Characterization of Decollement in the Nankai Trough Using Seismic Attributes Analysis

At plate convergent margins, the physical properties along the plate boundary fault (Decollement; a detachment that separates a deformed accretionary prism from undeformed underthrust sediments) are very important, considering the nature of the deformation features and of the earthquake mechanisms. The decollement and its seaward stratigraphic projection (Proto-decollement) are identified as a prominent reversed-polarity reflector on the seismic profile. In order to delineate the physical properties along the decollement/proto-decollement at the Nankai Trough off Muroto peninsula, we have applied seismic attributes analysis via a neural network to a 3-D seismic reflection data. This approach has been applied to characterize physical properties of hydrocarbon reservoirs in the oilfields. First of all, seismic attributes were computed from 3-D seismic reflection data. Then, seismic attributes form the input to a particular type of neural network called a Kohonen Self-Organizing Map (K-SOM), and the output of which divides the seismic profile into several regions with geologically similar features. In this study, Envelope, Instantaneous frequency, Instantaneous Q and Similarity are selected as input attributes. The result of classification along the decollement/proto-decollement shows a drastic change in the physical properties around the sole of the proto-thrust. Integrating with ODP data, the classification result may help understand the development of the decollement. The decollement develops at the sole of the proto-thrust. We infer that this localization relates to the release of the shear stress accompanied with the plate subducting process, because the proto-thrust is the most seaward thrust where the shear stress can be released to the seafloor. The release of shear stress can easily be responsible for plate boundary slip landward of the proto-thrust. Furthermore, we found the geometry of the proto-decollement/decollement supports above interpretation. The geometry of the proto-decollement/decollement seaward of the proto-thrust is not a continuous single plain, but two separated surfaces with an offset of ∼100m height. These two horizons converge and form a single detachment surface immediately landward of the proto-thrust. The plate boundary slip initiates from these two horizons of high fluid pressure layer, presumably due to the increase in overburden by the turbidite sedimentation and lateral tectonic loading. As a result of the plate boundary slip, these two horizons may connect to one horizon in order to become more stable condition (less frictional condition).