector changes across the seismic volume from NW to SE. The NW section of the reflector exhibits mainly high-amplitude, positive polarity. In the SE, the reflector decreases in relative amplitude, is more complex, and negative polarity becomes more prominent. Several models were built in order to determine the cause of these observed changes in polarity and waveform signature of the reflector along the fault. We performed synthetic waveform modeling of the fault reflectors, employing an averaged seafloor reflection to extract the source wavelet. Modeling to match the range of fault reflector waveform signatures allow us to evaluate the geometry and physical properties of the megasplay. Seismic impedance structure required to construct synthetic waveform models that accurately replicate the fault reflector waveform produces constraints on fault zone properties. IODP Site C0004, which was drilled and logged across the uppermost 400 m of the megasplay fault during Expedition 314 and 316, will be used to ground truth the impedance structure in an area of the fault characterized by strong reflectors and branching. Ground truthing from Site C0004 is necessary in order to gain confidence in the waveform models associated with other regions of this fault. The combination of analysis techniques employed documents the physical complexity of the megasplay fault zone and the range of reflector variability in this survey area.