The interpretation for structure of three-dimensional Splay Fault in Nankai trough

Abstract

The splay fault, referred to as ‘Mega-Splay’, in Nankai trough has been causing a lot of marine earthquakes and submarine landslides, which may have led to TSUNAMIs. The fault development history may have influenced its geometry, thus the detailed 3D geometry of the fault needs to be examined, with using 3D seismic data and logging data or coring data near the Mega-Splay Fault. The fault has a variation in the width, thus has been regarded as ‘fracture zone’. In this research, we traced the upper limit and the lower limit of the Mega-Splay Fault Zone on seismic profiles (3-D PSDM) as high amplitude reflection surfaces to reveal the three-dimensional spread of the fault width. Then, we constructed physical-property model around the Mega-Splay Fault, by appliying Acoustic Impedance Inversion (AI inversion) with Hampson-Russell STRATA. In the process of this inversion, synthetic waveforms are created by convolving a basic wave from seafloor and reflection coefficient array derived from logging data at C0004B. The synthetic waveforms are compared with real waveforms in seismic profiles (3-D PSTM), and then modifying the initial reflection coefficient array until minimizing the difference between synthetic waveforms and real seismic waveforms. Our results show that there is a low acoustic impedance layer in the upper zone of Splay Fault. 3-D distribution of the low acoustic impedance layer in the fault zone has a variation in the width, similar to the 3-D interpretation of the geometry. This suggests that there are a relationship between structural geometry and physical property along the Mega-Splay Fault.