A subducted oceanic ridge influencing the Nankai megathrust earthquake rupture

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. A subducted oceanic ridge influencing the Nankai megathrust earthquake rupture Jin-Oh Park Research Program for Plate Dynamics, Institute for Research on Earth Evolution (IFREE). Introduction The Nankai subduction zone may be divided into four discrete domains (A through D, Fig. ) marked by the megathrust earthquake rupture [Ando, 9], each of which roughly corresponds to a geologically well-defined forearc basin [Sugiyama, 99]. The two discrete domains (C and D) of the eastern Nankai (Fig. ) are often referred to as Tonankai and Tokai segments, respectively. The Zenisu ridge (Fig. ), presumed to be an old volcanic ridge at the western flank of the Izu-Bonin arc with overprinted crustal shortening [Lallemant et al., 99; Ishizuka et al., 99], is suggested to be one of the major structures associated with subduction processes in the Tokai segment [Le Pichon et al., 99]. Also, the presence of a subducted ridge (referred to as paleo- Zenisu) north of the current Zenisu ridge has been proposed by seafloor morphologic [Lallemand et al., 99; Okino and Kato, 99] and geomagnetic anomaly data [Le Pichon et al., 99]. Although there have been few seismic reflection images to clearly demonstrate the paleo-zenisu ridge, it was suggested that the ridge is presently located on top of the main décollement immediately adjacent to the backstop and its subduction below the Tokai margin has led to a reorganization of the margin with a sequence of uplift, erosion, and growth of a new accretionary wedge [Le Pichon et al., 99]. In order to determine the crustal structure of the eastern Nankai subduction zone, we have carried out extensive seismic surveys since 99. We present new multichannel seismic (MCS) reflection profiles and an Ocean Bottom Seismograph (OBS) wide-angle seismic profile, which reveal a subducted oceanic ridge immediately beneath the eastern Nankai accretionary wedge. In this paper, we describe the structural features related to the ridge subduction and demonstrate its seismotectonic implication.. Seismic survey data and interpretation We conducted detailed MCS surveys in the eastern Nankai Trough subduction zone, using R/V Kairei of the Japan Marine Science and Technology Center (JAMSTEC). On profile D (Fig. ) crossing the Nankai Trough, a strong reflection from oceanic crust of the PSP is continuously traceable at least km landward from the frontal thrust (FT), as the PSP subducts beneath the forearc accretionary wedge of Tertiary-Quaternary geologic age [e.g., Okuda, 9]. A clear décollement reflection is observed at least km landward from the FT. This profile reveals an oceanic basement high on the subducting PSP beneath the accretionary wedge at ~- km distance. The peak of the basement high is located apparently seaward of the outer ridge (~ km distance). Maximum height of the basement high is estimated to be ~. km, based on the averaged P-wave interval velocity (Vp) of ~. km/s around the basement high, which is derived from the MCS stacking velocity (this study) and wide-angle velocity of the adjacent OBS/MCS line KR9 [Nakanishi et al., ]. The base of the basement high is about km wide. A pronounced reflector (hereafter, referred to as R ) is observed at ~ sec two-way time (TWT) at ~- km distance. The almost horizontal reflector R is confined to the area just below the forearc basin covered by well-stratified sedimentary sequences. Compared with the adjacent OBS/MCS line KR9 [Nakanishi et al., ], the reflector R is roughly coincident with the topmost surface of seaward edge of the backstop that is mainly composed of Cretaceous- Tertiary accreted sediments. A similar basement high is also identified on other MCS profiles on dip lines D, KR9, D, D9,, and D. Not surprisingly, we also identify the oceanic basement high on strike profile S (Fig. ), which intersects all of the dip lines D through D. The topmost reflector of the oceanic crust is continuously observed over the entire line. This profile clearly exhibits a similar oceanic basement high at ~- km distance. On the whole, the décollement reflection is not continuous above the basement high, except for its western flank. This basement high with an irregular surface is similarly estimated to be ~. km high in maximum and ~ km long on this profile. As a result, several MCS profiles in the Tonankai segment reveal a buried elongate oceanic basement high on dip and strike lines, which subducts immediately beneath the accretionary wedge. We interpret this basement high as a trough-parallel subducted oceanic ridge attached to the descending PSP. A combined OBS/MCS survey on line TKY (Fig. ) imaged a double ridge (hereafter, referred to as south and north ridges) subducting beneath the forearc accretionary wedge, which were estimated to be ~ km and ~ km wide, respectively (see Kodaira et al. [] for detailed data acquisition and processing procedure). Particularly, the south ridge at the inner slope region almost coincides with the paleo-zenisu ridge distribution (Fig. ) that was suggested by the magnetic anomaly data [Le Pichon et al., 99].. Discussion: new ridge distribution and the possible impact on the 9 Tonankai earthquake rupture The magnetic anomaly signature [Le Pichon et al., 99] that was used to show the distribution of paleo-zenisu ridge, may help to infer the connection between the subducted Tonankai ridge and the subducted Tokai south ridge. The combination of the MCS images in the Tonankai segment, the OBS/MCS result showing the subducted south ridge [Kodaira et al, ] in the Tokai segment, and the magnetic data, we infer a spatial distribution of the elongate, trough-parallel subducted ridge in the eastern Nankai, as seen in Figure. As a result, our new seismic reflection and refraction data confirm the existence of the paleo-zenisu ridge, and moreover, expands its distribution to the west. The subducted ridge is located spanning roughly both the outer ridge region in

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. the Tonankai segment and the inner slope region in the Tokai segment. The ridge is estimated to be a maximum of ~. km high, ~- km wide, and ~ km long, which is similar to the current Zenisu ridge. Lateral diversity of the ridge height along the strike (e.g., line S in Fig. ) allows us to infer that it may be a complex of closely spaced isolated seamounts just like the current Zenisu ridge, rather than a uniformly elongate body with the same height. The spatial distribution of the subducted ridge (Fig. ) suggests that the ridge is located roughly at the seaward edge of the coseismic rupture zone of the 9 Tonankai earthquake. The ridge does not extend west beyond MCS line D. Instead, the coseismic rupture along the splay fault appears to have propagated farther seaward at the region (Fig. ), for example, on line (Fig. ). Combining the MCS data (reflector R on line D) with the wideangle OBS velocity structure on adjacent line KR9 [Nakanishi et al., ], the trough-parallel subducted ridge appears to be in close contact with the seaward end of the rigid backstop (Vp > km/s) in the Tonankai segment (Fig. ), leading us to speculate that there is strong mechanical coupling due to ridge-backstop collision in the segment. A numerical simulation result [Baba et al., ] indicates that tectonic shear stresses are easy to accumulate around a subducted seamount. In fact, a seismic swarm with focal depth of ~- km and a low-angle reverse-faulting earthquake (M =.9) [Fujinawa et al., 9] (Fig. ) have occurred within the subducted ridge, suggesting that the ridge often relaxes the tectonic stresses probably induced by the ridgebackstop collision. Both the spatial correlation and the ridge-backstop collision geometry suggest that the subducted ridge might be strongly mechanically coupled and may thus play a significant role as a seaward barrier inhibiting the 9 earthquake rupture from propagating farther seaward. The ridge-free region (lines, D, D,, and D), which is marked by the splay fault behavior, appears to have been ruptured farther seaward, supporting the ridge barrier hypothesis. A subducted seamount in the western Nankai [Kodaira et al., ] is proposed to have played a similar role during the 9 Nankaido earthquake. It was suggested that locally strong mechanical coupling occurred around the subducted seamount and thus the seamount might control lateral rupture propagation during the 9 event. Park et al. [] pointed out the slope angle of the splay fault in the Tonankai segment became steeper from the ridge-free region (with an to º dip) to the ridge region (with a to º dip). We speculate that the subducted ridge might influence the rupture behavior along the splay fault (Fig. ).. Conclusions Seismic reflection and refraction data clearly image a troughparallel subducted oceanic ridge immediately beneath the accretionary wedge in the eastern Nankai Trough subduction zone. The seismic survey data confirms the existence of subducted paleo- Zenisu ridge, and moreover, expands its distribution to the west. The newly constrained subducted ridge is located spanning roughly both the outer ridge region in the Tonankai segment and the inner slope region in the Tokai segment. The ridge is estimated to be a maximum of ~. km high, ~- km wide, and ~ km long, which is very similar to the current Zenisu ridge. Spatial mapping of the ridge shows that it is located roughly at the seaward edge of the coseismic rupture zone of the 9 Tonankai earthquake. This ridge appears to be in close contact with the seaward end of the rigid backstop in the Tonankai segment, leading us to suggest that the subducted ridge might act as a seaward barrier inhibiting the 9 Tonankai earthquake rupture from propagating farther seaward. Acknowledgements. The bathymetric data (Fig. ) were compiled by the Hydrographic and Oceanographic Department, Japan Coast Guard. We thank the captain, crew, and technical staff of the R/V Kairei of JAMSTEC for their support in acquiring the MCS data. References Ando M., Source mechanisms and tectonic significance of historical earthquakes along the Nankaki trough, Tectonophysics,, 9-, 9. Baba T., T. Hori, S. Hirano, P.R. Cummins, J.-O. Park, M. Kameyama and Y. Kaneda, Deformation of a seamount subducting beneath an accretionary prism: Constraints from numerical simulation, Geophys. Res. Lett,, -,. Fujinawa Y., T. Eguchi, M. Ukawa, H. Matsumoto, T. Yokota and M. Kishio, The 9 earthquake swarm off the Kii Peninsula observed by the ocean bottom seismometer array, J. Phys. Earth,, -, 9. Ishizuka O., K. Uto, M. Yuasa and A.G. Hochstaedter, K-Ar ages from seamount chains in the back-arc region of the Izu-Ogasawara arc, Island Arc,, -,99. Kikuchi M., M. Nakamura and K. Yoshikawa, Source rupture processes of the 9 Tonankai earthquake and the 9 Mikawa earthquake derived from low-gain seismograms, Earth Planet Space, in press,. Kodaira S., N. Takahashi, A. Nakanishi, S. Miura and Y. Kaneda, Subducted seamount imaged in the rupture zone of the 9 Nankaido earthquake, Science, 9, -,. Kodaira S., A. Nakanishi, J.-O. Park, A. Ito, T. Tsuru and Y. Kaneda, Cyclic ridge subduction at an inter-plate locked zone off central Japan, Geophys. Res. Lett.., doi:.9/gl9,. Lallemand S.E., J. Malavieille and S. Calassou, Effects of oceanic ridge subduction on accretionary wedges: Experimental modeling and marine observations, Tectonics,, -, 99. Nakanishi A., S. Kodaira, J.-O. Park and Y. Kaneda, Deformable backstop as seaward end of coseismic slip in the Nankai Trough seismogenic zone, Earth Planet. Sci. Lett.,, -,. Lallemant S., N. Chamot-Rooke, X. Le Pichon and C. Rangin, Zenisu ridge: A deep intraoceanic thrust related to subduction, off southwest Japan, Tectonophysics,, -, 99. Le Pichon X., S. Lallemant, H. Tokuyama, E. Thoue, P. Huchon and P. Henry, Structure and evolution of the backstop in the eastern Nankai trough area (Japan): Implications for the soon-to-come Tokai earthquake, Island Arc,, -, 99. Okino K. and Y. Kato, Geomorphological study on a clastic accretionary prism: the Nankai trough, Island Arc,, -9, 99. Okuda Y., 9, Tectonic evolution of the continental margin off southwest Japan during the late Cenozoic, Rep. Tech. Res. Center, Jap. Nat. Oil Corp., 9, -9, 9. Park J.-O., T. Tsuru, S. Kodaira, P.R. Cummins and Y. Kaneda, Splay fault branching along the Nankai subduction zone, Science, 9, -,. Seno T., S. Stein, and A.E. 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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. Philippine Sea plate consistent with NUVEL- and geological data, J. Geophys. Res., 9, 9-9, 99. Sugiyama Y., Neotectonics of Southwest Japan due to the rightoblique subduction of the Philippine Sea Plate, Geofisica Internacional,, -, 99. Taira A., T. Byrne and J. Ashi, Photographic Atlas of an Accretionary Prism, Geologic Structure of the Shimanto Belt, Japan, University of Tokyo Press, Tokyo,, 99.

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. Eurasian Plate Na ~ cm/yr Philippine Sea Plate Pacific Plate nk c Izu-Bonin Ar D C gh u A B ai Tro JAPAN D Izu Tokai C Kii 9 9 9 (M.) su Ri dg e S Izu-Bonin Arc h roug kai T Nan D D D Ze ni D D D9 D D NT- TKY NT- km NT- KR9 Bathymetry and topography (m) - - - Figure. Bathymetry map and locations of MCS (black thin and dotted) and OBS/MCS (blue thin) lines in the Nankai Trough margin off southwest Japan. Inset is a regional tectonic map showing the location of the study area (red box). The Philippine Sea Plate (PSP) subducts beneath the Eurasian Plate to the northwest with convergence rate ~ cm/yr [Seno et al., 99]. The Nankai subduction zone may be divided into four discrete domains (A through D) marked by distinct megathrust earthquake ruptures [Ando, 9; Sugiyama, 99]. The eastern Nankai subduction zone is composed of two domains (C and D), the Tonankai and Tokai segments, respectively. Contours with purple thin lines show the 9 Tonankai (M =.) coseismic slip amounts (meter) estimated from seismic inversion [Kikuchi et al., ]. The paleo-zenisu ridge distribution that was suggested by magnetic anomaly data (Le Pichon et al., 99) is shown by a polygon with heavy yellow line. Heavy black parts on lines D, S, TKY, and mark the MCS and OBS profiles shown in Figs.,, and. NW SE Two-way Time (sec) Outer ridge Forearc basin S Line D Cover sequence Accretionary prism collem R Oceanic crust 9 - Distance from frontal thrust (km) SW NE Line S Two-way Time (sec) t Oceanic crust Distance (km) Figure. Time-migrated MCS profiles of dip line D and strike line S showing the subducted ridge in the Tonankai segment. Subducting oceanic crust is shaded in light blue. Vertical exaggeration is about : at the seafloor.

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. Zenisu ridge Inner slope Nankai trough Depth (km) Line TKY Accretionary prism E Old accretionary prism (backstop) F South ridge North ridge Uppermost mantle Velocity (km/s) G........ Distance (km) Figure. Wide-angle OBS survey-derived crustal velocity structure model showing the subducted ridges in the Tokai segment, which was obtained from a first arrival refraction tomography (modified after Kodaira et al. []). Lighter colored regions shows no seismic ray sampling. Dotted lines indicate reflectors picked from a seismic reflection image (Figure b of Kodaira et al. []). obtained by prestack depth migrations of the OBS wide-angle and conventional MCS seismic data. The black (E), blue (F) and red (G) dotted lines are interpreted as reflectors from the top of an old accretionary complex, the top and the bottom of an igneous crust, respectively. NW SE Forearc basin Two-way Time (sec) Outer ridge Line Cover sequence Nankai Accretionary prism play S 9 Distance from frontal thrust (km) Figure. Time-migrated MCS profile of dip line showing the splay fault in the Tonankai segment. Subducting oceanic crust is shaded in light blue. Location of the splay fault's initial branching is marked by red dotted circle. Red arrows show motions of the splay fault slip. The splay fault branches upward from the plate-boundary interface at ~ km landward from the frontal thrust, approaching the seafloor just seaward of the outer ridge, breaking through the overriding accretionary wedge. Vertical exaggeration is about : at the seafloor.

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. LEGEND 9 9 coseismic slip (meter) Seaward end of backstop Splay fault D Subducted ridge Inferred subducted ridge distribution Izu Tokai Earthquake swarm (9) M.9 Earthquake (9) C Kii Izu-Bonin Arc K 9 (M.) S D km h ug ai Tro k n a N Bathymetry and topography (m) - - - Figure. Spatial distribution of the elongate, trough-parallel subducted ridge in the eastern Nankai subduction zone. The subducted ridge is shaded in light black. Heavy red and thin blue parts on the seismic survey lines mark the subducted ridge and the splay fault, respectively. The seaward end of backstop was constrained by several wide-angle OBS profiles [Nakanishi et al., ]. Uplifted outer ridge lt y la Sp u fa o r n? Sea ucted Subrdidge e Plat Ph r ath g Me ine ilipp ust g pl Ridge region Outer ridge Forearc basin Neogene-Quaternary accretionary prism Cretaceous-Tertiary accretionary prism lay Nankai Trough Sp? (Backstop) ine ilipp Sea e Plat Ph ust r ath g Me lt fau Ridge-free region Figure. Schematic cross sections showing the ridge-backstop collision in the Tonankai segment. All of the interseismic elastic strain at the updip portion of the seismogenic zone could be released by the coseismic splay fault slip alone, but it seems more likely that there would be slip partitioning between it and the subduction zone (i.e., plate-boundary interface) [Park et al., ]. The dotted and solid lines at the updip of the Tonankai megathrust indicate the possible slips along the splay fault and subduction zone, respectively. The subducting ridge may be responsible for the steeper slope angle of the splay fault.