Seismic reflection and bathymetric evidences for the Nankai earthquake rupture across a stable segment-boundary
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 10 August 2011
Accepted: 18 October 2011
Published: 12 March 2012
Seismic reflection profiles reveal steeply landward-dipping splay faults in segment B (the 1946 Nankai earthquake rupture, M 8.3) as well as segment C (the 1944 Tonankai earthquake rupture, M 8.1) of the Nankai subduction zone. The splay faults, branching upward from the plate-boundary interface, almost reach the seafloor, producing seafloor fault scarps. The swath-bathymetry map exhibits a ∼200-km-long, remarkable seafloor lineament with which the seafloor fault scarps align in the segments B and C. The seafloor lineament, which we believe is produced by repeating slips on the splay faults, is almost laterally continuous across a stable boundary off Kii Peninsula inbetween the two segments. These seafloor and subsurface features could be due to multiple, simultaneous coseismic slips across the B–C boundary, when subduction thrust earthquakes accompany the co-seismic slips on the splay fault. The splay faults are associated with (1) fluid expulsion, (2) dynamic deformation, and (3) tsunami generation.
It is well known that a subduction zone is divided into several discrete segments marked by a megathrust earthquake rupture. While the segment boundary limits the rupture area of an individual earthquake, sometimes a great earthquake across a segment boundary occurs, causing strong shaking and devastating tsunamis, e.g., Nankai (Ando, 1975), Cascadia (Satake et al., 1996), Chile (Campos et al., 2002), Kuril (Sawai et al., 2004), Sumatra (Lay et al., 2005), and Japan Trench (Ide et al., 2011). However, except for recent earthquakes such as the 2004 Sumatra (M 9.2), whether or not a great earthquake occurred across a segment boundary may be controversial, because it often depends solely on historical documents.
The Nankai Trough subduction zone off southwest Japan is one of the convergent margins best suited for studying large megathrust earthquakes, as well as the formation of accretionary prisms. At the Nankai Trough margin, the Philippine Sea plate (PSP) is being subducted beneath the Eurasian Plate to the northwest at a convergence rate of ∼4 cm/yr (Seno et al., 1993). The Shikoku Basin, the northern part of the PSP, is estimated to have opened between 25 and 15 Ma by backarc spreading of the Izu-Bonin arc (Okino et al., 1994).
Based on historical documents and observation on subduction thrust earthquake occurrence, the Nankai subduction zone is segmented into five different domains, i.e., A, B, C, D, and E (Ando, 1975), each of which roughly corresponds to a geologically well-defined forearc basin (Sugiyama, 1994). The earthquake rupture pattern over those five segments suggests that a boundary between segments A–B and C–D has been stable over the historic earthquake cycles. For example, the last 1944 Tonankai earthquake ruptured the segments C and D. About two years later, the 1946 Nankai (M 8.3) earthquake ruptured the segments A and B. Two events of the 1854 Ansei (M 8.4) earthquake were also separated by the B–C boundary. Tsunami waveform inversion results (Baba and Cummins, 2005) support the presence of the B–C boundary off Kii Peninsula. In contrast, the 1707 Hoei (M 8.7) earthquake is presumed to have extended across the B–C boundary and ruptured the segments A, B, C, and D at a single event, based on historical documents and numerical simulation (Kodaira et al., 2006). However, it has been controversial whether, or not, there has been an earthquake rupture across the stable B–C boundary, due to the lack of modern geophysical and geological evidence.
In this paper, we show multi-channel seismic (MCS) reflection profiles crossing the Nankai Trough, and Seabeam swath-bathymetry data off the Kii Peninsula, to resolve the controversy of whether, or not, coseismic rupture includes a previously-defined segment-boundary. Finally, we present the potential implications of the splay faults in the Nankai subduction zone.
2. Swath-Bathymetry Data and Their Interpretation
On the seafloor topography map (Fig. 1), the seafloor lineament looks roughly continuous over the segments B and C. In order to verify the continuity of the seafloor lineament along the strike, we have calculated the seafloor slope angle from the swath-bathymetry data. The slope-angle map (Fig. 2) highlights that the seafloor lineament of segment C is almost continuous to segment B, i.e., the 1946 Nankai earthquake rupture area, completely passing through the stable segment-boundary off Cape Shiono, Kii Peninsula. Most of the seafloor fault scarps, which are also denoted by “X” on the MCS profile 4, show a 15° to 25° dip. The seafloor lineament across the two segments is ∼200-km long. It enables us to deduce the existence of a similar splay-fault system in segment B. Accordingly, we have re-examined the MCS data off the Kii Peninsula.
3. Seismic Reflection Data and Interpretation
The MCS data that we have used for this study was collected in the Nankai Trough margin off the Kii Peninsula by R/V Kairei of the Japan Marine Science and Technology Center (JAMSTEC) in 2001. For deep-penetration seismic imaging, we used a large volume (∼200 liter) air gun array as the controlled sound source. The MCS data recording was made with a 4 km, 160-channel streamer with a 25-m group spacing. Figure 1 shows the positions of the MCS lines. Data processing included trace editing, pre-filtering, spherical divergence correction, signature deconvolution, CMP (Common Mid-Point) sort, NMO correction, multiple suppression by parabolic radon transform, CMP stack, and time-migration.
4. Implications of the Splay Faults
As a matter of fact, we do not know if the coseismic slip on the splay fault was regular over the historical earthquakes. Because it is not possible to account for all of the recent coseismic slips, it is difficult to tell if the along-strike continuity of the splay fault directly indicates the coseismic rupture across the B–C boundary. Even the seafloor lineament could be caused by aseismic slip. The splay fault system may play a significant role in rupture propagation across the B–C boundary only when a subduction thrust earthquake accompanies the coseismic slip on the splay fault.
Recent submersible observations (Ashi et al., 2009) in segment B, as well as in segment C, reported the presence of chemosynthetic benthic colonies around the seafloor fault scarp of the splay faults, indicating the presence of cold seeps (Figs. 1 and 5) and possible fluid expulsion along the fault.
Low-frequency tremors associated with reverse faults in a shallow accretionary prism were observed in segment C (Obana and Kodaira, 2009). Many of the low-frequency tremors were located near the shallowest part of a splay fault. The episodic activity of the low-frequency tremors suggests that the splay faults are conditionally stable and thus can become unstable under sufficiently strong dynamic loading. It is also well known that very low-frequency (VLF) earthquakes occur on the well-developed reverse fault system in the accretionary prism along the Nankai Trough (e.g., Obara and Ito, 2005). The activity of VLF earthquakes is considered to be the result of slips on the reverse fault system and, thus, the signature of a dynamic deformation process in the accretionary prism. The VLF earthquakes occurred around the splay fault in segment B in 2005 (Obara and Ito, 2005), as shown in Figs. 1 and 5. Most of the VLF earthquakes are located landward from the seafloor lineament. We infer that the VLF earthquakes occur along the splay faults as identified on MCS profiles 1 and 2, because the splay fault is the only major reverse fault in the accretionary prism. The splay faults could be under dynamic deformation during an interseismic period.
A tsunami waveform inversion study (Baba and Cummins, 2005) indicated that the 1946 Nankai coseismic rupture off Cape Shiono, Kii Peninsula, propagated anomalously seaward, compared to adjacent areas (Fig. 1). The splay fault system shown in Figs. 3 and 4, which is recognized as a first-order feature in the Nankai Trough margin off the Kii Peninsula, could be adopted to explain the anomalous rupture pattern.
Combining the seismic reflection and swath-bathymetry data provides evidence for the large earthquake rupture across a stable segment-boundary off the Kii Peninsula in the Nankai subduction zone. Seismic reflection profiles reveal steeply landward-dipping splay faults in the segments B and C. The splay faults, branching upward from the plate-boundary interface, almost reach the seafloor, producing the seafloor fault scarps. The swath-bathymetry map exhibits a ∼200-km-long, remarkable seafloor lineament with which the seafloor fault scarps align in the segments B and C. The seafloor lineament, which may be produced by repeating slips on the splay faults, is almost laterally continuous across the B–C boundary off the Kii Peninsula. These seafloor and subsurface features could be due to multiple, simultaneous coseismic slips across the B–C boundary when subduction thrust earthquakes accompany coseismic slips on the splay fault. The splay faults are related to fluid expulsion, dynamic deformation, and tsunami generation.
We are deeply indebted to Nathan Bangs and an anonymous reviewer for their constructive comments and suggestions to improve the manuscript. This work was supported by MEXT Grant-in-Aid for Scientific Research on Innovative Areas (21107002).
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