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Keywords:

  • faults;
  • subduction zone;
  • Nankai Trough;
  • earthquakes;
  • slow slip;
  • accretionary prism

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

Accretionary complexes host a variety of fault zones that accommodate plate convergence and internal prism deformation, including the décollement, imbricate thrusts, and out-of-sequence thrusts or splays. These faults, especially the décollement and major splay faults, are considered to be candidates for hosting slow slip events and large magnitude earthquakes, but it is not clear what modes of slip should be expected at shallow levels or how they are related to fault rock frictional properties. We conducted laboratory experiments to measure the frictional properties of fault and wall rock from three distinct fault zone systems sampled during Integrated Ocean Drilling Program Expedition 316 and Ocean Drilling Program Leg 190 to the Nankai Trough offshore Japan. These are (1) a major out-of-sequence thrust fault, termed the “megasplay” (Site C0004), (2) the frontal thrust zone, a region of diffuse thrust faulting near the trench (Site C0007), and (3) the décollement zone sampled 2 km from the trench (Site 1174). At 25 MPa effective normal stress, at slip rates of 0.03–100 μm/s, and in the presence of brine as a pore fluid, we observe low friction (μ ≤ 0.46) for all of the materials we tested; however, the weakest samples (μ ≤ 0.30) are from the décollement zone. Material from the megasplay fault is significantly weaker than the surrounding wall rocks, a pattern not observed in the frontal thrust and décollement. All samples exhibit primarily velocity-strengthening frictional behavior, suggesting that earthquakes should not nucleate at these depths. A consistent minimum in the friction rate parameter a-b at sliding velocities of ∼1–3 μm/s (∼0.1–0.3 m/d) is observed at all three sites, suggesting that these shallow fault zones may be likely to host slow slip events.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

In the Nankai Trough region offshore Japan (Figure 1), plate convergence is accommodated by numerous thrust faults developed within the accretionary prism [e.g., Moore et al., 2009]. These faults are commonly clay rich, with gouge derived from pelagic and hemipelagic sediments [Underwood, 2007]. This includes the décollement [Deng and Underwood, 2001; Moore et al., 2001], imbricate thrusts that accommodate deformation and thickening in the accreted wedge, and major out-of-sequence thrust (OOST) faults that branch from the décollement [Park et al., 2002; G. F. Moore et al., 2007].

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Figure 1. Map of the Nankai area showing location of drill sites C0004, C0007, and 1174 as well as the rupture areas (dashed boxes) and epicenters (stars) of the 1944 Tonankai and 1946 Nankaido earthquakes (modified from Kimura et al. [2008]).

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The Nankai subduction zone has hosted historical great (Mw > 8) earthquakes, including the 1944 Tonankai (Mw = 8.1) and 1946 Nankaido (Mw = 8.3) events [Ando, 1975; Sagiya and Thatcher, 1999]. The extent of coseismic rupture for the both the 1944 and 1946 earthquakes as determined by tsunami and seismic waveform inversions (Figure 2a) suggests that significant coseismic slip may have occurred along major splay faults in addition to, or instead of, the décollement [Kato, 1983; Sagiya and Thatcher, 1999; Cummins and Kaneda, 2000; Park et al., 2000; Tanioka and Satake, 2001; Kikuchi et al., 2003]. This inference is consistent with theoretical studies suggesting that rupture is likely to branch along such structures [Sekiguchi et al., 2000; Kame et al., 2003]. Seismic reflection data documents a major OOST, termed the “megasplay” fault, that is geologically active and has accommodated at least 1.3 km of slip over the past 1.95 million years (Ma) [Park et al., 2002; G. F. Moore et al., 2007; Strasser et al., 2009]. This structure, in particular, has been implicated in coseismic slip and tsunamigenesis [Tanioka and Satake, 2001; Kikuchi et al., 2003]. Seismic, geodetic, and hydrologic observations also suggest that faults in the outer fore arc, including the megasplay fault and the shallow décollement zone, host a range of slip behaviors, including very low frequency (VLF) earthquakes [Ito and Obara, 2006; Obana and Kodaira, 2009], slow slip [Davis et al., 2006], and postseismic relaxation [Sagiya and Thatcher, 1999]. Furthermore, recognition of pseudotachylytes from paleosubduction zones provides evidence of seismic slip occurrence on faults containing clay minerals [Rowe et al., 2005; Ujiie et al., 2007]. Thus, thrust faults within the prism are important for accommodating all types of slip.

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Figure 2. (a) Profile of the accretionary prism along the Kumano transect in Figure 1, showing IODP Sites C0004 and C0007. Also shown are estimates of the updip extent of coseismic slip inferred for the 1944 Tonankai earthquake (single asterisk, Tanioka and Satake [2001]; double asterisk, Kikuchi et al. [2003]), (modified from Kimura et al. [2008]). (b) Profile of the accretionary prism along the Muroto transect, showing ODP Site 1174 (modified from Shipboard Scientific Party [2001a]).

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Experimental and theoretical studies have shown that the mode of fault slip (e.g., creep, slow slip events, or earthquakes) can be explained in the context of constitutive friction laws, and in particular by the velocity dependence of sliding friction, which can be quantified by measuring the response of friction to sliding velocity perturbations in laboratory experiments [e.g., Marone, 1998; Scholz, 2002]. Previous work on synthetic mixtures used as analogs for natural fault gouge [e.g., Morrow et al., 1992; Saffer and Marone, 2003; Crawford et al., 2008; Ikari et al., 2009a] and limited studies of natural materials from subduction zone settings [Brown et al., 2003; Kopf and Brown, 2003; McKiernan et al., 2005; Ikari et al., 2009b] have shown that sheared clay-rich sediments are generally frictionally weak (μ < 0.5) and exhibit velocity-strengthening behavior, which is linked to stable sliding.

Here, we report on the results of shearing experiments conducted on material retrieved from the frontal thrust zone (Site C0007) and megasplay fault zone (Site C0004) of the Nankai Trough during Integrated Ocean Drilling Program (IODP) Expedition 316, including results previously reported by Ikari et al. [2009b] for material obtained from the megasplay fault zone, and from the décollement zone sampled during Ocean Drilling Program (ODP) Leg 190 (Figures 12). For each site, we tested samples from depth transects that include both wall rock and key fault zones. Our results allow detailed comparison of the frictional properties and strength of natural fault materials, and provide insight into the mechanical behavior across three distinct fault systems in an active accretionary complex. These measurements also provide important constraints for modeling studies that aim to assess fault slip and rupture processes [e.g., Liu and Rice, 2005; Perfettini and Ampuero, 2008].

2. Geologic Setting

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

The Nankai accretionary complex off the coast of SW Japan is formed by subduction of the Philippine Sea plate beneath the Eurasian plate (Figure 1). It is one of the most extensively studied subduction zones in the world, and has been the focus of numerous ODP and IODP drilling expeditions, 3-D seismic imaging, and geodetic surveys [e.g., Thatcher, 1984; Ozawa et al., 1999; Sagiya and Thatcher, 1999; Mazzotti et al., 2000; Moore et al., 2001; Park et al., 2002; Bangs et al., 2004]. Scientific drilling in this region includes Deep Sea Drilling Project (DSDP) Legs 31 and 87, ODP Legs 131 and 190, which sampled the décollement zone at Sites 1174 and 808 along the Muroto transect (Figures 1 and 2b), and IODP Expeditions 314, 315, and 316. Expedition 316 sampled the megasplay fault at Site C0004 and the frontal thrust zone at Site C0007 along the Kumano transect, located ∼200 km to the northeast of the Muroto transect (Figures 1 and 2a). The two transects exhibit marked differences in heat flow [Yamano et al., 2003; Kinoshita et al., 2003] and prism structure, including differences in bathymetric slope, taper angle, and frontal thrust geometry [Moore et al., 2001, 2009].

2.1. Megasplay Thrust Fault

The megasplay is a major out-of-sequence splay fault bounding the seaward edge of the Kumano fore-arc basin (Figure 2). It was drilled at Site C0004 during IODP Expedition 316, as part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) [Kimura et al., 2008]. The megasplay branches from the main décollement ∼50–55 km landward of the deformation front and terminates near the seafloor ∼25 km from the trench [G. F. Moore et al., 2007, 2009] (Figure 2a). Our samples span a depth transect from 119 to 439 mbsf (meters below seafloor) (Figure 3). The hanging wall (lithologic Unit II, Figure 3) is mostly composed of late to middle Pliocene hemipelagic mudstones, and extends from 78 to 258 mbsf. Lithologic Unit III is a fault-bounded package between 258 and 308 mbsf, defined by two biostratigraphic age reversals and a zone of fracturing and brecciation [Kimura et al., 2008; Expedition 316 Scientists, 2009a]. This is interpreted as the main fault zone of the megasplay, and is composed of fractured and brecciated mid-Pliocene hemipelagic muds with minor interbedded volcanic ash. The footwall (lithologic Unit IV) is composed of overridden slope apron sediment, consisting of early Pleistocene fine-grained hemipelagic claystones with some sandy turbidites [Kimura et al., 2008; Expedition 316 Scientists, 2009a]. Within the depth interval of our samples, total clay mineral abundance generally ranges from ∼40–70 wt%, combined quartz and plagioclase content ranges from ∼25–55 wt%, and minor calcite is present throughout the section (Figure 3). The highest clay content is found within the fault zone, and a distinct reduction in clay and corresponding increase in quartz and plagioclase abundances are observed at the lower boundary of Unit III (Figure 3) [Expedition 316 Scientists, 2009a]. We conducted friction and permeability measurements on two samples from the hanging wall, four from the fault zone, and two from the footwall.

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Figure 3. (first panel) Summary of sample locations and lithostratigraphy, (second panel) seismic reflection data, and (third, fourth, and fifth panels) mineralogy data at Site C0004. The lithostratigraphy is modified from M. B. Underwood (unpublished data, 2008). The lithostratigraphy and seismic reflection profile are modified from Expedition 316 Scientists [2009a]. Symbols show the depth location and physical state (remolded or intact) of samples used in this study.

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2.2. Frontal Thrust System

Drilling at IODP Site C0007 sampled the frontal thrust zone near the toe of the accretionary prism along the same transect as Site C0004 (Figure 2a) [Kimura et al., 2008]. At this site, drilling penetrated several thrust faults within the depth interval from 235 to 450 mbsf (Figure 4), including three fault zones located at depths of 237.5–259.3 mbsf, 341.5–362.3 mbsf, and 399–446 mbsf [Kimura et al., 2008; Expedition 316 Scientists, 2009b]. The trench wedge facies (Units II and III) is a coarsening upward sequence of mudstones with interbedded sand and gravel extending from 34 to 362 mbsf. Unit III is composed of Pliocene hemipelagic sediments with small ash layers found primarily at the top of the unit, and extends from 362 to 439 mbsf. This unit is interpreted as the stratigraphic equivalent of the Upper Shikoku Basin facies identified along strike at Site 1174 and extending throughout most of the Shikoku Basin [Moore et al., 2001; Ike et al., 2008; Expedition 316 Scientists, 2009b]. In the upper ∼150 m of the section at Site C0007, quartz + plagioclase abundance ranges from ∼50–75 wt%, and clay content is ∼25–50 wt% (Figure 4) [Kimura et al., 2008]. Below ∼200 mbsf, quartz + plagioclase content declines approximately linearly from ∼50% to ∼30%, and clay content increases from ∼40% to near 70%. Calcite content is low throughout the section. Our samples from Site C0007 span a depth range of 104–437 mbsf and include one sample from the shallowest fault zone (Fault Zone 1) and two samples from the deepest fault zone (Fault Zone 3) in addition to four wall rock samples.

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Figure 4. (first panel) Summary of sample locations and lithostratigraphy, (second panel) seismic reflection data, and (third, fourth, and fifth panels) mineralogy data at Site C0007. USB, Upper Shikoku Basin facies. The lithostratigraphy is modified from Underwood (unpublished data, 2008). The lithostratigraphy and seismic reflection profile are modified from Expedition 316 Scientists [2009b]. Symbols show the depth location and physical state (remolded or intact) of samples used in this study.

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2.3. Décollement Zone

ODP Site 1174, drilled during ODP Leg 190, penetrated the décollement zone along the Muroto Transect (Figure 2b). The samples tested from this site are significantly deeper than those from Sites C0004 and C0007, ranging from 660 to 968 mbsf (Figure 5). The shallowest sample we tested comes from near the base of the Upper Shikoku Basin facies (USB), which is underlain by the Lower Shikoku Basin facies (LSB), a homogeneous section of Miocene to Pliocene silty claystone/clayey siltstone extending from 661 to 1102 mbsf [Moore et al., 2001; Shipboard Scientific Party, 2001b]. The décollement zone localizes within the LSB, and is identified as a ∼30 m thick zone of fractured and brecciated mudstone from 808 to 840 mbsf, and by a sharp increase in porosity and decrease in bedding dip across its base. From ∼500–1100 mbsf, quartz + plagioclase content decreases slightly, from ∼55 to ∼30 wt%, with some scattered lower values (Figure 5). Clay content increases slightly from ∼40 to ∼60 wt% within this depth range. Calcite content is generally low throughout the section, but exhibits some values as high as ∼25 wt% above the décollement and ∼75 wt% below. We tested three hanging wall samples, three samples from within the décollement zone, and four footwall samples, including one sample within 1.3 m of the décollement zone.

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Figure 5. (first panel) Summary of sample locations and lithostratigraphy, (second panel) seismic reflection data, and (third, fourth, and fifth panels) mineralogy data at Site 1174. The lithostratigraphy and seismic reflection profile are modified from Shipboard Scientific Party [2001b]. Symbols show the depth location and physical state (remolded or intact) of samples used in this study.

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3. Experimental Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

We tested samples as intact wafers trimmed from whole-round cores when possible, and sheared them in a direction perpendicular to the core axis (i.e., parallel to bedding). Other samples were obtained as brecciated mudstone fragments; these samples were either remolded or tested as powdered gouge. Samples tested as powdered gouge were dried for ∼24 h at ∼40°C (to prevent clay mineral dehydration reactions during drying that may occur at higher temperatures), disaggregated by hand, and sieved to a grain size of <106 μm. Prior to shearing, layer thickness of the saturated gouge was ∼1.5 mm under normal load (Table S1 in the auxiliary material). The fragile nature of the intact wafers required the use of thicker samples (2.5–4 mm initial thickness).

We conducted friction experiments within a pressure vessel housed in a biaxial testing apparatus with servo-hydraulic control [Ikari et al., 2009a, Samuelson et al., 2009]. In this configuration, normal stress is applied horizontally to three steel forcing blocks that hold two sample layers (Figure 6a, inset). Shear is induced in the sample by driving the center block vertically past the side blocks. The forcing blocks are jacketed so that confining pressure (Pc) and pore pressure (Pp) can be applied to the sample, resulting in a true triaxial stress state. Effective normal stress was maintained at 25 MPa, and represents the combined effects of confining pressure, externally applied normal load, and pore pressure. We chose 25 MPa because it is similar to the anticipated in situ effective normal stress, and because the frictional properties of clay-rich gouge (such as steady state friction and friction rate dependence) do not vary greatly at effective normal stresses from 25 MPa up to ∼60 MPa [Ikari et al., 2009a], which facilitates extrapolation of frictional behavior to greater depths.

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Figure 6. (a) Example of friction shear strain data from a typical experiment, showing measurement of the coefficient of friction μ, velocity step test, and slide-hold-slide sequence. Dashed box shows the velocity step in Figure 3c. Inset shows double-direct shear configuration with applied stresses and pore fluid ports. (b) Idealized illustration of the response of friction to an increase in load point velocity, indicating a, b, and Dc. (c) Example of a velocity step with the model inversion superimposed on experimental data.

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During shear, Pc was held constant at 6 MPa, pore pressure at the upstream end of the sample (Ppa) was held constant at 5 MPa, and the downstream pore pressure (Ppb) was set to a no-flow (undrained) condition in order to monitor pore pressure in the layer and assess any changes during shearing [e.g., Ikari et al., 2009a]. In order to approximate in situ pore chemistry conditions, we used 3.5 wt% NaCl brine as pore fluid [Expedition 316 Scientists, 2009a]. In each experiment, shear was implemented as a displacement rate boundary condition (∼10 μm/s) at the gouge layer boundary. After attainment of steady state shear stress (typically at shear strain of ∼5), a velocity-stepping sequence over a range from 0.03 to 100 μm/s was performed to measure friction constitutive parameters [e.g., Marone, 1998]. Total accumulated shear strain ranged from ∼4–30.

We measured the steady state shear stress (τ) prior to the initiation of velocity steps (Figure 6a) to determine the coefficient of sliding friction (μ), assuming that cohesion is negligible for our samples of disaggregated gouge and weakly consolidated mudstones:

  • equation image

[Handin, 1969] where σn is the effective normal stress, computed using the average of the pore pressures at the drained and undrained boundaries, which typically differ by <∼1% [Ikari et al., 2009a]. We quantify the rate dependence of friction with the parameter a-b:

  • equation image

where Δμss is the change in steady state coefficient of friction upon an instantaneous change in sliding velocity from Vo to V [Tullis and Weeks, 1986; Marone, 1998] (Figures 6b and 6c). A material exhibiting a positive value of a-b is considered to be velocity strengthening, and will tend to slide stably and inhibit propagation of seismic rupture. A material with negative a-b is termed velocity weakening, a property that is considered a prerequisite for frictional instability and earthquake nucleation [Scholz, 2002].

We determined values of the friction rate parameter a-b and other constitutive parameters using an inverse modeling technique with an iterative least squares method, using Dieterich's [1979, 1981] constitutive law for friction with two state variables:

  • equation image
  • equation image

where a, b1 and b2 are empirically derived constants (unitless), and Dc1 and Dc2 are the critical slip distances that evolve over time periods measured as Θ1 and Θ2, the state variables (Figure 6b). The state variables are inferred to represent the average lifetime of contacts that control friction, and the critical slip distance is the displacement over which those contacts are renewed.

The inclusion of two state variables where appropriate represents a more detailed quantification of velocity-dependent friction parameters than in most other, similar studies that use a single state variable model [e.g., Ikari et al., 2009b]. In many cases, however, the data from an individual velocity step are well fit using a single state variable, and in these cases equations (3) and (4) are simplified by setting Θ2 = 0. In our use of the parameter a-b, we consider b = b1 + b2 to account for the possibility of using either 1 or 2 state variables. Similarly, we consider Dc = Dc1 + Dc2. These equations are coupled with an expression describing machine stiffness:

  • equation image

where K is the stiffness of the fault surroundings (in this case the testing apparatus and sample blocks) normalized by normal stress (K = ∼3 × 10−3μm−1 at 25 MPa normal stress), Vlp is the load point velocity, and V is the true slip velocity [Reinen and Weeks, 1993; Saffer and Marone, 2003; Ikari et al., 2009a]. An example of a velocity step in friction data and the corresponding modeled friction is shown in Figure 6c.

4. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

4.1. Frictional Strength

For all three sites, most samples exhibit friction coefficients μ ≤ 0.46 (Figure 7). From the megasplay (Site C0004), the lowest friction samples are from within the fault zone (0.36 ≤ μ < 0.44); the surrounding wall rocks exhibit consistently higher friction coefficients (0.41 ≤ μ ≤ 0.46) [Ikari et al., 2009b] (Figure 7). Samples from the frontal thrust (Site C0007) are consistently weaker than those from the megasplay, with 0.32 ≤ μ ≤ 0.40 for all samples except sample C0007C-11X-4 (μ = 0.54). This sample is especially clay poor (∼30 wt% clay) and is the only sample with μ > 0.46. Unlike the megasplay, we observe no difference in frictional strength between fault zone material and wall rock at the frontal thrust. There is a general trend of slightly decreasing friction as a function of depth (Figure 7). The samples from Site 1174 are significantly weaker than those from the other two sites, with 0.20 ≤ μ ≤ 0.28. As at Site C0007, the samples from wall rock and within the décollement do not exhibit systematic differences in friction coefficient. The coefficient of friction decreases slightly with depth, but this effect is less pronounced than in the frontal thrust region. For samples from all three sites, there is no significant difference in friction coefficient between samples tested as intact wafers and granular gouge, with a maximum difference of 0.05 observed between two samples of specimen C0007D-24R-1.

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Figure 7. Coefficient of friction μ as a function of depth at Sites C0004, C0007, and 1174. Legend indicates the physical state (remolded or intact) of the samples we tested.

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4.2. Velocity Dependence of Friction

Samples from all three fault settings exhibit similar and generally velocity-strengthening (a-b > 0) frictional behavior, with a few exceptions: one instance of velocity weakening in the megasplay footwall (C0004D-42R-3, a-b = −0.0001) and three instances of velocity weakening in the frontal thrust samples, two from wall rock and one from within fault zone 3 (C0007D-24R-1 and C0007D-27R-1, minimum a-b = −0.0005) (Table S2 and Figure 8). The décollement zone samples and all samples from Site 1174 exhibit strictly velocity-strengthening behavior. Values of a-b range up to ∼0.006 for all three locations.

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Figure 8. Friction rate parameter a-b as a function of depth for Sites C0004, C0007, and 1174. Legend indicates the physical state (remolded or intact) of the samples we tested.

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We observe no systematic dependence of a-b on depth, lithification state, or position across the fault zones. However, we do observe a strong dependence of a-b on slip velocity (Figure 9). For all three sites, we measure minimum values of a-b at slip velocities of 1–3 μm/s. These velocity steps are also better fit by a two state variable constitutive model (Table S2). At velocities > 3 μm/s, a-b shows a strong positive dependence on slip velocity. In general, there is no difference in values of a-b between wafers and gouges for samples from Sites C0004 and C0007. In contrast, in the deeper samples from Site 1174, a-b is generally lower for intact wafers than for gouges (Figure 9).

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Figure 9. Friction rate parameter a-b as a function of upstep load point velocity for samples from Sites C0004, C0007, and 1174. Legend indicates the physical state (remolded or intact) of the samples we tested.

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Constitutive modeling of velocity steps allows extraction of the individual rate and state friction parameters a, b, and Dc [e.g., Marone et al., 1990; Reinen and Weeks, 1993]. Values of a range from 0.0046 to 0.00108 at Site C0004, from 0.0035 to 0.00101 at Site C0007, and from 0.0022 to 0.0088 at Site 1174 (Figure 10 and Table S2). Values of b exhibit a larger range than that of a, from −0.0004 to 0.0084 at Site C0004, from 0.0003 to 0.0083 at Site C0007, and from −0.0009 to 0.0073. Similar to a-b, we observe no discernable trend of a, b, or Dc with either depth or structural position. However, b shows a trend as a function of slip velocity that is generally the opposite of that for a-b, with maximum values for sliding velocities of 1–3 μm/s.

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Figure 10. Friction rate parameters a and b as a function of upstep load point velocity for Sites C0004, C0007, and 1174. Legend indicates the physical state (remolded or intact) of the samples we tested.

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5. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

5.1. Relations Between Composition, Friction, and Prism Structure

All samples in this study have bulk clay contents of at least ∼45 wt%, with the exception of sample C0007D-11X4, which contains only ∼30 wt% [Shipboard Scientific Party, 2001b; Expedition 316 Scientists, 2009a, 2009b]. Based on available XRD analyses of clay mineral separates from Site 1174, the clay sized fraction of the bulk sediment is composed primarily of smectite and illite, but chlorite + kaolinite comprise as much as 30 wt% [Steurer and Underwood, 2005]. The overall values of friction coefficient we observe for our clay-rich Nankai samples are 0.20 to 0.46, which compare favorably with previous experimental studies of saturated clay-rich gouges [e.g., Morrow et al., 1992; Crawford et al., 2008; Ikari et al., 2009a; Smith and Faulkner, 2010] including studies of samples recovered during ODP Leg 190 [Brown et al., 2003; Kopf and Brown, 2003; Bourlange et al., 2004; McKiernan et al., 2005]. Morrow et al. [1992] sheared layers of powdered gouge in a standard triaxial configuration, and obtained friction coefficients of ∼0.38 to 0.48 for a powdered illite shale and ∼0.18 to 0.29 for pure montmorillonite; for mixtures of kaolinite and quartz, Crawford et al. [2008] reported residual friction values < 0.45 under water saturated conditions (after accounting for strain hardening effects observed in the triaxial testing apparatus). Kopf and Brown [2003] and Brown et al. [2003] sheared remolded natural samples from the incoming sedimentary sections at Nankai and Barbados using a ring (rotary) shear system, and report friction values of μ = 0.10–0.30 and μ = 0.17–0.27, respectively. Friction coefficients of 0.20–0.30 were measured on samples from Site 1174 by McKiernan et al. [2005] in a triaxial configuration, and values of 0.37–0.40, also measured in a triaxial cell, were reported by Bourlange et al. [2004] on mudstone samples from Site 1173, which is a reference site outboard of the trench along the Muroto transect.

We observe significant differences in friction coefficient between specimens from the three sites we studied, although the samples we tested are primarily hemipelagic silty claystones/clayey siltstones and therefore lithologically similar. Clay content is known to exert a control on friction coefficient [e.g., Lupini et al., 1981; Logan and Rauenzahn, 1987; Brown et al., 2003; Ikari et al., 2007], so it is possible that second-order variations in composition drive some of the patterns in friction that we observe. These include (1) the step increase μ from the fault zone to the footwall at Site C0004, which coincides with an increase in quartz + plagioclase content (and decrease in clay content) (Figures 3 and 7), (2) the overall decrease in μ and corresponding increase in clay content with depth at Site C0007 (Figures 4 and 7), and (3) the slight overall decrease in μ and corresponding slight increase in clay content with depth at Site 1174 (Figures 5 and 7).

However, the range in bulk clay content is similar for all three sites, suggesting that other factors may also strongly influence friction coefficient. For example, the gradual decrease in μ with depth observed in intact wafers from Sites 1174 and C0007 (Figure 7) suggests that fabric development associated with consolidation and early diagenesis may affect friction coefficient at burial depths greater than ∼400–500 m. Overall, however, we observe little difference in friction coefficient between samples trimmed as wafers with minimal disturbance to in situ fabric and consolidation state, and those tested either as powders or remolded sediment (Figure 7). This applies to both Site C0007 and 1174, is consistent with previous findings from C0004, and probably indicates that at depths of up to ∼1 km, fabric development and consolidation have not advanced to the point that they exert a significant influence on frictional strength.

In the megasplay fault zone, material from within the fault zone is frictionally weaker than the wall rocks. In contrast, our measurements document little difference between fault rock and wall rock friction coefficient at Sites 1174 or C0007. This is consistent with the observation that the clay contents of the décollement and fault zones within the frontal thrust region are similar to those of the wall rock. However, due to sampling limitations, it is also possible that very thin, localized zones of slip in a material with very low frictional strength could exist at all three sites, but were not sampled [e.g., Collettini et al., 2009].

Variations in friction coefficient between the three sampling locations are also broadly consistent with observed large-scale along-strike differences in the accretionary complex structure. Samples obtained from the megasplay are consistently stronger than those from both the frontal thrust zone and the décollement zone, and the samples from Site 1174 are the weakest in this study. The lower friction of the samples from Site 1174 compared to that of the frontal thrust samples from Site C0007 is consistent with the higher taper angle of the outermost accretionary wedge along the Kumano transect (∼8–10°) than along the Muroto transect (∼4°) [e.g., Davis et al., 1983; Davis and von Huene, 1987; Dahlen, 1990; Kimura et al., 2007], assuming that the frontal thrust is similar in strength and composition to the décollement along the Kumano transect (which was not sampled). However, the differences in taper angle could also be explained by lower basal shear strength along the Muroto transect due to higher pore fluid pressures there, as suggested by several previous studies [Kopf and Brown, 2003; Moore et al., 2005; Screaton et al., 2009; Skarbek and Saffer, 2009; Tobin and Saffer, 2009; Saffer, 2010], or by the influence of seamounts on the subducting plate that perturb the overriding accretionary wedge [Park et al., 1999; Bangs et al., 2006]. Taken together, our data and the results of previous analyses suggest that differences in friction coefficient play a role in driving along-strike differences in taper angle, although they are probably not the only factor.

5.2. Rate- and State-Dependent Friction and Fault Slip Behavior

In order to better quantify frictional rate dependence, we expand on previous studies [e.g., Saffer and Marone, 2003; Ikari et al., 2009a] by allowing use of a two state variable constitutive law. In contrast to friction coefficient, the velocity dependence of friction shows little dependence on mineralogic variation or position across the fault zones (Figure 8). This is consistent with previous work showing that a-b is less sensitive to changes in clay mineral content than is friction coefficient [Ikari et al., 2007]. We do, however, observe that at Site 1174, values of a-b are generally higher for samples that were tested as powdered or remolded specimens than for those tested as intact wafers. This is not observed in the samples from Sites C0004 and C0007, where burial depths are significantly shallower and the in situ temperature is lower. This may indicate that the lower values of a-b are linked to more advanced consolidation and lithification at the higher in situ pressures and temperatures at Site 1174. If true, this would support the hypothesis that lithification is one key factor in promoting slip instability [Moore and Saffer, 2001; Saffer and Marone, 2003; J. C. Moore et al., 2007].

Our observation of mostly velocity-strengthening behavior is in agreement with several previous experimental studies on clay-rich gouges [e.g., Logan and Rauenzahn, 1987; Morrow et al., 1992; Saffer and Marone, 2003; Ikari et al., 2009a; Tembe et al., 2010], and with analyses suggesting that seismic slip is not expected to occur at shallow depths [Marone and Scholz, 1988; Hyndman, 2007]. We observe a few occurrences of velocity weakening, although the magnitude is small (a-b ≥ −0.0005). Fitting velocity steps that yield the lowest values of a-b (including all instances of velocity weakening we observe) generally requires a two state variable constitutive law. This may indicate a change in underlying frictional mechanisms at the grain scale, but verification of this will require further research because the physical processes represented by the two state variable friction law remain speculative [Marone and Cox, 1994; Blanpied et al., 1998].

Analysis of the rate-dependent friction parameters obtained from constitutive modeling show that trends in the parameter b as a function of slip velocity are inversely correlated with a-b; values of a-b are lowest for sliding velocities in the range 1–3 μm/s, and values of b are highest in this range. This is consistent with previous observations suggesting that the parameter b is the primary control on the evolution of friction and its velocity dependence [e.g., Saffer and Marone, 2003; Ikari et al., 2009a]. The parameter b is interpreted to be a measure of the change in friction resulting from a change in the real area of contact at the grain scale during grain boundary sliding [e.g., Dieterich and Kilgore, 1994]. Low values of b imply that contact area is already maximized, perhaps due to the presence of soft and/or platy clay grains. In this case, and because the parameter a is always positive, a-b is more likely to be positive, leading to velocity-strengthening behavior. This process may provide an explanation for the common observations of a-b > 0 in clay-rich gouges [e.g., Morrow et al., 1992; Saffer and Marone, 2003; Ikari et al., 2009a] and low values of b that can be negative in some cases [Ikari et al., 2009a; Smith and Faulkner, 2010]. In contrast, large rates of contact area growth may lead to high values of b, and thus to reduced values of a-b. It is unclear why a minimum would occur within a specific velocity range, however this could be related to the average grain size in the gouge, which would modulate the lifetime of grain-scale contacts (i.e., Θ).

Due to the overall velocity-strengthening behavior of the shallow fault zone material, seismic slip is not expected to nucleate at the depths of the fault zones examined in this study. At slip velocities of 1–100 μm/s, we observe that all samples become increasingly velocity strengthening (Figure 9). This suggests that propagation of coseismic slip initiated deeper on the subduction megathrust will likely be decelerated by shallow clay-rich sediments as slip reaches this velocity range, making it difficult for seismic slip to reach the seafloor [Rubin, 2008]. However, recent vitrinite reflectance and geochemical studies have documented significant frictional heating within fault rocks of the shallow megasplay and frontal thrust, suggesting that coseismic (rapid) slip may in fact propagate to within a few hundred meters of the seafloor [Sakaguchi et al., 2011]. High-velocity friction experiments, including recent work on samples from the megasplay fault [Ujiie and Tsutsumi, 2010] have also demonstrated that considerable weakening occurs at slip rates in the m/s range, driven by a number of processes including frictional melting and thermal pressurization of pore fluid [Di Toro et al., 2004; Hirose and Bystricky, 2007; Brantut et al., 2008; Ujiie et al., 2009; Ferri et al., 2010].

Velocity-strengthening behavior would be enhanced in low-permeability sediments and gouge, due to undrained conditions during dilation that accompanies increasing slip rate (dilatancy hardening) [Segall and Rice, 1995; Samuelson et al., 2009]. However, thermal pressurization is also important and can dominate the transient pore pressure signal during slip once sliding velocities reach ∼1 mm/s [Segall and Rice, 2006]. Previous studies have reported low-permeability values (<1 × 10−18 m2) for clay-rich Nankai sediments from Sites C0004 and 1174, including both unsheared and sheared material [Bourlange et al., 2004; Gamage and Screaton, 2006; Ikari et al., 2009b]. This is likely to result in competition between thermal depressurization and dilatancy hardening; detailed modeling of these processes informed by laboratory measurements like those reported here will help to resolve their relative importance and their ultimate effects on fault slip and rupture propagation [Segall and Rice, 2006; Segall et al., 2010].

The velocity range over which we observe minimum values of a-b in all three fault zone settings (1–3 μm/s, or 0.1–0.3 m/d) corresponds to estimated slip rates for slow slip events in both the Nankai area and on the San Andreas fault in California [Linde et al., 1996; Ide et al., 2007]. In a general sense, we expect that of all types of slip events will propagate further in a material characterized by lower values of a-b. Within this particular velocity range, a-b reaches negative values at Site C0004 and C0007 and is near velocity neutral at Site 1174 (Figure 9). Several numerical modeling studies have shown that transitional frictional behavior (near velocity neutral) is necessary for slow slip events to arise [Shibazaki and Iio, 2003; Liu and Rice, 2005; Lowry, 2006; Shibazaki and Shimamoto, 2007]. The models used by Shibazaki and Iio [2003] and Shibazaki and Shimamoto [2007] are based on friction experiments in halite, which exhibits velocity-weakening behavior and a minimum in friction coefficient for slip velocities in the range of ∼0.1–3 μm/s [Shimamoto, 1986]. This coincides with the velocity range over which we observe a minimum in a-b for natural clay-rich fault zone materials. Other numerical modeling studies have shown that slow slip events or earthquake afterslip can occur on faults with near velocity-neutral [Rubin, 2008] or velocity-strengthening [Perfettini and Avouac, 2007; Perfettini and Ampuero, 2008] frictional behavior. The occurrence of slow slip is not considered to be solely driven by rate-dependent frictional behavior, but also may depend on high overall pore pressures [Kodaira et al., 2004; Shelly et al., 2006; Liu and Rice, 2007], pore pressure transients due to dilatancy [Rubin, 2008; Segall et al., 2010], or specific elastic stiffness conditions [Perfettini et al., 2001; Lowry, 2006]. Additionally, the majority of studies investigating slow slip focus on events that occur at the downdip (deeper) limit of the seismogenic zone rather than shallow portions.

Nevertheless, velocity-dependent friction likely plays a role in controlling slow slip occurrence, and in particular a transition from velocity-strengthening to velocity-weakening or near velocity-neutral behavior may be required. Our data show that such behavior occurs at sliding velocities similar to those for some slow slip events (∼1–3 μm/s) in materials from all of the fault zones in our study. This indicates that that slow slip events such as low frequency earthquakes [Ito and Obara, 2006] and earthquake afterslip [Thatcher, 1984; Marone et al., 1991; Perfettini and Ampuero, 2008] may be likely to occur in all shallow fault zone settings within the accretionary prism, including the décollement [e.g., Davis et al., 2006] and the frontal thrust zone.

6. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

We observe low values of friction coefficient for nearly all samples in this study; however friction coefficients measured on samples from the décollement zone at Site 1174 are consistently lower than those from the frontal thrust area at Site C0007, which in turn are consistently lower than those from the megasplay fault zone at Site C0004. Mineralogy is one control on the frictional strength of these materials, as variations in clay content are broadly consistent with the following: (1) the reduced friction of the megasplay fault zone relative to the wall rocks, (2) the absence of such a trend for the frontal thrust and décollement zones, and (3) the slight decrease in friction as a function of depth in the frontal thrust and décollement environments. The slight decreases in friction coefficient with depth at Sites C0007 and 1174 may also be linked to the development of initial compaction related fabric at burial depths > ∼400–500 mbsf. In contrast to the absolute strength, frictional rate dependence is largely independent of mineralogic composition, depth, or position across the fault zones. Values of a-b are generally positive, indicating velocity-strengthening behavior. This precludes earthquake nucleation, and may indicate resistance to seismic slip propagation. However, a minimum in frictional stability is observed in the range 1–3 μm/s at all three sites, suggesting that shallow fault settings in accretionary prisms are potentially susceptible to slow slip events or afterslip. This observed minimum appears to be primarily controlled by the rate-dependent friction parameter b.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

This work was supported by NSF awards EAR-0746192, EAR-0752114, and OCE-0648331. We thank Matt Knuth for help with samples and running experiments. We also thank Kohtaro Ujiie, Editor Thorsten Becker, an anonymous AE, and an anonymous reviewer for helpful suggestions that improved this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Geologic Setting
  5. 3. Experimental Methods
  6. 4. Results
  7. 5. Discussion
  8. 6. Conclusions
  9. Acknowledgments
  10. References
  11. Supporting Information

Auxiliary material for this article contains tables that describe friction experiments performed on sediments sampled from fault zones in the Nankai Trough, collected during IODP Expedition 316 and ODP Leg 190.

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ggge1916-sup-0001-readme.txtplain text document4Kreadme.txt
ggge1916-sup-0002-ts01.txtplain text document2KTable S1. Experiment Parameters.
ggge1916-sup-0003-ts02.txtplain text document17KTable S2. Constitutive Friction Parameters: Sites C0004, C0007, and 1174.

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