Modern and Ancient Out-of-Sequence Thrust in the Nankai Trough: Insight from Laboratory-Derived Properties and Seismic Data

At convergent plate margins, characteristics of out-ofsequence thrusts (OOSTs) are important for understanding the nature of earthquake mechanisms and deformation of the accretionary prism. To understand the characteristics of  OOSTs, Integrated Ocean Drilling Program (IODP) plans to  drill through the seismogenic OOST in the Nankai Trough. In this phase, however, the seismogenic OOST is too deep to penetrate, so its properties have only been estimated from seismic data. Seismic reflection data from the Nankai Trough off the Kii Peninsula in southwest Japan (Figs. 1 and 2) image  a strong negative polarity OOST (or splay fault) reflection branching from the major plate boundary fault (Park et al.,  2002; Tsuru et al., 2005). This OOST might have ruptured  during the 1944 Tonankai earthquake and associated tsunami (Tanioka and Satake, 2001). The negative polarity  reflection of the OOST has been interpreted to indicate elevated fluid pressure in the fault zone (Park et al., 2002).  From seismic data alone, the reflection polarity is highly useful information for estimating fault zone properties.  However, polarity of deep seismic reflections is also affected by acoustic dispersion, and the fracture zone  causes wavelet tuning (Costain and  Coruh, 2004). Therefore, full charac terization of OOST solely using  reflection polarity is not possible. To investigate characteristics of a seismogenic OOST, we determined acoustic properties of discrete samples obtained from the fossil OOST outcrop in Nobeoka, and compared them with those of an active Kumano OOST imaged on seismic profile off the Kii  peninsula. Because the Nobeoka OOST is interpreted as a fossil OOST, preserves in situ structure, and crops out (Kondo et al., 2005), it is ideal for  evaluating characteristics of a seismogenic OOST.


Introduction
At convergent plate margins, characteristics of out-ofsequence thrusts (OOSTs) are important for understanding the nature of earthquake mechanisms and deformation of the accretionary prism.To understand the characteristics of of OOSTs, Integrated Ocean Drilling Program (IODP) plans to , Integrated Ocean Drilling Program (IODP) plans to drill through the seismogenic OOST in the Nankai Trough.In this phase, however, the seismogenic OOST is too deep to penetrate, so its properties have only been estimated from seismic data.Seismic reflection data from the Nankai Trough off the Kii Peninsula in southwest Japan (Figs. 1 and 2) image s. 1 and 2) image 1 and 2) image a strong negative polarity OOST (or splay fault) reflection branching from the major plate boundary fault (Park et al., (Park et al., Park et al., 2002;Tsuru et al., 2005).This OOST might have ruptured ).This OOST might have ruptured .This OOST might have ruptured during the 1944 Tonankai earthquake and associated tsunami (Tanioka and Satake, 2001).The negative polarity (Tanioka and Satake, 2001).The negative polarity Tanioka and Satake, 2001).The negative polarity ).The negative polarity .The negative polarity reflection of the OOST has been interpreted to indicate elevated fluid pressure in the fault zone (Park et al., 2002).(Park et al., 2002).Park et al., 2002).). .From seismic data alone, the reflection polarity is highly useful information for estimating fault zone properties.
for estimating fault zone properties.estimating fault zone properties.ing fault zone properties.fault zone properties.However, polarity of deep seismic reflections is also affected by acoustic dispersion, and the fracture zone the fracture zone fracture zone causes wavelet tuning (Costain and (Costain and Costain and Coruh, 2004).Therefore, full charac-).Therefore, full charac-.Therefore, full characterization of OOST solely using of OOST solely using solely using reflection polarity is not possible.To investigate characteristics of a seismogenic OOST, we determined acoustic properties of discrete samples obtained from the fossil OOST outcrop in Nobeoka, and compared them with those of an active Kumano OOST imaged on seismic profile off the Kii the Kii Kii peninsula.Because the Nobeoka OOST is interpreted as a fossil OOST, preserves in situ structure, and crops out (Kondo et al., 2005), it is ideal for (Kondo et al., 2005), it is ideal for Kondo et al., 2005), it is ideal for ), it is ideal for , it is ideal for evaluating characteristics of a seismogenic OOST.

Results and Discussions
P-wave and S-wave velocities of discrete (minicore-shaped) samples were measured in dry conditions because in saturated conditions fluid dispersion masks pressure effects (Tsuji et al., (Tsuji et al., Tsuji et al., 2006).We observed anisotropy of velocity in the hanging ).We observed anisotropy of velocity in the hanging .We observed anisotropy of velocity in the hanging wall of the Nobeoka OOST attributed to foliation of politic the Nobeoka OOST attributed to foliation of politic Nobeoka OOST attributed to foliation of politic phyllite (Fig. 3).Foliation-normal P-wave velocities in the hanging wall are ~700 m s s s -1 slower than the foliation-parallel velocities.In contrast, the footwall is composed of brittle, , deformed, chaotic shales and fine sandstones, and velocities in the footwall are lower than those in the hanging wall.Furthermore, velocities increase just above the fault core (gray zones in Fig. 3), where velocity anisotropy at atmospheric pressure is weak and velocity increase with pressure is less apparent.These characteristics may be caused by the filling of most cracks with quartz, which is observed just above the fault core in the Nobeoka OOST.The shear-related dewatering and consolidation may also affect -related dewatering and consolidation may also affect related dewatering and consolidation may also affect elastic moduli just above the fault core.
Comparison of the acoustic properties of the fossil Nobeoka OOST (determined from outcrop samples) and the OOST (determined from outcrop samples) and the (determined from outcrop samples) and the (determined from outcrop samples) and the determined from outcrop samples) and the ) and the and the the active Kumano OOST (calculated from seismic reflection OOST (calculated from seismic reflection (calculated from seismic reflection (calculated from seismic reflection calculated from seismic reflection data) is warranted by the two thrusts' ) is warranted by the two thrusts' is warranted by the two thrusts' similar tectonic settings and lithologies.Weathering has affected properties of the Nobeoka OOST, but overall results from the hanging wall and the footwall should reflect trends of in situ properties.Furthermore, for urthermore, for or the comparison of the fossil Nobeoka and active Kumano OOSTs, we calculated water-saturated P-wave velocities from velocities determined under dry conditions via Gassmann's relation.If we assume an average bulk density of 2.5 g cm cm cm -3 3 , a seafloor depth of 2 km, an OOST depth of 7 km, and hydrostatic conditions, effective pressure in the Kumano OOST is calculated to be ~73 MPa.Assuming that velocities at 55 MPa represent those at in situ effective pressures, the contrast between foliation-normal P-wave velocities in hanging wall samples and footwall samples is >600 m s s s -1 .The change in P-wave velocities across the fault core of the Nobeoka OOST (<600 m s s s -1 ) is larger is larger is larger

Figure 1 .
Figure 1.Locations of the Nobeoka OOST and the seismic line off Kumano.

Figure 2 .Figure 3 .
Figure 2. Out-of-sequence thrust on seismic reflection profile off Kumano.This profile crosses the planned IODP site.

Figure 4 .
Figure 4. Comparison of angle-dependent reflection coefficients of discrete Nobeoka OOST samples (thick line)(Tsuji et al., 2006); the active Kumano OOST from seismic reflection data (solid dots) (Zone B inTsuru  et al., 2005).Thin lines represent reflection coefficients parameterized by Poisson's ratio.Poisson's ratios in the footwall of the Nobeoka OOST are lower than that of the active Kumano OOST, suggesting high fluid pressures in the footwall of the active Kumano OOST.