Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand

    Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand Matt J. Ikari, Brett M. Carpenter, Achim J. Kopf, Chris Marone PII: DOI: Reference:

S0040-1951(14)00242-X doi: 10.1016/j.tecto.2014.05.005 TECTO 126306

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

22 January 2014 26 April 2014 2 May 2014

Please cite this article as: Ikari, Matt J., Carpenter, Brett M., Kopf, Achim J., Marone, Chris, Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand, Tectonophysics (2014), doi: 10.1016/j.tecto.2014.05.005

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ACCEPTED MANUSCRIPT Frictional strength, rate-dependence, and healing in DFDP-1 borehole

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samples from the Alpine Fault, New Zealand

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Matt J. Ikari1*, Brett M. Carpenter2,3, Achim J. Kopf1, Chris Marone3

MARUM, Center for Marine Environmental Sciences, University of Bremen, Germany

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Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

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Department of Geosciences, The Pennsylvania State University, USA

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*Corresponding author: [email protected]

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Abstract

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The Alpine Fault in southern New Zealand is a major plate-boundary fault zone that,

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according to various lines of evidence, may be nearing the end of its seismic cycle and approaching earthquake failure. In order to better characterize this fault and obtain a better understanding of its seismic potential, two pilot boreholes have been completed as part of a larger drilling project. Samples representative of the major lithologic subdivisions of the fault zone at shallow (~100 m) depths were recovered and investigated in laboratory friction experiments. We show here that materials from within and very near the principal slip zone (PSZ) tend to exhibit velocity strengthening frictional behavior, and restrengthen (heal) rapidly compared to samples of the wall rock recovered near the PSZ. Fluid saturation causes the PSZ to be noticeably weaker than the surrounding cataclasites (µ = 0.45), and eliminates the velocityweakening behavior in these cataclasites that is observed in dry tests. Our results indicate that the PSZ has the ability to regain its strength in a manner required for repeated rupture, but its ability to nucleate a seismic event is limited by its velocity-strengthening nature. We suggest

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ACCEPTED MANUSCRIPT that seismogenic behavior at depth would require alteration of the PSZ material to become velocity weakening, or that earthquake nucleation occurs either within the wall rock or at the

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interface between the wall rock and the PSZ. Coseismic ruptures which propagate to the surface

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have been inferred for previous earthquakes on the Alpine Fault, and may be facilitated by the slightly weaker PSZ material and/or high rates of healing in the PSZ which support fault locking

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and eventual stress drops.

1. Introduction

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Illuminating the underlying mechanisms controlling the seismogenic potential of major plate boundary faults, especially those near populated areas, is a primary goal of geoscience.

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Seismology, numerical modeling, and laboratory experiments on synthetic fault materials have provided great insight, but validation of these techniques requires observations and

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measurements of natural fault zone and wall rock material. To this end, several high-profile scientific drilling projects are currently focused on major continental faults capable of largemagnitude earthquakes, such as the San Andreas Fault Observatory at Depth (SAFOD) (e.g. Zoback et al., 2011), and the Taiwan Chelungpu-fault Drilling Project (TCDP) (e.g. Ma et al., 2006). The Deep Fault Drilling Project (DFDP) is designed to investigate the Alpine Fault in New Zealand and has a similar concept to these projects, but aims to incorporate a larger suite of boreholes penetrating the fault at different depths and include a more comprehensive monitoring array (Townend et al., 2009). The seismic behavior of major faults depends on their frictional properties, specifically frictional strength and how this strength changes as a function of slip history (memory or state effects) and slip velocity (friction velocity dependence). Here, we present results of laboratory

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ACCEPTED MANUSCRIPT friction measurements, including frictional strength, velocity-dependent friction, and frictional healing, using samples recovered from two pilot holes of a multi-phase scientific drilling project

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on the Alpine Fault. The data we present here characterize the frictional properties of the active

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fault zone and surrounding wall rock in support of future drilling, and represent a link with earlier studies testing outcrop samples from exhumed portions of the Alpine Fault in the vicinity

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of the drill site (Boulton et al., 2012a) and ~100 km to the south (Barth et al., 2013), as well as

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recent studies using borehole samples (Sutherland et al., 2012; Boulton et al., 2014; Carpenter et

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al., 2014).

2. Geologic Setting

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2.1 The Alpine Fault, South Island, New Zealand The Alpine Fault is a major, dextral-reverse fault located along the West coast of New It forms part of the boundary between the Pacific Plate and the

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Zealand’s South Island.

Australian Plate and links two subduction zones: the top-to-northeast-verging Hikurangi subduction zone to the northeast, and the top-to-westward-verging Puysegur Trench to the southwest (Figure 1).

The fault has accumulated ~400-500 km of cumulative offset at a

displacement rate of ~20-35 mm/yr, as determined by offset of geologic markers and radiocarbon dating (Cooper and Norris, 1994; Norris and Cooper, 2000; Sutherland et al., 2006). Geodetic measurements in the central South Island indicate that the locked zone of the Alpine Fault (> 5-8 km depth) is being loaded from below by strain in the lower crust representing 50-70% of the plate convergence rate (Beavan et al., 1999; Norris and Cooper, 2000).

Furthermore, no

measurable historic creep has been found to occur at the surface (Beavan et al., 1999), which indicates that much of the strain release occurs during earthquakes. A major earthquake (Mw >

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ACCEPTED MANUSCRIPT 7.0) has not occurred on the Alpine Fault in the last ~300 years, but such events are inferred to have occurred over the past 8000 years with a recurrence of ~260-400 years (Bull, 1996; Evidence of earthquake slip includes pseudotachylytes observed in

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Berryman et al., 2012).

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surface outcrops of hanging wall mylonites (Toy et al., 2011) and buried fault scarps offset by previous surface ruptures (De Pascale and Langridge, 2012). Observations of near-surface

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seismic rupture combined with indications that the Alpine Fault appears to be in the late stages of

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the seismic cycle suggest that a hazardous, large-magnitude earthquake may be imminent (Sutherland et al., 2007, 2012; Townend et al., 2009, 2013).

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2.2 Deep Fault Drilling Project and Sample Description

The Alpine Fault was chosen for scientific drilling based on several criteria, including

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well-exposed outcrops, exhumation of representative lithologies from depth, well-constrained Quaternary slip rates, the presence of a seismic monitoring network, and a fault dip of ~45° in

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the selected drilling location that allows penetration by simple vertical boreholes (Townend et al., 2009). Drilling on the central portion of the Alpine fault commenced in 2011 as part of the Deep Fault Drilling Project (DFDP). Two pilot boreholes were drilled, the first (DFDP-1A) to 96 m and the second (DFDP-1B) to 151 m. The boreholes are located ~ 80 m apart and both successfully crossed the fault zone core. The hanging wall consists of five major lithologies; a dark gray highly-fractured mylonite, a laminated brown-green-black ultramylonite, foliated and unfoliated greenish-gray cataclasites, and gouges.

The fault zone proper is identified in

recovered drill cores as a zone composed of cm-scale layers of fine-grained, clay-rich, uncemented ultracataclasite interpreted to be a principal slip zone (PSZ) (Sutherland et al., 2012; Townend et al., 2013). In borehole DFDP-1A, this zone separates hanging wall cataclasites from Quaternary gravel and is noticeably thinner (~3 cm) than the main PSZ identified in DFDP-1B

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ACCEPTED MANUSCRIPT (~20 cm). In DFDP-1B, the main PSZ is located at a depth of 128 m as measured in the recovered core. This PSZ separates hanging wall and footwall cataclasites; a second PSZ is

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located ~17 m below; however, which of these zones was most recently active is unknown. The

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footwall at the DFDP-1B borehole consists of granitic cataclasites underlain by gneisses. We tested 8 samples from DFDP-1A: three mylonites and four cataclasites from the hanging wall,

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and one sample of footwall gravel. The character of the cataclasite samples changes with

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proximity to the PSZ, with the cataclasite sample from 89.7 m depth having significantly larger proportions of clay minerals than cataclasites at greater distances from the PSZ. We were unable

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to obtain the PSZ from DFDP-1A, however we tested a sample of the upper PSZ from DFDP1B. Mineralogical analysis indicates that our sample of the PSZ from DFDP-1B is composed of

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25% quartz. 4% orthoclase, 25% albite, 9% calcite, 12% illite-muscovite, 18% smectite, 6% kaolinite (Boulton et al., 2012b; Schleicher et al., 2014). Compositionally, this gouge is similar

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to a different sample of the DFDP-1B PSZ studied by Boulton et al. (2014) in terms of total phyllosilicates (~55-60%). However, our PSZ sample exhibits differences in clay mineralogy, including elevated illite-muscovite (+7%) and reduced smectite (-8%), the presence of kaolinite and absence of chlorite. This highlights subtle, but important variations in composition that can occur within the PSZ despite its 10’s of cm thickness. 3. Experimental Methods We tested our samples as powdered gouges, which were dried for ~24 hours at ~40°C in order to prevent thermally-induced clay mineral transformation, then crushed in a disk mill apparatus and sieved to a grain size of <125 µm. Prior to shearing, layer thickness of the gouge was ~1.5-2 mm under normal load (Table 1). We conducted two types of frictional shearing experiments in the double-direct shear apparatus at Penn State University: constant velocity

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ACCEPTED MANUSCRIPT experiments over a range of normal stresses, and constant normal stress experiments employing both velocity-stepping and slide-hold-slide tests. Most of these experiments were conducted at

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room temperature and humidity (sub-saturated conditions). In the constant velocity tests, we

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sheared the samples at 11 µm/s, while increasing the normal stress in steps within the range 10100 MPa. For velocity-stepping tests, we sheared the samples at 30 MPa (effective) normal

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stress, with an initial background velocity of 11 µm/s to an approximate engineering shear strain

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γ ≥ 5 in order to reach steady state shear strength, and to allow microstructure to fully develop (Haines et al., 2009, 2013). We then increased the shearing velocity in 3-fold increments from 1-

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300 µm/s in order to quantify the velocity-dependence of friction. Following this test, we conducted slide-hold-slide tests, in which the loading velocity was alternated between 10 µm/s

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and zero. We studied hold durations from 3-1000 s in order to measure time- and creepdependent frictional healing (e.g. Dieterich, 1972; Beeler et al., 1994; Marone, 1998a,b).

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We conducted a subset of velocity-stepping tests under fluid-saturated conditions with controlled pore pressure. For these experiments we used a true-triaxial pressure vessel, within which a jacketed sample assembly can be subjected to controlled normal stress, shear stress, confining pressure, and pore fluid pressure (Ikari et al., 2009, Samuelson et al., 2009; Carpenter et al., 2012). Effective normal stress, which was maintained at 30 MPa, represents the combined effects of confining pressure (6 MPa), externally applied normal load, and pore pressure (5 MPa). Previous experiments under similar conditions have shown that the frictional properties of clay-rich gouge do not vary greatly at effective normal stresses from 25 MPa to ~60 MPa (Ikari et al., 2009), which suggests our results are also applicable at higher pressure conditions. We measured the steady-state shear stress () at a constant velocity of 11 µm/s prior to the initiation of velocity steps, in order to calculate the coefficient of sliding friction (µ) as:

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ACCEPTED MANUSCRIPT 

 n'

(1)

disaggregated samples (Handin, 1969).

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where σn’ is the effective normal stress, and assuming that cohesion is negligible in our From velocity-step tests, we quantify the rate

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 ss ln V   Vo 

(2)

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a b 

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dependence of friction with the parameter a-b:

Where ∆µss is the change in steady state coefficient of friction upon an instantaneous change in

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sliding velocity from Vo to V (e.g. Marone, 1998b) (Figure 2). A material exhibiting a positive value of a-b is considered to be velocity strengthening, and will tend to slide stably and resist

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propagation of seismic rupture. A material with negative a-b is termed velocity weakening, a

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(Scholz, 2002).

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property that is considered a prerequisite for frictional instability and earthquake nucleation

Using an inverse modeling technique with an iterative least-squares method, we calculated values of rate-dependent friction constitutive parameters following Dieterich’s (1979, 1981) friction law:

V  V   V    b1 ln  o 1   b2 ln  o 2     D  Vo   Dc1   c2

   o  a ln 

   

di Vi  1 , i  1,2 dt Dc i

(3)

(4)

where a, b1 and b2 are empirically derived constants (unitless) that scale the direct effect and two evolution effects, respectively. 1 and 2 are termed the state variables, interpreted to be the average lifetime of grain-scale asperities (Dieterich and Kilgore, 1994) and/or the critical granular porosity during shear (e.g., Marone and Kilgore, 1993). The critical slip distances Dc1

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ACCEPTED MANUSCRIPT and Dc2 are the displacements required to re-establish a steady-state real area of contact following the velocity perturbation (Figure 2c).

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Employing two state variables (1 and 2) provides a better fit to our data from velocity

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step tests compared to a single state variable model (e.g., Ikari et al., 2009), however the physical

al., 1998).

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mechanisms represented by the number of state variables are not well known (e.g. Blanpied et In some cases, distinction between the two evolution effects is minimal or

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unconstrained and those cases are fit using a single state variable. In such cases, Equations 3 and 4 are simplified by setting b2 = 0. In our use of the parameter a-b, we consider b = b1 + b2 to

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account for the possibility of using either 1 or 2 state variables. Recovery of friction constitutive

d  K(Vlp  V ) dt

(5)

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account for this via the relation:

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parameters from velocity-step tests requires knowledge of the testing apparatus stiffness. We

Where K is the apparatus stiffness normalized by normal stress (K = ~3x10-3 µm-1 at 25 MPa), Vlp is the load point velocity, and V is the true slip velocity in the sample (Reinen and Weeks, 1993; Saffer and Marone, 2003; Ikari et al., 2009). For slide-hold-slide tests (Figure 2d), the difference in friction coefficient between the pre-hold steady-state value and the maximum (peak) friction upon re-initiation of shear is quantified as Δµ . Because this quantity depends logarithmically on the hold time, we calculate rates of frictional healing β for each sample:    log t

(6)

where t is the duration of the hold period (e.g. Dieterich, 1972; Marone, 1998b).

4. Results

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ACCEPTED MANUSCRIPT Measured shear strength (τ) varies approximately linearly with increasing normal stress for all samples (Figure 3). Friction coefficients for dry samples range from 0.51 to 0.68 with

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most values lying between 0.60 and 0.65 (Figures 3 and 4). Lower values for these samples tend

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to be observed for the mylonites. Fluid-saturated samples exhibit µ = 0.45-0.59, consistently weaker than samples tested at room humidity. Weakening due to fluid saturation is most

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pronounced for the PSZ sample, which is not weaker than the adjacent cataclasites in the dry

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case (Figure 4a).

The velocity-dependence of friction, quantified by the parameter a-b, ranges from 0 to -

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0.002 for the majority of the samples, indicating velocity-weakening behavior (Figure 4). Exceptions include the PSZ and the cataclasite sample closest to the 1A fault zone, which exhibit The PSZ is clearly much more velocity

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exclusively velocity-strengthening behavior.

strengthening (a-b = 0.003-0.006) than the cataclasites (a-b = 0-0.001). Fluid saturation has a

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strong effect on velocity dependence, significantly increasing a-b values for two cataclasite samples and the footwall gravel (Figures 4, 5). Although we only tested a small number of the total samples under fluid-saturated conditions, all those tested yielded positive values of a-b. In the case of the cataclasite from 85.4 m and the gravel, the presence of fluid resulted in a switch from velocity-weakening to velocity-strengthening behavior. However, the friction velocity dependence of the PSZ sample is essentially unaffected by the presence of fluid (Figure 5). As a function of increasing velocity, a-b tends to decrease if the samples are velocity weakening overall, the reverse is true for velocity-strengthening samples. Interestingly, when sheared under fluid-saturated conditions the cataclasite from 89.7 m becomes highly sensitive to increasing velocity, exhibiting a strong positive relationship (Figure 5). Rates of frictional healing are narrowly constrained within β = 0.005-0.007 decade-1 for both room-humidity and fluid-

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ACCEPTED MANUSCRIPT saturated samples. The exception is the PSZ, which exhibits elevated healing rates (β = 0.009

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dry, 0.010 wet) compared to the rest of the sample suite.

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5. Discussion 5.1. Observed Frictional Behavior

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Our experimental results show clear differences in the frictional behavior of the PSZ

around the Alpine Fault.

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compared to the mylonites, cataclasites, and gravel that make up the hanging wall and footwall Under fluid-saturated conditions the PSZ is weaker than the

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surrounding rock, and under dry conditions the wall rock exhibits velocity-weakening behavior. Our results are consistent with recent measurements which show µ = 0.49 for the DFDP-1B PSZ

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under similar conditions to our water-saturated test (Boulton et al., 2014), and also previous work on surface samples of the Alpine Fault recovered near Gaunt Creek, which showed that the

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fault gouge is both significantly weaker than surrounding cataclasites and exhibits velocitystrengthening friction under fluid-saturated conditions (Boulton et al., 2012a, Barth et al., 2013). Because we use disaggregated gouge powders, the differences in observed frictional behavior can be attributed solely to compositional variations, rather than rock fabric (e.g., Collettini et al., 2009; Ikari et al., 2011). Compositional analyses of the DFDP-1B PSZ as well as outcrop samples indicate that illite and chlorite are common in the Alpine Fault, but that the PSZ has a significant smectite content not found in the wall rocks (Boulton et al., 2012b, 2014; Schleicher et al., 2014). Increased lower overall strength are consistent with elevated clay mineral content in general and smectite in particular, especially under the influence of fluids (e.g. Saffer and Marone, 2003; Ikari et al., 2007).

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ACCEPTED MANUSCRIPT The PSZ exhibits significantly higher rates of frictional healing compared to the cataclasites and mylonites. The healing rates we observe for the PSZ material are notably higher

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than previously measured values under similar testing conditions on samples from other major

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fault zones (Figure 6). A smectite (saponite)-rich sample of the active deforming zone of the San Andreas Fault from the SAFOD borehole exhibited negligible frictional healing (Carpenter et al.,

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2012), while β < 0.002 decade-1 was reported for phyllosilicate-rich (~50%) samples from the

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Zuccale Fault in central Italy (Tesei et al., 2012), and 0.004 < β < 0.008 decade-1 was reported for samples from within a major splay fault and the frontal thrust zone at the Nankai Trough

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subduction zone, southwest Japan (Ikari et al., 2012). Healing rates for the PSZ sample are more consistent with β = 0.008-0.009 decade-1 observed in quartz sand (Marone, 1998a). Because

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stress drops occur during earthquakes, elevated fault healing rates facilitate the strength recovery

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necessary for repeated earthquake rupture. Under fluid-saturated conditions, our observed β =

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0.010 for the PSZ represents a strength increase of approximately 2% decade-1, which matches healing rates of 1-3% decade-1 inferred from small magnitude (Mw = ~1.5) repeating earthquakes on the Calaveras Fault, a San Andreas Fault branch located ~150 km northwest of the SAFOD site (Vidale, 1994; Marone et al., 1995). 5.2. Implications for the Slip Behavior of the Alpine Fault The velocity-strengthening behavior we observe for the PSZ, especially when fluid saturated, indicates that it is unlikely to generate seismicity if similar material exists at depth on the Alpine Fault (Barth et al., 2013). For earthquake nucleation at seismogenic depth on the Alpine Fault, this means that either: (1) the PSZ does not exist at several km depth and coseismic slip initiates in either mylonites or cataclasites, (2) earthquake nucleation occurs on the boundary of the fault core where the PSZ material is in contact with either mylonites or cataclasites, or (3)

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ACCEPTED MANUSCRIPT that the PSZ material has been altered by elevated pressure and temperature at several km depth to become frictionally unstable, as has been observed for clay (illite)-rich material (e.g. den

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Hartog and Spiers, 2013) and other gouge samples from DFDP boreholes (Boulton et al., 2014).

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We favor the second and/or third hypotheses (Figure 7) because fluid-rock interaction that results in significant alteration of the mineral assemblage is common in major fault zones (Chester et al.,

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1993; Wintsch et al., 1995; Vrolijk and van der Pluijm, 1999), including the Alpine Fault (Warr

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and Cox, 2001). Therefore we expect that the core of the Alpine Fault at seismogenic depths would have a composition distinct from the mylonite/cataclasite protolith, which may be similar

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to the PSZ from Borehole 1B (i.e., having higher clay content).

Furthermore, rupture

propagation is expected to be influenced by competency contrasts in the Alpine Fault as

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demonstrated by Townend et al. (2013), who used P-wave velocity and fracture density measurements from wireline logging data in both DFDP boreholes to show that the hanging wall

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is stiffer than the footwall. Additional laboratory determinations of elastic moduli for material in and around the PSZ, which suggest that the fault zone material is significantly more compliant and thus the Alpine Fault exhibits a strong elastic contrast between the fault zone and wall rock (Carpenter et al., 2014). This evidence is supports our second hypothesis, although we expect the rupture to be controlled by the wall rock – gouge boundary considering a fault core of finite width rather than an infinitely thin surface (e.g., Marone et al., 2009). Earthquake slip on plate boundary faults is expected to occur within narrow zones that are likely of mm- to cm-scale thickness and therefore the entire width of the PSZ may not necessarily deform coseismically (Sibson, 2003; Rowe et al., 2013). Furthermore, friction experiments simulating coseismic (m/s) slip rates have shown that shear deformation tends to localize at the boundary between the gouge and the wall rock (e.g. Ujiie and Tsutsumi, 2010; De Paola et al., 2011).

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ACCEPTED MANUSCRIPT Elevated healing rates are observed for PSZ material, which is favorable for interseismic strength recovery that is necessary for repeating earthquakes. However, the velocity-

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strengthening nature of the PSZ indicates that negative stress drops, or strength increases which

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hinder coseismic slip, should be expected which supports the inference that seismicity originates either on the PSZ boundary or within an altered PSZ. Although it is difficult to project how

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healing rates would evolve as seismogenic depths are approached, slide-hold-slide tests

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simulating hydrothermal conditions generally show that greater healing is to be expected (e.g. Karner et al., 1997; Tenthorey et al., 2003; Yasuhara et al., 2005). Thus, the high healing rates

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we observe in the PSZ sample are favorable for repeated stress drops in large earthquakes. High rates of healing in the PSZ also suggest that the fault has the ability to become

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locked or partially locked at the near surface and accumulate strain energy, which is supported by the inferred absence of recent aseismic fault creep from GPS measurements (Beavan et al.,

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1999). Although evidence of fluid-rock interactions are prevalent in the DFDP-1 core samples (Sutherland et al., 2012), our dry tests of mylonite and cataclasite samples recovered from very shallow depths (< 100 m) indicate that velocity-weakening behavior could occur under certain (unusual) circumstances. This may suggest that the Alpine Fault could be frictionally unstable throughout the brittle crust and may therefore lack the upper aseismic zone within a few km of the surface that is commonly observed on other faults (e.g. Marone and Scholz, 1988). This is consistent with seismological observations show that earthquake hypocenters are distributed throughout the upper 12 km on the Alpine Fault, including up to the surface (Leitner et al., 2001). In the case of an earthquake which nucleates at depth and propagates upward, coseismic rupture and slip may therefore be likely to propagate to the surface, most likely at the boundary between the weaker PSZ and the velocity-weakening mylonites or cataclasites at shallow depths

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ACCEPTED MANUSCRIPT (Figure 7). This picture is complicated somewhat by the velocity-strengthening behavior of the PSZ itself, and because although the PSZ and adjacent cataclasites may be moderately weak

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under fluid-saturated conditions (µ = 0.45-0.53) compared common crustal rocks (e.g. Byerlee,

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1978) they are stronger than gouges with very high phyllosilicate contents (≥ 50%), which are very weak at low velocity (e.g. Brown et al., 2003; Ikari et al., 2009; Carpenter et al., 2011,

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2012) and achieve dynamic weakening quickly (Faulkner et al., 2011). However, field studies

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suggest that surface-rupturing events occur regularly on the Alpine Fault with an estimated 330 year recurrence (Berryman et al., 2012). Given the potential for slip instability that we document

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and the elapsed time since major events in 1717 (Sutherland et al., 2007; De Pascale and Langridge, 2012) and 1748 (Bull, 1996), it is reasonable to expect an increased likelihood of

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6. Conclusions

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such an event in the next 10’s of years.

Based on laboratory friction measurements conducted on samples collected from two pilot holes drilled through the Alpine Fault during the initial phase of DFDP drilling, we find that the PSZ is velocity strengthening, and displays increased rates of frictional healing compared to wall rock cataclasites and mylonites under both dry and wet conditions. Further more, the PSZ is significantly weaker than the surrounding rock when fluid saturated. These characteristics are likely related to differences in mineralogic composition from the wall rock, such as increased smectite content. Velocity-strengthening behavior of the PSZ material suggests that it may be unable to host earthquake nucleation in its recovered form. Thus, earthquake nucleation on the Alpine Fault likely occurs either within the velocity-weakening cataclasites or mylonites that bound the PSZ, at the boundary between these rocks and the PSZ, or in a highly altered form of

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ACCEPTED MANUSCRIPT the shallow PSZ at greater depth. The slight weakness of the PSZ material relative to wall rock at shallow depth could, however, facilitate localization of coseismic slip within this zone instead

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of the adjacent, stronger cataclasites. Elevated healing rates indicate that shear stress on the

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Alpine Fault would be easily recovered in the interseismic period following coseismic stress drops (which is necessary for repeating earthquakes) and support fault locking, even at shallow

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depths. In conjunction with field, seismologic, and geodetic observations our results indicate

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that the Alpine Fault displays a tendency for slip instability that likely extends to the surface.

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Acknowledgements

We thank the DFDP-Alpine Fault team lead by Rupert Sutherland, John Townend and

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Virginia Toy, and especially thank Virginia Toy for helpful discussions. We also thank Steve Swavely and Marco Scuderi for assistance in the laboratory. Heather Savage and Patrick Fulton

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are gratefully acknowledged for providing helpful and constructive reviews. This work was supported by National Science Foundation grants OCE-0648331, EAR-0746192, and EAR0950517 to C.M. and Deutsche Forschungsgemeinschaft grant DFG KO2108/14-1 to A.J.K.

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ACCEPTED MANUSCRIPT Zoback, M., Hickman, S., Ellsworth, W., SAFOD Science Team, 2011. Scientific drilling into the San Andreas fault zone – An overview of SAFOD’s first five years. Scientific

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Figure Captions

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Figure 1: (A) Map of the New Zealand area showing the location of DFDP drilling. (B)

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Geologic cross-section showing DFDP-1A and DFDP-1B boreholes. Yellow circles indicate the depth location of samples tested in this study. Figure modified from Sutherland et al. (2012).

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Figure 2: (A) Example of experimental data from a load-stepping experiment at constant velocity. Inset shows double-direct shear configuration. (B) Example of experimental data from

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Example of friction data from an individual velocity step (black) overlain by an inverse model (blue). (D) Example of friction data from an individual hold, showing the frictional healing Δµ and frictional relaxation Δµc.

Figure 3: Steady-state shear strength as a function of normal stress under room temperature and humidity for: (A) all samples, and (B) the DFDP-1B PSZ with a linear fit to the data. Figure 4: (A) Coefficient of friction, and (B) friction rate parameter a-b as a function of depth in the DFPD-1A borehole under both room humidity and fluid-saturated conditions. The PSZ in DFDP-1A (91 m) is represented by data from the DFDP-1B sample. Figure 5: Friction rate parameter a-b as a function of upstep load point velocity under (A) room humidity, and (B) fluid-saturated conditions.

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data from the DFDP-1B sample. For comparison, shaded areas show healing values from the

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San Andreas Fault (SAF; Carpenter et al., 2012), Zuccale Fault (ZF; Tesei et al., 2012), Nankai Trough (NT; Ikari et al., 2012), and quartz sand (QS; Marone, 1998b).

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Figure 7: Schematic illustration of possible paths of earthquake rupture nucleating at

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seismogenic depths. Stars indicate nucleation either within the PSZ or at the PSZ/cataclasite boundary at depth. Left inset shows image of the core sample containing the PSZ (outlined in

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Table 1: Sample and Experiment Details Test Type

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Constant V Constant V Constant V Constant V Constant V Constant V Constant V Constant V Constant V VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS VS + SHS

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Shearing Velocity (mm/s) 11 11 11 11 11 11 11 11 11 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300 1-300

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67.5 73.0 79.4 83.7 85.4 87.5 89.7 95.7 127.0* 67.5 73.0 79.4 83.7 85.4 87.5 89.7 95.7 127.0* 85.4 89.7 95.7 127.0

(Effective) Normal Stress (MPa) 10-100 10-100 10-100 10-100 10-100 10-100 10-100 10-100 10-100 30 30 30 30 30 30 30 30 30 30 30 30 30

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Mylonite Mylonite Mylonite Cataclasite Protocataclasite Cataclasite Cataclasite Gravel Principal Slip Zone Mylonite Mylonite Mylonite Cataclasite Protocataclasite Cataclasite Cataclasite Gravel Principal Slip Zone Protocataclasite Cataclasite Gravel Principal Slip Zone

Depth (m)

Condition

Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Room Dry Water Saturated Water Saturated Water Saturated Water Saturated

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p3529 p3513 p3530 p3512 p3511 p3559 p3515 p3509 p3671 p3558 p3520 p3557 p3518 p3519 p3556 p3517 p3516 p3672 p3776 p3782 p3781 p3783

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ACCEPTED MANUSCRIPT Highlights: “Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand (Matt J. Ikari, Brett M. Carpenter, Achim J. Kopf, Chris Marone) We present friction measurements on Alpine Fault, NZ samples from DFDP-1 boreholes

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Frictional healing in the fault zone is favorable for repeating earthquakes

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Velocity-strengthening in the fault zone is unfavorable for earthquake nucleation

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Earthquakes from depth likely propagate along the fault zone/wall rock boundary

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