Deep seismic imaging of the eastern Nankai trough, Japan, from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration

Abstract

[1] The easternmost segment of the Nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench. It is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the Izu collision zone. We investigate this area with densely sampled active seismic data. One hundred ocean bottom seismometers, spaced only 1 km apart, were deployed along a line perpendicular to the trough. A two-step tomographic approach, designed for this type of acquisition, is applied: first, a long-wavelength velocity model is built by a linearized inversion of first arrival travel times, its resolution is assessed, and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested; second, this tomographic model is used to exploit late reflected arrivals with a prestack depth migration, the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model. Thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge. Our results bring new insights on the deep crustal zones of the margin and its evolution. The existence of presently inactivated major thrusts is very clearly evidenced in the backstop. A general timing for its evolution and deformation is proposed. Farther seaward, the presence of a subducted ridge beneath the accretionary wedge is confirmed. Its compressive structure and origin are demonstrated. It exhibits a thickened lower crust which nature is discussed. Besides, strong analogies between this ''Paleo-Zenisu'' subducting ridge and the Zenisu ridge, observed farther south, are also emphasized, bringing evidences for a steady state crustal deformation pattern.

IODP Expedition 333: Return to Nankai Trough Subduction Inputs Sites and Coring of Mass Transport Deposits

“…The lower part of the Shikoku Basin displays a complex geometry influenced by basement topography (Ike et al, 2008b). Basement highs are also imaged within the subduction zone (Kodaira et al, 2003;Dessa et al, 2004), where they influence margin structure (Lallemand et al, 1992;Le Pichon et al, 1996;Park et al, 1999;Mazzotti et al, 2002;Bangs et al, 2006;Moore et al, 2009) and seismicity (Kodaira et al, 2000a;Park et al, 2004). The NanTroSEIZE expeditions focused on the Kumano transect, while earlier Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) sites were drilled on the Muroto transect and the Ashizuri transects, respectively, in the central and western part of Shikoku Basin ( Fig.…”

Wavefront picking for 3D tomography and full-waveform inversion

“…smoothing operator), δs is the slowness update, and y is a temporary vector. The adjoint of the Jacobian matrix J † smears the traveltime residuals along the raypaths computed using the traveltime tables (Dessa et al, 2004). This back-projection approach is preferred to the back projection using the adjoint-state method (Sei and Symes, 1994;Leung and Qian, 2006;Taillandier et al, 2009), due to the small number of rays, which is faster to trace.…”

“…Traveltime inversion is the traditional method for generating accurate starting models (Bording et al, 1987;Dessa et al, 2004;Woodward et al, 2008;Prieux et al, 2013), whereas firstbreak picking is the first step toward shallow tomographic inversion (Zhu et al, 1992;Zelt and Barton, 1998).…”

Onshore-offshore seismic transect from the eastern Nankai Trough to central Japan crossing a zone of the Tokai slow slip event

“…Our imaged south ridge is the eastern most member of this ridge. Dessa et al (2004) also imaged a subducted ridge immediately west of the south ridge using high resolution OBS data. Park et al (2003) concluded from mapping that the western half of the ridge, which is located off the Kumano basin, is colliding with the back stop.…”