Frictional Strength, Rate-Dependence, and Healing in DFDP-1 Borehole Samples from the Alpine Fault, New Zealand

TECTO-126306; No of Pages 8 Tectonophysics xxx (2014) xxx–xxx

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

Matt J. Ikari a,⁎, Brett M. Carpenter b,c, Achim J. Kopf a, Chris Marone c a MARUM, Center for Marine Environmental Sciences, University of Bremen, Germany b Istituto Nazionale di Geoﬁsica e Vulcanologia, Rome, Italy c Department of Geosciences, The Pennsylvania State University, USA article info abstract

Article history: The Alpine Fault in southern New Zealand is a major plate-boundary fault zone that, according to various lines of Received 22 January 2014 evidence, may be nearing the end of its seismic cycle and approaching earthquake failure. In order to better char- Received in revised form 26 April 2014 acterize this fault and obtain a better understanding of its seismic potential, two pilot boreholes have been com- Accepted 2 May 2014 pleted as part of a larger drilling project. Samples representative of the major lithologic subdivisions of the fault Available online xxxx 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 Keywords: Fault frictional behavior, and restrengthen (heal) rapidly compared to samples of the wall rock recovered near the PSZ. Friction Fluid saturation causes the PSZ to be noticeably weaker than the surrounding cataclasites (μ = 0.45), and elim- Earthquake inates the velocity-weakening behavior in these cataclasites that is observed in dry tests. Our results indicate that Seismicity the PSZ has the ability to regain its strength in a manner required for repeated rupture, but its ability to nucleate a Alpine Fault seismic event is limited by its velocity-strengthening nature. We suggest that seismogenic behavior at depth ﬁ Scienti c drilling would require alteration of the PSZ material to become velocity weakening, or that earthquake nucleation occurs either within the wall rock or at the interface between the wall rock and the PSZ. Coseismic ruptures which propagate to the surface 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 and eventual stress drops. © 2014 Elsevier B.V. All rights reserved.

1. Introduction as a function of slip history (memory or state effects) and slip velocity (friction velocity dependence). Here, we present results of laboratory Illuminating the underlying mechanisms controlling the seismogenic friction measurements, including frictional strength, velocity-dependent potential of major plate boundary faults, especially those near populated friction, and frictional healing, using samples recovered from two pilot areas, is a primary goal of geoscience. Seismology, numerical modeling, holes of a multi-phase scientific drilling project on the Alpine Fault. The and laboratory experiments on synthetic fault materials have provided data we present here characterize the frictional properties of the active great insight, but validation of these techniques requires observations fault zone and surrounding wall rock in support of future drilling, and rep- and measurements of natural fault zone and wall rock material. To resent a link with earlier studies testing outcrop samples from exhumed this end, several high-profile scientific drilling projects are currently portions of the Alpine Fault in the vicinity of the drill site (Boulton et al., focused on major continental faults capable of large-magnitude earth- 2012a) and ~100 km to the south (Barth et al., 2013), as well as recent quakes, such as the San Andreas Fault Observatory at Depth (SAFOD) studies using borehole samples (Sutherland et al., 2012; Boulton et al., (e.g. Zoback et al., 2011), and the Taiwan Chelungpu-fault Drilling Pro- 2014; Carpenter et al., 2014). ject (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 2. Geologic setting similar concept to these projects, but aims to incorporate a larger suite of boreholes penetrating the fault at different depths and include a 2.1. The Alpine Fault, South Island, New Zealand more comprehensive monitoring array (Townend et al., 2009). The seismic behavior of major faults depends on their frictional The Alpine Fault is a major, dextral-reverse fault located along the properties, specifically frictional strength and how this strength changes West coast of New Zealand's South Island. It forms part of the boundary between the Pacific Plate and the Australian Plate and links two subduc- ⁎ Corresponding author. tion zones: the top-to-northeast-verging Hikurangi subduction zone to E-mail address: [email protected] (M.J. Ikari). the northeast, and the top-to-westward-verging Puysegur Trench to the

Please cite this article as: Ikari, M.J., et al., Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand, Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.05.005 2 M.J. Ikari et al. / Tectonophysics xxx (2014) xxx–xxx southwest (Fig. 1). The fault has accumulated ~400–500 km of cumula- 2.2. Deep Fault Drilling Project and sample description tive offset at a displacement rate of ~20–35 mm/yr, as determined by offset of geologic markers and radiocarbon dating (Cooper and Norris, The Alpine Fault was chosen for scientific drilling based on several 1994; Norris and Cooper, 2000; Sutherland et al., 2006). Geodetic criteria, including well-exposed outcrops, exhumation of representative measurements in the central South Island indicate that the locked lithologies from depth, well-constrained Quaternary slip rates, the zone of the Alpine Fault (N5–8 km depth) is being loaded from below presence of a seismic monitoring network, and a fault dip of ~45° in by strain in the lower crust representing 50–70% of the plate conver- the selected drilling location that allows penetration by simple vertical gence rate (Beavan et al., 1999; Norris and Cooper, 2000). Furthermore, boreholes (Townend et al., 2009). Drilling on the central portion of the no measurable historic creep has been found to occur at the surface Alpine Fault commenced in 2011 as part of the Deep Fault Drilling (Beavan et al., 1999), which indicates that much of the strain release Project (DFDP). Two pilot boreholes were drilled, the first (DFDP-1A) occurs during earthquakes. A major earthquake (Mw N 7.0) has not to 96 m and the second (DFDP-1B) to 151 m. The boreholes are located occurred on the Alpine Fault in the last ~300 years, but such events ~ 80 m apart and both successfully crossed the fault zone core. are inferred to have occurred over the past 8000 years with a recurrence The hanging wall consists of five major lithologies; a dark gray highly- of ~260–400 years (Bull, 1996; Berryman et al., 2012). Evidence fractured mylonite, a laminated brown-green-black ultramylonite, of earthquake slip includes pseudotachylytes observed in surface foliated and unfoliated greenish-gray cataclasites, and gouges. The outcrops of hanging wall mylonites (Toy et al., 2011) and buried fault zone proper is identified in recovered drill cores as a zone fault scarps offset by previous surface ruptures (De Pascale and composed of cm-scale layers of fine-grained, clay-rich, uncemented Langridge, 2012). Observations of near-surface seismic rupture com- ultracataclasite interpreted to be a principal slip zone (PSZ) bined with indications that the Alpine Fault appears to be in the late (Sutherland et al., 2012; Townend et al., 2013). In borehole DFDP-1A, stages of the seismic cycle suggest that a hazardous, large-magnitude this zone separates hanging wall cataclasites from Quaternary gravel earthquake may be imminent (Sutherland et al., 2007, 2012; Townend and is noticeably thinner (~3 cm) than the main PSZ identified in et al., 2009, 2013). DFDP-1B (~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 located ~17 m below; however, which of these zones was most recently active is unknown. The footwall at the DFDP-1B borehole consists of granitic cataclasites A 170°E underlain by gneisses. We tested 8 samples from DFDP-1A: three mylonites and four cataclasites from the hanging wall, and one sample of footwall gravel. The character of the cataclasite samples changes with proximity to the PSZ, with the cataclasite sample from 89.7 m depth having significantly larger proportions of clay minerals than Australian Plate cataclasites at greater distances from the PSZ. We were unable to obtain the PSZ from DFDP-1A, however we tested a sample of the upper PSZ from DFDP-1B. Mineralogical analysis indicates that our sample of the PSZ from DFDP-1B is composed of 25% quartz. 4% orthoclase, 25% albite, 9% calcite, 12% illite–muscovite, 18% smectite, 6% kaolinite (Boulton Hikurangi Margin et al., 2012b; Schleicher et al., in review). Compositionally, this gouge Alpine Fault is similar to a different sample of the DFDP-1B PSZ studied by Boulton ~40mm/yr et al. (2014) in terms of total phyllosilicates (~55–60%). However, 25mm/yr 45°S 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, 600 km but important variations in composition that can occur within the PSZ Margin Pacific Plate despite its 10s of cm thickness. Puysegur

3. Experimental methods NW Gravel SE We tested our samples as powdered gouges, which were dried for B PSZ ~24 h at ~40 °C in order to prevent thermally-induced clay mineral transformation, then crushed in a disk mill apparatus and sieved to a

Gravel DFDP-1A grain size of b125 μm. Prior to shearing, layer thickness of the gouge

DFDP-1B Ultramylonite 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 experiments over a range of normal stresses, and constant normal stress experiments HW Ca Western Province employing both velocity-stepping and slide–hold–slide tests. Most of Granite and Gneiss taclasit these experiments were conducted at room temperature and humidity e (sub-saturated conditions). In the constant velocity tests, we sheared ~50m the samples at 11 μm/s, while increasing the normal stress in steps FW Cataclasite within the range of 10–100 MPa. For velocity-stepping tests, we sheared the samples at 30 MPa (effective) normal stress, with an initial back- ground velocity of 11 μm/s to an approximate engineering shear strain Fig. 1. (A) Map of the New Zealand area showing the location of the DFDP drilling. γ ≥ 5 in order to reach steady state shear strength, and to allow micro- (B) Geologic cross-section showing DFDP-1A and DFDP-1B boreholes. Yellow circles indi- structure to fully develop (Haines et al., 2009, 2013). We then increased cate the depth location of samples tested in this study. (For interpretation of the refer- μ ences to color in this ﬁgure legend, the reader is referred to the web version of this article.) the shearing velocity in 3-fold increments from 1 to 300 m/s in order to Figure modiﬁed from Sutherland et al. (2012). quantify the velocity-dependence of friction. Following this test, we

Table 1 Sample and experiment details.

Experiment Lithology Depth (m) (Effective) Normal stress (MPa) Shearing velocity (mm/s) Test type Condition

p3529 Mylonite 67.5 10–100 11 Constant V Room dry p3513 Mylonite 73.0 10–100 11 Constant V Room dry p3530 Mylonite 79.4 10–100 11 Constant V Room dry p3512 Cataclasite 83.7 10–100 11 Constant V Room dry p3511 Protocataclasite 85.4 10–100 11 Constant V Room dry p3559 Cataclasite 87.5 10–100 11 Constant V Room dry p3515 Cataclasite 89.7 10–100 11 Constant V Room dry p3509 Gravel 95.7 10–100 11 Constant V Room dry ⁎ p3671 Principal slip zone 127.0 10–100 11 Constant V Room dry p3558 Mylonite 67.5 30 1–300 VS + SHS Room dry p3520 Mylonite 73.0 30 1–300 VS + SHS Room dry p3557 Mylonite 79.4 30 1–300 VS + SHS Room dry p3518 Cataclasite 83.7 30 1–300 VS + SHS Room dry p3519 Protocataclasite 85.4 30 1–300 VS + SHS Room dry p3556 Cataclasite 87.5 30 1–300 VS + SHS Room dry p3517 Cataclasite 89.7 30 1–300 VS + SHS Room dry p3516 Gravel 95.7 30 1–300 VS + SHS Room dry ⁎ p3672 Principal slip zone 127.0 30 1–300 VS + SHS Room dry p3776 Protocataclasite 85.4 30 1–300 VS + SHS Water saturated p3782 Cataclasite 89.7 30 1–300 VS + SHS Water saturated p3781 Gravel 95.7 30 1–300 VS + SHS Water saturated ⁎ p3783 Principal slip zone 127.0 30 1–300 VS + SHS Water saturated

V: velocity, VS: velocity steps, SHS: slide–hold–slide. ⁎ Depth in the DFDP-1B borehole.

conducted slide–hold–slide tests, in which the loading velocity was al- Using an inverse modeling technique with an iterative least-squares ternated between 10 μm/s and zero. We studied hold durations from 3 method, we calculated values of rate-dependent friction constitutive to 1000 s in order to measure time- and creep-dependent frictional parameters following Dieterich's (1979, 1981) friction law: healing (e.g. Dieterich, 1972; Beeler et al., 1994; Marone, 1998a,b). fl Θ Θ We conducted a subset of velocity-stepping tests under uid- μ ¼ μ þ V þ V o 1 þ V o 2 ð Þ o a ln b1 ln b2 ln 3 saturated conditions with controlled pore pressure. For these experi- V o Dc1 Dc2 ments we used a true-triaxial pressure vessel, within which a jacketed sample assembly can be subjected to controlled normal stress, confining pressure, and pore fluid pressure (Ikari et al., 2009; Samuelson et al., dΘ VΘ i ¼ 1− i ; i ¼ 1; 2 ð4Þ 2009; Carpenter et al., 2012). Effective normal stress, which was main- dt Dci tained at 30 MPa, represents the combined effects of confining pressure

(6 MPa), externally applied normal load, and pore pressure (5 MPa). where a, b1 and b2 are empirically derived constants (unitless) that scale Previous experiments under similar conditions have shown that the the direct effect and two evolution effects, respectively. Θ1 and Θ2 are frictional properties of clay-rich gouge do not vary greatly at effective termed the state variables, interpreted to be the average lifetime of normal stresses from 25 MPa to ~60 MPa (Ikari et al., 2009), which sug- grain-scale asperities (Dieterich and Kilgore, 1994) and/or the critical gests our results are also applicable at higher pressure conditions. granular porosity during shear (e.g., Marone and Kilgore, 1993). The τ We measured the steady-state shear stress ( ) at a constant velocity critical slip distances Dc1 and Dc2 are the displacements required to re- of 11 μm/s prior to the initiation of velocity steps, in order to calculate establish a steady-state real area of contact following the velocity per- the coefficient of sliding friction (μ)as: turbation (Fig. 2C). Employing two state variables (Θ and Θ ) provides a better fittoour τ 1 2 μ ¼ ð Þ data from velocity step tests compared to a single state variable model σ 0 1 n (e.g., Ikari et al., 2009), however the physical mechanisms represented by the number of state variables are not well known (e.g. Blanpied σ ′ where n is the effective normal stress, and assuming that cohesion is et al., 1998). In some cases, distinction between the two evolution ef- negligible in our disaggregated samples (Handin, 1969). From fects is minimal or unconstrained and those cases are fitusingasingle velocity-step tests, we quantify the rate dependence of friction with state variable. In such cases, Eqs. (3) and (4) are simplified by setting the parameter a − b: b2 = 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. Recovery Δμ a−b ¼ .ss ð2Þ of friction constitutive parameters from velocity-step tests requires ln V knowledge of the testing apparatus stiffness. We account for this via V o the relation: where Δμ is the change in steady state coefficient of friction upon an μ ss d ¼ − ð Þ KVlp V 5 instantaneous change in sliding velocity from Vo to V (e.g. Marone, dt 1998b)(Fig. 2). A material exhibiting a positive value of a − b is considered to be velocity strengthening, and will tend to slide stably and resist where K is the apparatus stiffness normalized by normal stress (K = −3 −1 propagation of seismic rupture. A material with negative a − b is ~3×10 μm at 25 MPa), Vlp is the load point velocity, and V is the termed velocity weakening, a property that is considered a prerequisite true slip velocity in the sample (Reinen and Weeks, 1993; Saffer and for frictional instability and earthquake nucleation (Scholz, 2002). Marone, 2003; Ikari et al., 2009).

0.64 0.43 Δμ

0.62 a: 0.0126 b1: 0.0043 0.425 b2: 0.0021 μ 0.6 Coefficient of Friction Dc1: 5.2 m Coefficient of Friction Dc2: 36.4 μm a-b: 0.0063 0.58

0.42 12.4 12.6 12.8 13 13.2 -50 0 50 100 150 Load Point Displacement (mm) Time (s)

Fig. 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 a constant effective normal stress test with a velocity-stepping and slide–hold–slide sequence. In this case the test is run under fluid-saturated conditions, inset shows pressure vessel sys- tem. (C) 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. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For slide–hold–slide tests (Fig. 2D), the difference in friction coeffi- The velocity-dependence of friction, quantified by the parameter cient between the pre-hold steady-state value and the maximum a − b,rangesfrom0to−0.002 for the majority of the dry samples, in- (peak) friction upon re-initiation of shear is quantified as Δμ. Because dicating velocity-weakening behavior (Fig. 4). Exceptions include this quantity depends logarithmically on the hold time, we calculate the PSZ and the cataclasite sample closest to the 1A fault zone, which rates of frictional healing β for each sample: exhibit exclusively velocity-strengthening behavior. The PSZ is clearly much more velocity strengthening (a − b =0.003–0.006) than the Δμ ¼ β logt ð6Þ cataclasites (a − b =0–0.001). Fluid saturation has a strong effect on velocity dependence, significantly increasing a − b values for two where t is the duration of the hold period (e.g. Dieterich, 1972; Marone, cataclasite samples and the footwall gravel (Figs. 4, 5). Although we 1998b). 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 4. Results of the cataclasite from 85.4 m and the gravel, the presence of fluid re- sulted in a switch from velocity-weakening to velocity-strengthening Measured shear strength (τ) varies approximately linearly with behavior. However, the friction velocity dependence of the PSZ sample increasing normal stress for all samples (Fig. 3). Friction coefficients is essentially unaffected by the presence of fluid (Fig. 5). As a function of for dry samples range from 0.51 to 0.68 with most values lying between increasing velocity, a − b tends to decrease if the samples are velocity 0.60 and 0.65 (Figs. 3 and 4). Lower values for these samples tend to be weakening overall, the reverse is true for velocity-strengthening sam- observed for the mylonites. Fluid-saturated samples exhibit μ =0.45– ples. Interestingly, when sheared under fluid-saturated conditions the 0.59, consistently weaker than samples tested at room humidity. cataclasite from 89.7 m becomes highly sensitive to increasing velocity, Weakening due to fluid saturation is most pronounced for the PSZ exhibiting a strong positive relationship (Fig. 5). Rates of frictional − sample, which is not weaker than the adjacent cataclasites in the dry healing are narrowly constrained within β = 0.005–0.007 decade 1 case (Fig. 4A). for both room-humidity and fluid-saturated samples. The exception is

Mylonites Principal Slip Zone 60 Cataclasites Gravel (DFDP-1B) 1B PSZ 50

2 30 R = 0.997

Shear Stress (MPa) 20

0 0 20 40 60 80 100 0 20406080100120 Normal Stress (MPa) Normal Stress (MPa)

Fig. 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. the PSZ, which exhibits elevated healing rates (β = 0.009 dry, 0.010 samples indicate that illite and chlorite are common in the Alpine wet) compared to the rest of the sample suite. Fault, but that the PSZ has a significant smectite content not found in the wall rocks (Boulton et al., 2012b; Schleicher et al., in review). 5. Discussion lower overall strength is consistent with elevated clay mineral content in general and smectite in particular, especially under the influence of 5.1. Observed frictional behavior fluids (e.g. Saffer and Marone, 2003; Ikari et al., 2007). The PSZ exhibits significantly higher rates of frictional healing com- Our experimental results show clear differences in the frictional be- pared to the cataclasites and mylonites. The healing rates we observe havior of the PSZ compared to the mylonites, cataclasites, and gravel for the PSZ material are notably higher than previously measured that make up the hanging wall and footwall around the Alpine Fault. values under similar testing conditions on samples from other Under fluid-saturated conditions the PSZ is weaker than the surrounding major fault zones (Fig. 6). A smectite (saponite)-rich sample of the rock, and under dry conditions the wall rock exhibits velocity-weakening active deforming zone of the San Andreas Fault from the SAFOD bore- behavior. Our results are consistent with recent measurements which hole exhibited negligible frictional healing (Carpenter et al., 2012), show μ = 0.49 for the DFDP-1B PSZ under similar conditions to our while β b 0.002 decade−1 was reported for phyllosilicate-rich (~50%) water-saturated test (Boulton et al., 2014), and also previous work on samples from the Zuccale Fault in central Italy (Tesei et al., 2012), and surface samples of the Alpine Fault recovered near Gaunt Creek, which 0.004 b β b 0.008 decade−1 was reported for samples from within a showed that the fault gouge is both significantly weaker than surround- major splay fault and the frontal thrust zone at the Nankai Trough sub- ing cataclasites and exhibits velocity-strengthening friction under fluid- duction zone, southwest Japan (Ikari et al., 2012). Healing rates for the saturated conditions (Boulton et al., 2012a; Barth et al., 2013). PSZ sample are more consistent with β = 0.008–0.009 decade−1 Because we use disaggregated gouge powders, the differences in ob- observed in quartz sand (Marone, 1998a). Because stress drops served frictional behavior can be attributed solely to compositional var- occur during earthquakes, elevated fault healing rates facilitate the iations, rather than rock fabric (e.g., Collettini et al., 2009; Ikari et al., strength recovery necessary for repeated earthquake rupture. Under 2011). Compositional analyses of the DFDP-1B PSZ as well as outcrop fluid-saturated conditions, our observed β = 0.010 for the PSZ

AB 65 Dry Wet 70 Mylonites 75 Mylonites

85 Cataclasites Cataclasites Depth (m)

90 PSSPSZ (1B)(1B) PSZ (1B)

95 Gravel Gravel

100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 -0.005 0 0.005 0.01 0.015 0.02 Coefficient of Friction a-b

Fig. 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.

1B PSZ Dry Wet 0.01 Cataclasite 89.7 m All other samples

0.005 a-b

1 10 100 1000 1 10 100 1000 Upstep Load Point Velocity (μm/s) Upstep Load Point Velocity (mic/s)

Fig. 5. Friction rate parameter a − b as a function of upstep load point velocity under (A) room humidity, and (B) ﬂuid-saturated conditions. represents a strength increase of approximately 2% decade−1, which where the PSZ material is in contact with either mylonites or matcheshealingratesof1–3% decade−1 inferred from small magnitude cataclasites, or (3) that the PSZ material has been altered by elevated

(Mw = ~1.5) repeating earthquakes on the Calaveras Fault, a San pressure and temperature at several km depth to become frictionally Andreas Fault branch located ~150 km northwest of the SAFOD site unstable, as has been observed for clay (illite)-rich material (e.g. den (Vidale et al., 1994; Marone et al., 1995). Hartog and Spiers, 2013) and other gouge samples from the DFDP boreholes (Boulton et al., 2014). We favor the second and/or third hypotheses (Fig. 7)becausefluid–rock interaction that results in signif- 5.2. Implications for the slip behavior of the Alpine Fault icant alteration of the mineral assemblage is common in major fault zones (Chester et al., 1993; Wintsch et al., 1995; Vrolijk and van der The velocity-strengthening behavior we observe for the PSZ, espe- Pluijm, 1999), including the Alpine Fault (Warr and Cox, 2001). There- cially when fluid saturated, indicates that it is unlikely to generate seis- fore we expect that the core of the Alpine Fault at seismogenic depths micity if similar material exists at depth on the Alpine Fault (Barth et al., would have a composition distinct from the mylonite/cataclasite 2013). For earthquake nucleation at seismogenic depth on the Alpine protolith, which may be similar to the PSZ from Borehole 1B (i.e., having Fault, this means that either: (1) the PSZ does not exist at several km higher clay content). Furthermore, rupture propagation is expected to depth and coseismic slip initiates in either mylonites or cataclasites, be influenced by competency contrasts in the Alpine Fault as demon- (2) earthquake nucleation occurs on the boundary of the fault core strated by Townend et al. (2013), who used P-wave velocity and frac- ture density measurements from wireline logging data in both DFDP boreholes to show that the hanging wall is stiffer than the footwall. SAF Additional laboratory determinations of elastic moduli for material in ZF NT QS and around the PSZ, which suggest that the fault zone material is signif- 65 icantly more compliant and thus the Alpine Fault exhibits a strong elas- Dry tic contrast between the fault zone and wall rock (Carpenter et al., 70 Wet NW SE 75 PSZ Mylonites Stable Unstable PSZ Frictional Stability 80 EQ Rupture Path

85 Depth (m) Cataclasites

90 PSZ (1B) Ultra- mylonite 95 Gravel Cataclasites 100 0 0.004 0.008 0.012 0.016 Healing Rate (decade-1) Western Provence Granite & Gneiss

Fig. 6. Frictional healing rate as a function of depth in the DFPD-1A borehole under both Fig. 7. Schematic illustration of possible paths of earthquake rupture nucleating at room humidity and ﬂuid-saturated conditions. The PSZ in DFDP-1A (91 m) is represented seismogenic depths. Stars indicate nucleation either within the PSZ or at the PSZ/ by data from the DFDP-1B sample. For comparison, shaded areas show healing rates from cataclasite boundary at depth. Left inset shows image of the core sample containing the the San Andreas Fault (SAF; Carpenter et al., 2012), Zuccale Fault (ZF; Tesei et al., 2012), PSZ (outlined in red) from the DFDP-1A borehole. (For interpretation of the references Nankai Trough (NT; Ikari et al., 2012), and quartz sand (QS; Marone, 1998b). to color in this ﬁgure legend, the reader is referred to the web version of this article.)

2014). This evidence supports our second hypothesis, although we related to differences in mineralogic composition from the wall rock, expect the rupture to be controlled by the wall rock–gouge boundary such as increased smectite content. Velocity-strengthening behavior of considering a fault core of finite width rather than an infinitely thin the PSZ material suggests that it may be unable to host earthquake surface (e.g., Marone et al., 2009). Earthquake slip on plate boundary nucleation in its recovered form. Thus, earthquake nucleation on the faults is expected to occur within narrow zones that are likely of mm- Alpine Fault likely occurs either within the velocity-weakening to cm-scale thickness and therefore the entire width of the PSZ may cataclasites or mylonites that bound the PSZ, at the boundary between not necessarily deform coseismically (Sibson, 2003; Rowe et al., 2013). these rocks and the PSZ, or in a highly altered form of the shallow PSZ Furthermore, friction experiments simulating coseismic (m/s) slip at greater depth. The slight weakness of the PSZ material relative to rates have shown that shear deformation tends to localize at the bound- wall rock at shallow depth could, however, facilitate localization of ary between the gouge and the wall rock (e.g. Ujiie and Tsutsumi, 2010; coseismic slip within this zone instead of the adjacent, stronger De Paola et al., 2011). cataclasites. Elevated healing rates indicate that shear stress on the Elevated healing rates are observed for PSZ material, which is favor- Alpine Fault would be easily recovered in the interseismic period able for interseismic strength recovery that is necessary for repeating following coseismic stress drops (which is necessary for repeating earthquakes. However, the velocity-strengthening nature of the PSZ in- earthquakes) and support fault locking, even at shallow depths. In con- dicates that negative stress drops, or strength increases which hinder junction with field, seismologic, and geodetic observations our results coseismic slip, should be expected which supports the inference that indicate that the Alpine Fault displays a tendency for slip instability seismicity originates either on the PSZ boundary or within an altered that likely extends to the surface. PSZ. Although it is difficult to project how healing rates would evolve as seismogenic depths are approached, slide–hold–slide tests under Acknowledgments simulated hydrothermal conditions generally show that greater healing is to be expected (e.g. Karner et al., 1997; Tenthorey et al., 2003; We thank the DFDP-Alpine Fault team lead by Rupert Sutherland, Yasuhara et al., 2005). Thus, the high healing rates we observe in the John Townend and Virginia Toy, and especially thank Virginia Toy for PSZ sample are favorable for repeated stress drops in large earthquakes. helpful discussions. We also thank Steve Swavely and Marco Scuderi High rates of healing in the PSZ also suggest that the fault has the for assistance in the laboratory. Heather Savage and Patrick Fulton are ability to become locked or partially locked at the near surface and accu- gratefully acknowledged for providing helpful and constructive re- mulate strain energy, which is supported by the inferred absence of re- views. This work was supported by National Science Foundation grants cent aseismic fault creep from GPS measurements (Beavan et al., 1999). OCE-0648331, EAR-0746192, and EAR-0950517 to C.M. and Deutsche Although evidence of fluid–rock interactions are prevalent in the DFDP- Forschungsgemeinschaft grant DFG KO2108/14-1 to A.J.K. 1coresamples(Sutherland et al., 2012), our dry tests of mylonite and b cataclasite samples recovered from very shallow depths ( 100 m) Appendix A. Supplementary data indicate that velocity-weakening behavior could occur under certain (unusual) circumstances. This may suggest that the Alpine Fault could Supplementary data to this article can be found online at http://dx. be frictionally unstable throughout the brittle crust and therefore lack doi.org/10.1016/j.tecto.2014.05.005. the upper aseismic zone within a few km of the surface that is common- ly observed on other faults (e.g. Marone and Scholz, 1988). This is con- References sistent with seismological observations which show that earthquake hypocenters are distributed throughout the upper 12 km on the Alpine Barth, N.C., Boulton, C.J., Carpenter, B.M., Batt, G.E., Toy, V.G., 2013. Slip localization on the Fault, including up to the surface (Leitner et al., 2001). In the case of an southern Alpine Fault, New Zealand. Tectonics 32, 1–21. http://dx.doi.org/10.1002/ tect.20041. earthquake which nucleates at depth and propagates upward, coseismic Beavan, J., Moore, M., Pearson, C., Henderson, M., Parsons, B., Bourne, S., England, P., rupture and slip may therefore be likely to propagate to the surface, Walcott, D., Blick, G., Darby, D., Hodgkinson, K., 1999. Crustal deformation during most likely at the boundary between the weaker PSZ and the velocity- 1994–1998 due to oblique continental collision in the central Southern Alps, New weakening mylonites or cataclasites at shallow depths (Fig. 7). 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