Characterization of Damage in Sandstones along the Mojave Section of the San Andreas Fault: Implications for the Shallow Extent of Damage Generation

Pure appl. geophys. 166 (2009) 1747–1773 0033–4553/09/101747–27 DOI 10.1007/s00024-009-0516-z Ó Birkhäuser Verlag, Basel, 2009 Pure and Applied Geophysics Characterization of Damage in Sandstones along the Mojave Section of the San Andreas Fault: Implications for the Shallow Extent of Damage Generation ORY DOR,1,5 JUDITH S. CHESTER,2 YEHUDA BEN-ZION,1 JAMES N. BRUNE,3 and THOMAS K. ROCKWELL4 Abstract—Following theoretical calculations that suggest shallow generation of rock damage during an earthquake rupture, we measure the degree of fracture damage in young sedimentary rocks from the Juniper Hills Formation (JHF) that were displaced 21 km along the Mojave section of San Andreas Fault (SAF) and were not exhumed significantly during their displacement. In exposures adjacent to the fault, the JHF typically displays original sedimentary fabrics and little evidence of bulk shear strain at the mesoscopic scale. The formation is, however, pervasively fractured at the microscopic scale over a zone that is about a 100 m wide on the southwest side of the SAF near Little Rock. The abundance of open fractures, the poor consolidation, and the shallow inferred burial depth imply that the damage was generated close to the surface of the Earth. The spatial correlation of this damage with a seismically active trace of the SAF suggests that it was generated by SAF slip events that by assumption were of a seismic nature throughout the displacement history of the JHF. Thus the JHF provides a very shallow upper bound for the generation of brittle damage in a seismic fault zone. The fracture fabric is characterized by preferred orientations of fractures that split grains between contact points and is consistent with overall deformation under directed compression. However, the available results cannot be used to distinguish between proposed off-fault damage mechanisms. Fracture orientations are compatible with a maximum compressive stress oriented at a high angle to the fault at about 10 m, and at a lower, more variable angle farther away from the fault. The fracture distribution and fabric are consistent with observations made of the microscale damage characteristics of the Hungry Valley Formation in the northwestern section of the SAF in the Mojave, and with previous observations of exhumed, ancestral strands of the SAF. Key words: Fault-zone structure, rock damage, San Andreas fault, earthquake rupture mechanism, mode I fractures, Sedimentary rocks. 1 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, U.S.A. E-mail: dor@brown.edu 2 Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115, U.S.A. 3 Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557, U.S.A. 4 Department of Geological Sciences, San Diego State University, San Diego, CA 92182-1020, U.S.A. 5 Now at Department of Geological Sciences, Brown University, Providence, RI 02912, U.S.A 1748 O. Dor et al. Pure appl. geophys., 1. Introduction 1.1. The Origin and Depth of Pulverized Fault Zone Rocks The key characteristics of pulverized fault zone rocks found along the San Andreas Fault (SAF) and elsewhere are (WILSON et al., 2005; DOR et al., 2006a; ROCKWELL et al., 2009) reduced grain size by fracture with minimal distortion of the primary rock structure on many scales. WILSON et al. (2005) and ROCKWELL et al. (2009) have shown that the pulverized Tejon Lookout Granite at Tejon Pass lacks significant weathering products, suggesting that the pulverization occurred primarily by a mechanical process. DOR et al. (2006a) mapped the distribution of pulverized crystalline rocks along the Mojave section of the SAF and found that they occupy a tabular zone parallel to the fault that is 100–300 m wide. Based on evidence for the exhumation depth of the SAF in the Mojave area, they inferred that the observed pulverization occurred in the top 1–3 km, in contrast to the conclusion reached earlier by WILSON et al. (2005). Our investigation of potential geological signals constraining the depth and mechanism of rock pulverization discussed in this paper is driven by theoretical expectations that we review here briefly. Lab-constrained damage rheology (LYAKHOVSKY et al., 1997; HAMIEL et al., 2004; LYAKHOVSKY and BEN-ZION, 2008), analytical work (RICE et al., 2005), and numerical simulations of spontaneous damage generation during dynamic rupture on a bimaterial interface (BEN-ZION and SHI, 2005) indicate that pervasive fracturing is likely confined to the top few km of the crust. In the case of ruptures along faults that separate different elastic materials at depth, the shallowgenerated damage is expected to have an asymmetric pattern and be concentrated on the side of the fault with higher seismic velocity at depth (BEN-ZION and SHI, 2005). These predictions are compatible with properties of the pulverized rock layer observed by DOR et al. (2006a), and with the symmetry properties of smaller-scale damage products mapped by DOR et al. (2006b) along several fault sections of the San Andreas system in southern California (including the Mojave section of the SAF). Brittle fractures in the damage zone adjacent to large displacement seismogenic faults can result from numerous loading conditions at dynamic and quasi-static deformation rates. In particular, the orientation of microfractures (given that the fracture pattern was not extensively overprinted by various other processes) may reflect the orientation of the near-field stresses developed during the passage of an earthquake rupture (e.g., DI TORO et al., 2005; RICE et al., 2005). BRUNE et al. (1993) and Brune (2001) suggested that the crushed rocks adjacent to the SAF resulted from fault-normal loading and unloading due to a dynamic reduction of normal stress associated with the passage of successive rupture tips. Ruptures propagating on a bimaterial interface may produce local fault-normal openings associated with transient absolute tension (BEN-ZION, 2001; BEN-ZION and HUANG, 2002), which may also lead to variability in the orientation of stresses, and fractures, throughout the damage zone. Directed stress concentrations may develop along a rough fault surface during cyclic frictional sliding, Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1749 depending on the far field stress direction and the friction on the fault (CHESTER and CHESTER, 2000). Damage developed due to the geometrical effect of the fault roughness will fall off linearly as a function of the log-distance from the fault (e.g., SCHOLZ et al., 1993). A summary of several models and their predicted microfracture orientations is given by WILSON et al. (2003). Damage generated during the propagation of successive earthquakes will overshadow the initial damage produced by the growth of the fault (CHESTER et al., 2004b). 1.2. Research Rational To further assess the shallow depth extent and origin of pulverization in the damage structure of the SAF, and test model predictions, we examine the presence and type of damage in sedimentary rocks, mainly from the Plio-Pleistocene Juniper Hills Formation (JHF). These sedimentary rocks were never deeply buried while being displaced along the SAF (Section 1.3). A spatial correlation between the level of damage in those rocks, if found, and the trace of the SAF, should indicate that the damage is associated with SAF slip events. Paleoseismic studies show evidence for large seismic events on the SAF in the Mojave during the last 6000 years (WELDON et al., 2004). We assume that this was the case during the entire displacement history of the JHF (total of 19–21 km of strike-slip, BARROWS et al., 1985). The presence of SAF-related damage in these sandstones can therefore provide an estimate for the upper boundary of the depth range of damage generation during seismic faulting events. The properties of the damage can help identify the possible damage mechanisms operating during seismic rupture. 1.3. Characteristics and Exhumation of the Juniper Hills Formation The JHF was derived, in part, from the Punchbowl Formation and contains several distinctive clasts not found in older sedimentary rocks (BARROWS, 1980; 1985). It consists of elongated tectonostratigraphic units that parallel the SAF and its subsidiary faults. Distinctive JHF units containing the unique clasts are offset 13 to 16 km by the Northern Nadeau fault, and 19 to 21 km by the SAF in this and the surrounding regions. Samples were collected from the TQjh member (Juniper Hills formation, undifferentiated), a poorly to moderately indurated fluvial deposit of coarse arkosic sandstone, lesser conglomerate and thin-bedded shale that commonly exhibits distinct bedding and often is poorly sorted (BARROWS et al., 1985). Clasts are subangular to well-rounded and varicolored. The JHF is characterized by considerable variation in bedding, sorting and induration properties between exposures and even between adjacent layers. Compositional variations also have been noted (BARROWS, 1980, 1985). The poorly indurated layers are sometimes powdery even far from a fault. Therefore, and in contrast to crystalline rocks studied previously (e.g., DOR et al., 2006a), the texture in a hand sample of the JHF or its appearance in the field does not provide a systematic indication of microscopic fracture damage. 1750 O. Dor et al. Pure appl. geophys., Exhumation of the SAF in the study area was inferred by DOR et al. (2006a) to be minimal, and certainly below the maximum uplift of 2–4 km inferred for the Punchbowl fault (CHESTER et al., 1993). SPOTILA et al. (2007) show that exhumation of the SAF in the vicinity of the San Gabriel Mountains ranges 0.03 to 0.5 mm/y with an exceptional rate of up to 1 mm/y in a few places based on theormochronological data. Similar rates are proposed for blocks bounding the SAF in the relevant time frame (BLYTHE et al., 2002). These rates imply tens of meters up to about 1 km of uplift of the SAF in the last 2 Ma. While there is no direct evidence for the maximum burial depth of the JHF, the poor consolidation state is consistent with shallow burial. WEBER (1999; see also KENNY, 2000) suggests that uplift of the southeast portion of the San Gabriel mountains, initiating 0.75 to 0.5 Ma, raised beds of the middle (?) Pleistocene Shoemaker Gravel Formation (SGF) in the Big Pine area 1100 m with respect to equivalent units near Big Rock Creek (inset in Fig. 1). The easternmost zone of our sampling area is 12 km farther to the northwest of Big Rock Creek, and 25 km northwest of Big Pine. Although the JHF is older than the SGF, its distance from the locus of uplift (farther away from the correlative lower body of the SGF), and the inference that uplift had intensified toward the end of the Pleistocene also support considerably lower exhumation values for the JHF. 1 km 8e - 10 N E. 106th St. Map after Barrows et al. (1985) San Andreas fault 28 - 115 31 - 65 N. Nadeau fault Palmade Little Rock N 27 - 335 25 km Fault / dashed where inferred Big Rock Creek SGM 26 - 670 Juniper Hills Fm. (member TQjh) Big Pine SA F Sample used for analysis 33 - 1455 Sample used for characterization 6a - 1930 Sample name and distance from SAF Road 6a - 1930 Figure 1 The inner-inset (lower left corner) shows a map of California with main faults (thick black lines). Rectangle marks the section of the SAF shown in the larger inset. Larger inset: Digital elevation model of the southwestern Mojave with the SAF marked as a white line. Location of geological map is shown as a rectangle near the town of Little Rock. The star indicates the location of sample 6a. SGM: San Gabriel Mountains. The geological map (after BARROWS et al., 1985) shows the distribution of some of the JHF bodies in the area with the location of our sampling sites. Samples 6a and 33 are marked not in their geographic location. Note that the JHF here is bounded by the SAF and the N. Nadeau fault. Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1751 Our two sampling points farthest from each other are near E. 106th St. (e.g., point 28– 115 in Figure 1) and near the intersection of Pearblossom Hwy and Sierra Hwy (point 6a, shown as a star in the inset in Fig. 1), with the first being 16 km east-southeast and 190 m higher than the second one. An additional 22 km to the southeast, the same JHF member is exposed near Jackson Lake (shown as a triangle in the inset of Fig. 1), 790 m higher than the E. 106th St. sampling point. The elevation gradient changes from 12 m/km to 34 m/km between these two sections, and the trace of the SAF responds to it by an abrupt change in its elevation gain 12 km to the southeast of 106th St., immediately east of Big Rock Creek. These topographical features are compatible with a substantial drop in uplift rates in the vicinity of Big Rock Creek and to the northwest, also consistent with minimal exhumation in the study area. 2. Methodology 2.1. Research Approach The microscale damage we observe in the JHF is characterized by an inhomogeneous fragmentation of grains without apparent distortion of their shape, while the matrix includes detrital grain fragments that are not considered damage products. In addition, there are variations in the original modal and grain size distribution between the sampling locations (Section 3.1). Consequently, damage analysis of the JHF rocks cannot take advantage of ‘whole rock’ particle size distribution analyses methods (e.g., image segmentation, particle laser analyzer, etc.) as a means of comparison between the samples, and therefore targets the individual grains by comparing their current damage content to their original, predamage state. Since most of the microfractures observed are open, it is impossible to separate the grains from the matrix without breaking them apart, and so the damage analysis is best done optically on photomicrographs using the image analysis methods discussed below. 2.2. Sampling Damage estimates are based on 13 JHF samples collected at seven stations along a fault-normal traverse southwest of the SAF (Fig. 1). We compare the results to observations from three samples of the Hungry Valley Formation collected at distances of 125, 1055 and 3380 m from the SAF in the northwest portion of the Mojave Desert and in Ridge Basin (BARROWS et al., 1985; CROWELL, 1982). To ensure as much uniformity as possible between our samples, specimens were collected from layers having similar grain sizes and colors, and as far as possible from mesoscale faults, veins, fine layering, large clasts and other obvious structural and sedimentary elements in the TQjh member of the JHF. Samples were impregnated in the lab with a low viscosity epoxy (Epo-Tek 301Ó) under a vacuum and then sectioned and polished for optical analysis. 1752 O. Dor et al. Pure appl. geophys., 2.3. Image Analysis The fracture damage analysis was performed on a representative population of grains in each thin section under cross-polarized, plane-polarized and reflected-light conditions. We first mapped the original grain boundaries of 170–320 grains per section on planepolarized light images. We confined our analysis to a grain size range of *150 – 2000 microns (with some exceptional larger grains), avoiding large rock fragments, grains showing significant alteration and grains with many subgrains. The minimum size was set to avoid host-rock matrix particles and because grains smaller than 150-micron show very little fracturing. The maximum size is the actual maximum original grain size in the samples, excluding large (up to 3–4 mm) rock fragments. We digitized the overall outlines of the mapped qualified grains to estimate the original (prefracturing) grain size distribution for this range using Image J (http://rsbweb.nih.gov/ij/). A random selection of grains for the analysis from the entire mapped grain population would result in a non-representative subset of mainly small grains. Such a subset would underestimate the degree of damage. To address this, we divided the grain population into four size bins and chose 10–12 random grains from each bin (Fig. 2a) to obtain a subset of the initial population that consists of 40–48 grains per thin section. Each selected grain was photographed at 100X magnification under reflected light (Fig. 2b). We used this photograph to generate a grayscale image of the fractured grain, masked from the background (Fig. 2c), and a second image showing a digitized and filled trace of the original grain to represent the intact grain in its predamaged form (Fig. 2d). The image of the masked grain was transformed into a bitmap (binary) using a 50% threshold value and then inverted so that the grain and its fragments are black, and the Figure 2 Intermediate and final products of the analysis process: a. Forty grains in four size bins (color coded) are chosen from a population of mapped grains (light transparent gray). b. An image of a single quartz grain taken under reflected light, optical microscope. c. Grayscale version of the same grain, masked from its environment. d. A map of the grain’s outline, filled, representing the predamaged state of the grain. e. The product of (c): binary (bitmap) image of the fragments that belong to the same grain. Their total perimeter length is divided in the perimeter length of the ‘‘intact’’ grain in (d) to give the FIPL (see text). Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1753 background is white (Fig. 2e). Epoxy-filled gaps that passed the threshold process were masked manually in Adobe Photoshop. The total perimeter length of each grain was determined using Image J. The ‘‘intact’’ grain image (Fig. 2d) gives the length of the original grain perimeter before it was fractured, and the ‘‘damaged’’ grain image (Fig. 2e) gives the cumulative perimeter length of all the fragments that belong to the grain. We define a Factor of Increase in Perimeter Length (FIPL) as the ratio between the ‘‘damaged’’ and the ‘‘intact’’ perimeter lengths, and the FIPL for the entire sample is calculated as the area-weighted average FIPL of all grains that were analyzed in that sample. The perimeter length for the original grains is not corrected for dilation of the grain caused by fracture opening, consequently our FIPL measurements slightly underestimate the actual values. For modal distribution analysis we used backscatter images of the thin sections acquired by a Cameca SX100 Electron-probe with a current of 15KeV. We used Image SXM (http://www.liv.ac.uk/wsdb/ImageSXM/) to generate bitmap images of each of the phases in the sample by density slicing of the grayscale images at the corresponding gray levels, and calculated the proportions between the phases from their relative area on the images. ‘‘Noise’’ (very small particles) was subtracted from the bitmap of each phase and was added to the total area of the matrix bitmap. The trends of microfractures were determined for selected samples on a flat microscope stage. In these samples, we measured the orientation of fractures in all the grains chosen for the perimeter length analysis. We classified the measured fractures into the fracture types described in Section 3.2. 3. Observations 3.1. Juniper Hills Formation Host Rock A representative exposure of the JHF at 670 m from the fault shows a light brown to yellow arkosic sandstone with layering defined by grain size variations. The formation is tilted shallowly at this location and the surrounding regions, and is cut by calcite veins and scattered mesoscale faults. Overall, the host rock is friable and most layers can be disaggregated fairly easily down to the grain scale with a rock hammer (sometimes yielding a powder), reflecting poor cementation. The formation typically is grain supported, containing angular to subrounded quartz and feldspar grains that are either intact or cut by open fractures and by fluid inclusion plains. The granulometric and mineral composition properties of the five samples used for the FIPL and fracture orientation measurements (Sections 3.2–3.5) are shown in Table 1. The grain size data are based on the population of grains that were mapped for the image analysis purpose (i.e., data include the original, ‘‘intact’’ size of qualified grains) and is not an absolute measure for particle-size distribution in the samples. The smaller grains tend to be more angular, while larger, more rounded grains dominate the thin section area 1754 O. Dor et al. Pure appl. geophys., Table 1 Grain Size and Composition of Samples Used for FIPL Measurements Sample 8E-b 31 28 27 6a Distance from SAF Granulometric properties No. grains analyzed DiameterI range (lm) Mean grain diameter (lm) Standard deviation (lm) Skewness of distribution 10 65 115 335 1930 189 160-2296 816 423 1.29 247 151-1680 488 241 1.76 210 198-1444 530 224 1.33 205 187-1957 566 289 1.77 310 151-2526 366 229 4.15II Phases area percent on thin sections (normalized to 100% without rock fragments, epoxy)III Quartz Plagioclase K-Feldspar Others phasesIV MatrixV Total 36 9 8 2 45 100 38 14 11 1 36 100 31 16 11 1 41 100 46 5 12 1 36 100 No No No No No No data data data data data data Epoxy gaps Rock fragments 2 18 2 9 8VI 3 2 6 No data No data Comments: I The diameter of a circle with an area identical to that of the particle II The distribution of mapped grains in sample 6a is more linear on a log-log scale than in the other samples, with a slope of *1.7 III Due to the large size of individual rock fragments with respect to the analyzed image frame, their percentage area may not represent their actual proportion in the sample. Their percentage area (before normalization) is given as a rough estimation for their area proportion IV Mainly biotite and iron oxides V The original proportion of matrix in the samples is probably larger since it likely was a major component of the material replaced by epoxy VI The area used for this analysis included a macroscopic wide fracture filled with epoxy, hence this anomalous proportion of each sample. The matrix is partially opaque to transmitted light, but has an average visible grain size of 5–20 microns including, in addition to quartz and feldspar fragments, grains of micas and oxides. The JHF was deposited in a local fluvial system in the vicinity of the source rocks. As such, some granulometric and compositional variations are expected between its various exposures, and this is manifested in the data in Table 1. 3.2. Microscale Damage Characteristics The dominant microscale damage observed is the in situ shattering of grains without significant distortion of the original grain shape. Fragments fit together in a hierarchical fashion, and grains, even when extensively shattered, have uniform extinction in crosspolars (Fig. 3a and 3b). These features suggest that minimal or no shear and rotation are involved in the microscale deformation process of the grains. Similar damage was observed by ROCKWELL et al. (2009) in samples from the pulverized Tejon Lookout Granite in Tejon Pass on the SAF and in Tejon Ranch on the Garlock fault. The Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1755 Figure 3 Microscale damage features: a. Partially fractured grain. b. Heavily fractured grain. Note the three columns of fragments: may be caused by impingement, like in (e, f). The grains in (a) and (b) are identified due to simultaneous extinction of the fragments, suggesting minimal or no shear and rotation of the grains. c. Crosspolarized photo of bended and sheared mica grain (marked ‘‘M’’), in this case clearly due to an impingement between the two grains bounding the mica grain: Note the fractures radiating from the contact point between the grains (white arrow). d. Grain-scale fractures emanating from a contact point (white arrow) between two quartz grains, BSE image. Note intense small scale fragmentation near contact point. e, f. Cross-polarized (e) and reflected light (f) photos of fractures radiating in a Hertzian pattern from a contact zone between two grains. White arrow marks one of those fractures. distribution of fracturing is heterogeneous in all samples, with some grains fragmented down to the micron scale whereas others are intact. This type of damage is somewhat similar to fault zone cataclasites classified by SIBSON (1986) as crushed and implosion 1756 O. Dor et al. Pure appl. geophys., breccias. In many samples there is clear evidence of intragranular and intergranular cracks that have relatively sharp traces displaying opening-mode, relatively narrow apertures. The amount of fracture opening requires some dilation of the grains. These fractures often emanate from grain-grain contact points and have parallel or radiating geometries that sometimes link opposing points of contacts (Figs. 3 c-f, e.g., arrows in Fig. 3d, 3f); they often are associated with small conical cracks at contact sites (arrow in Fig. 3c). These features are consistent with Hertzian-type stressing in unconsolidated porous aggregates (e.g., CHESTER et al., 2004; GALLAGHER et al. ,1974). Some mica crystals (‘‘M’’ in Fig. 3c) are kinked and squeezed due to a displacement of another more rigid grain. Several types of microfractures are observed in the JHF. These are distinguished according to their relations to grain size and to their hierarchical internal arrangement within and with respect to the grain (Fig. 4): Type I: Type Type Type Type Type Transgranular fractures, extending beyond grain boundaries into the matrix and neighboring grains (not common). II: Intragranular fractures that cut an entire grain. They are frequently parallel to each other, dissecting the grain or part of it to elongated columnar fragments. III: Intragranular fractures that cut at least half of the grain’s width in the direction of the fracture, and terminate within a grain. Some of them taper into the grain while others connect fractures to other fractures or to grain boundaries. IV: Intragranular fractures that are considerably shorter than the grain’s average axis length. Those are usually connecting fractures that terminate against type II or III fractures. They often cut elongated columnar fragments into rectangular or otherwise angular fragments, creating webs in crisscrossing relationships (e.g., category II fractures of LAUBACH, 1997). V: Subgrain boundary and grain boundary fractures. VI: Fluid inclusion planes (always intragranular); these are shown as linear traces of bubbles, marking the location of healed (or sealed) fractures (e.g., TUTTLE, 1949). The vast majority of all fractures observed optically are joints (mode I fractures), associated with dilation normal to the fracture wall. Most fractures appear to be open (not healed or sealed), except fractures of Type VI. For our fracture orientation analysis (Section 3.5) we measured the orientation of fracture Types II-IV because these are less likely inherited from the source rock. The average orientation of Type I fractures is recorded, even though the number captured in thin sections is not statistically significant. We also measured the orientation of Type VI fractures. Type V fractures are curved in most cases, making their strike determination problematic. Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1757 Figure 4 Fracture types (see text for complete definition): a. Type I — transgranular fractures. Not common in our samples and therefore were not considered for the fracture analysis (such possible fracture is marked with a red arrow. These, however, could also be two aligned intragranular fractures). b. Type II (red) — fractures that cut the entire grain; Type III (blue, doted were projected) — cut at least half of the grain’s length in the direction of the fracture, taper into the grain or terminate against other fracture; IV (green) — short, connecting fractures. c. Type V — sub grain and grain boundary fractures. d. Type VI — fluid inclusion plains (marked by an arrow), typically cross-cut by other (open) types of fractures. Type V fractures were not included in our analysis because they are hard to measure and to interpret. 3.3. Statistical Significance of the FIPL Method The statistical significance of the FIPL method was tested on a population of 104 grains out of the 189 grains that were mapped from sample 8E-b. This sample was taken 10 m from the fault and displays a wide range of grain sizes and a heterogeneous fracture distribution. The area-weighted average FIPL for the 104 grains is 8.14 and the areaweighted standard deviation is 5.34 (Table 2). A histogram for the FIPL values from this population is presented in Figure 5. FIPL values are plotted against original grain size in Figure 6. These data show that the original grain size limits the FIPL value, although there is no strong correlation between them (R2 = 0.37). We assume that the damage content of the 104-grain population represents the damage state of the rock because it consists of more than half of the ‘qualified grains’ in the sample. To determine if a smaller subset of the population is sufficient to define the 1758 O. Dor et al. Pure appl. geophys., Table 2 Area-weighted Average FIPL and Standard Deviation for the 104-grain Population and the ten 40-grain subsets from sample 8E-b 40 grains Set 1 2 3 4 5 6 7 8 9 10 Average all 104 grains FIPL 7.40 8.06 7.09 6.10 8.72 8.40 7.99 7.15 8.92 7.29 7.71 A. weighted STDV 4.45 5.84 4.35 3.48 6.01 5.25 5.03 4.37 5.02 4.18 4.8 t-test P value 0.8 0.92 0.86 0.86 0.82 0.95 0.72 0.28 0.42 0.54 8.14 5.34 Figure 5 Frequency-distribution of FIPL values for 104 grains in sample 8E-b. damage we randomly picked 10 subsets, each composed of 40 grains. We then calculated the area-weighted average FIPL and area-weighted standard deviation for each subset (Table 2). We performed a t-test between the FIPL result of each of the 40-grain subsets and the FIPL of the 104-grain population in order to verify that the chosen subsets are indeed representative of the larger population. The results (Table 2) show that the subsets are not statistically different than the parent population. The P values of all are larger than 0.05, and all subsets pass the Shapiro-Wilk and Shapiro-Francia W normality tests with over 95% probability (Appendix 1). The statistical tests suggest that our sampling method captures the damage properties of the entire grain population. 3.4. FIPL as a Function of Distance from the SAF A qualitative examination of the thin sections before the measurements suggested that fracturing and fragmentation of grains are very intense in the samples taken 10 m from the fault, high in the sample taken 65 m from the fault, and significantly less farther away from the fault. We observe similar patterns in other sedimentary formations along the Mojave section of the SAF. The FIPL measurements for five samples taken at 10 m, 65 m, 115 m, 335 m, and 1930 m from the fault confirm this trend (Table 3). The FIPL values decrease with Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1759 Figure 6 FIPL as a function of original grain diameter for 104 grains in sample 8E-b. Although FIPL values have no strong correlation with the grain sizes (note small R2 value), grain size seems to control the upper limit of the FIPL values. distance from the fault in all samples except the one at 335 m. A t-test between the FIPL values of each sample and the next sample closer to the fault shows that the samples taken at 10, 65 and 115 m are statistically different from each other, while the samples taken at 115, 335 and 1930 m are statistically indistinguishable from each other in their FIPL values (Table 3). All sample results passed the Shapiro-Wilk and Shapiro-Francia W normality tests with over 95% probability (Appendix 1). These results suggest that the damage gradient is significant from 0 to *100 m, after which FIPL values probably approach a regional background damage state consistent with previous studies (e.g., WILSON et al., 2003). To test the effect of the mineralogy of the measured grain population on the results, we obtained FIPL values for each sample using only quartz grains from the subset of the grain population used for the analysis presented in Table 3. The existence of elevated densities of fractures in the damage zone may enhance weathering processes, particularly along the cleavage planes of feldspar grains. If this process is significant in extending existing fractures or creating new fractures, FIPL values obtained using quartz grains Table 3 FIPL values for JHF samples Sample 8E-b 31 28 27 6a distance from fault (m) FIPL FIPL (not weighted)I Section orientation t-test P valueII 10 8.14 5.61 28/240 65 5.21 3.85 70/028 0.00 115 2.42 2.16 80/256 0.00 335 3.81 2.43 46/145 0.4 1930 2.38 2.28 83/175 0.38 Comments: I Calculated as a simple average between FIPL values of the individual grains II For each sample with respect to the next sample closer to the fault 1760 O. Dor et al. Pure appl. geophys., only (FIPLquartz) should be substantially lower with respect to FIPL values obtained from all mineral phases (FIPLall-phases). The results, presented in Table 4, reveal no significant difference between FIPLall-phases and FIPLquartz. This is particularly true in sample 8E-b where more than 1/3 of the grains in the analyzed subset are feldspars. Quartz grains can be fragmented due to weathering alone (e.g., FRAZIER and GRAHAM, 2000). In such cases, products of pedogenesis, such as clay minerals, will be present between fragments; for the most part, we do not see these products in the open fractures cutting quartz in our samples (e.g., Fig. 3d). The similarity between the FIPLall-phases and the FIPLquartz values (Table 4), and the lack of significant weathering products in open fractures in quartz grains suggest that the damage we observe is mostly due to mechanical breakage. We note that the proportions between the mineral phases in the analyzed subsets (Table 4) are different than the apparent absolute proportions of those phases in the samples (Table 1), usually with higher proportion of quartz in the subset (except for sample 8E-b). One reason for this is that during the initial mapping of grains on the thin sections we disqualified grains that are especially weathered, mostly feldspars, so that the proportion of quartz in the mapped grain population is larger in most cases than its proportion in the sample. The mean grain diameter in sample 8E-b is larger than the mean grain diameter in the other samples (Table 1). A weak positive dependence of FIPL values in grain size is shown in Figure 6, although the relationships presented there alone cannot explain the significantly higher FIPL value in sample 8E-b with respect to the other samples. 3.5. Microfracture Trends The orientation of fracture types II and III was measured on three mutually perpendicular sections made from sample 8E (8E-a, 8E-b and 8E-c), and on a single section from samples 31, 28, 27 (the four samples correspond to distances of 10, 65, 115 and 335 m from the fault). The orientation of fracture types IV and VI was measured on sections 8E-b and 31. All measurement results are presented in Figure 7. Section 8E-c is vertical with a strike of N60E, and the data for this section in Figure 7 correspond to the inclination of the fractures on the plane of the section, so that ‘‘north’’ in the projection is vertical. Besides fracture type VI (fluid inclusion planes) and fracture type II in sections 8Ea and 28, fractures in all other sections show preferred orientations with various degrees of Table 4 The Partition of Analyzed Grains Between Quartz, Plagioclase and K- feldspar, and its Effect on FIPL values Sample Number of grains analyzed Quartz Palgioglase K-Feldspar FIPLquartz FIPLquartz as a fraction of FIPLall-phases 8E-b 31 28 27 31 8 8 7.9 0.97 40 2 3 4.89 0.94 45 1 1 2.33 0.96 41 1 6 3.78 0.99 6a No No No No No data data data data data Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1761 scattering. Fractures in sections 8E-a, 31, 28 and 27 are oriented at a low angle with respect to the SAF (13°– 31°). Fractures in section 8E-b are oriented with a high angle to the fault (74°–88°) and have the lowest 95% confidence margins (least scattered). The measurements on the thin sections of sample 8E allow deciphering the approximate three-dimensional mean orientation of fractures at 10 m from the fault. The orientations of the three thin sections from sample 8E are projected in Figure 8 as gray lines. The combined mean orientation for fracture types II and III for each thin section is marked as a black circle and its 95% confidence interval serves as an error bar and is marked as a thick black line. The best-fit great circle representing the inferred mean fracture orientation for the sample is constrained by the mean orientations and error ranges from each of the thin sections, and is marked on the projection as a black line. Its orientation is 57/132, with its strike 14° off the normal to the strike of the SAF in this area (black arrow in Figure 8). 3.6. The Hungry Valley Formation The majority of the Hungry Valley Formation (HVF) is widespread in Ridge Basin and is truncated on the north by the SAF. Tectonic slivers of the formation appear north of the SAF west of Three Points. The HVF is highly variable in texture and composition (BARROWS et al., 1985). Both the age and displacement of the formation are poorly constrained and highly debated in the literature. The age may range between late Miocene (MILLER and DOWNS, 1974) to Pleistocene (KAHLE, 1979) and the displacement is estimated to be on the order of 12 to 27 km by BARROWS et al. (1985) and about 220 km by RAMIREZ (1983). There are no direct constraints on the maximum burial depth, nonetheless it is probably not significant as there are indications for only moderate exhumation of the area since the late Miocene (DOR et al., 2006a; SPOTILA et al., 2007). In addition, consolidation of the formation in most places is poor. We compare three samples of the HVF (Fig. 9a): One collected 125 m northeast of the fault west of Three Points (22a); one collected 1055 m southwest of the fault in a road cut of Hwy 138 near its intersection with the I-5 (24); and one collected in the heart of the Ridge Basin 3380 m southwest of the SAF and in the immediate vicinity of no other known faults (25a). The HVF west of Three Points is an extremely heterogeneous fine- to coarse-grained arkosic sandstone that is poorly to moderately cemented (although it can be well indurated locally), bedded to massive, and poorly to moderately sorted (BARROWS et al., 1985). Typically, the formation contains subrounded quartz and feldspar grains, and large varicolored, sub- to well-rounded clasts of granite, volcanic rock and some metamorphic pebbles and cobbles. We find this description appropriate for the area from which the sample at 125 m from the fault (22a) was collected. Other locations sampled are more similar to each other and are well-bedded, wellsorted, and show less lateral lithologic variation when compared to the Three Points location. The samples at 1055 and 3380 m from the fault were collected from very poorly cemented layers of uniform material with mm-size clasts of mainly quartz and feldspar. 1762 O. Dor et al. Type II Type III Pure appl. geophys., Type II + III Type IV Type VI #8E-a 10 m Section orientation 62/060 No. of data: 57 Mean Direction: 094° 95% confidence: +/- 90° No. of data: 156 Mean Direction: 098° 95% confidence: +/- 37° No. of data: 213 Mean Direction: 097° 95% confidence: +/- 32° No. of data: 39 Mean Direction: 011° 95% confidence: +/- 22° No. of data: 94 Mean Direction: 025° 95% confidence: +/- 27° No. of data: 133 Mean Direction: 018° 95% confidence: +/- 18° No. of data: 30 Mean Direction: 080° 95% confidence: +/- 26° No. of data: 69 Mean Direction: 073° 95% confidence: +/- 21° No. of data: 99 Mean Direction: 077° 95% confidence: +/- 17° No. of data: 51 Mean Direction: 115° 95% confidence: +/- 54° No. of data: 90 Mean Direction: 139° 95% confidence: +/- 44° No. of data: 141 Mean Direction: 130° 95% confidence: +/- 38° No. of data: 35 Mean Direction: 103° 95% confidence: +/- 47° No. of data: 59 Mean Direction: 104° 95% confidence: +/- 90° No. of data: 94 Mean Direction: 105° 95% confidence: +/- 39° No. of data: 32 Mean Direction: 148° 95% confidence: +/- 32° No. of data: 62 Mean Direction: 139° 95% confidence: +/- 33° No. of data: 94 Mean Direction: 143° 95% confidence: +/- 24° #8E-b 10 m Section orientation 28/240 No. of data: 428 Mean Direction: 027° 95% confidence: +/- 16° No. of data: 84 Mean Direction: 95% confidence: +/- 90° #8E-c 10 m Section orientation 90/150 #31 65 m Section orientation 70/128 No. of data: 500 Mean Direction: 145° 95% confidence: +/- 16° No. of data: 41 Mean Direction: 95% confidence: +/- 90° #28 115 m Section orientation 80/256 #27 335 m Section orientation 46/145 Figure 7 Rose diagrams (area weighted projections) of fracture orientation data, classified according to fracture type. Dashed lines show the strike orientation of the SAF. Section 8E-c is vertical and therefore data represent an apparent dip of the fractures (or their 2-D projection on a vertical plane), with north in the projections being ‘‘up’’. Figure 9b shows cross-polarized and reflected light photomicrographs of samples 22a and 25a (125 and 3380 m from the fault, respectively). At 3380 m from the fault the grains are subrounded to subangular, with quartz and feldspar as main constituents. The rock is grain supported, containing a significant volume of calcite cement. The grains are Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1763 Normal to SAF -a 8E -c 8E 32 /1 57 8E-b -c 8E Thin section name and orientation 2D mean orientation of fracture types II+III with 95% confidence error sector Best fit great circle (3D mean orientation) for fractures types II+III Figure 8 A stereonet (equal area) showing the orientation of the mutually perpendicular sections from sample 8E (gray lines), the combined mean preferred direction of fracture types II and III from each of these sections (black circles), the 95% confidence interval for this mean direction (thick black lines) and the best-fit combined mean orientation for fracture types II and III in the sample (black lines). intact or only weakly fractured. Some post-cementation compaction has occurred as is evident by the presence of open fractures and twinned calcite (marked by green arrows on Fig. 9b). The host rock mineralogy at 125 m (Fig. 9b) is similar to that at 3380 m, but the intensity of fracturing is significantly greater, and the volume of calcite cement is considerably lower. Original variations in the porosity between the various parts of the formation are possible, although it is likely that at least some of these differences reflect damage-related volumetric strain. The sample from 1055 m shows characteristics intermediate to the other samples bounding it in terms of its fragmentation intensity and volume of cement. The existence of additional fault strands parallel to the SAF up to about 1 km southwest of its currently active trace (BRUNE, personal comm.), folding in the Ridge Basin area or thrusting in Frazier Park could affect the intermediate and minor damage contents of samples 24 and 25a, respectively. In such a case, the SAF slip-related damage gradient suggested by our observations may be an overestimate. In comparison with the JHF, the intensity of damage in the HVF is substantially higher. At 115 m from the fault, damage in the JHF is interpreted to reflect background levels (Table 2). In contrast, the HVF sample collected at 125 m displays fracture levels 1764 O. Dor et al. Pure appl. geophys., -118.80° Tejon Pass San North And I-5 reas Fau lt Hwy 138 Quail Lake 24 34.75° 25a 22a Three Points (a) 2 km The Hungry Valley Formation sample 25a - 3380 m SW of SAF (b) sample 22a - 125 m NE of SAF 1000 µm Figure 9 a. Location map of the HVF sampling points. Arrow on the small index map shows the location of the sampling area on the SAF. b. Cross-polarized (upper panels) and reflected light (lower panels) photomicrographs of Hungry Valley formation samples taken 3380 m from the fault (left) and 125 m from the fault (right). The farfield sample is only mildly damaged, containing some post-depositional compression features such as open cracks associated with Hertzian-type contacts and calcite twinning of the cement (green arrows). The near-field sample is intensely damaged although grain boundaries are not distorted. well above the background and appears to be more damaged than the JHF sample at 10 m from the fault. The lack of a direct relationship between degree of fragmentation and shape distortion, however, is shared by both formations. Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1765 4. Discussion 4.1. Precision of Measurements To evaluate the precision of the semi-automated image analysis technique, we compare the total perimeter length obtained for a single fractured grain (Fig. 10a) by using the technique described above (Fig. 10b), with the total perimeter length obtained by manual digitization of the grain (Fig. 10d). Figures 10c and 10e are the automatic and manually digitized grain, respectively, from which the noise (epoxy spots that passed the threshold filter in the automatic process and were also manually digitized) was removed. The FIPL obtained from the automatically digitized image (Fig. 10b) is higher than the FIPL obtained from the manually digitized image (Fig. 10d) because the manual digitization does not account for very small pixel clusters of gray values (could also be epoxy spots) that passed the automatic thresholding process (the amount of particles in Figure 10b is 1161 vs. 337 particles that were manually digitized in Figure 10d). An opposite trend occurs in the images that were cleaned from noise: The total perimeter length of all the fragments in the manually digitized image without the noise (Fig. 10e) is 7% higher than the corresponding value obtained for the autodigitized, noise-free image (Fig. 10c). Once the noise (pixel-scale spots as well as larger epoxy spots that were digitized in Fig. 10b) is removed and only actual rock fragments are analyzed for perimeter length, the image obtained by manual digitization yields a higher FIPL value because the automatic detection thresholding process tends to shrink fragments (as they are darker at the edges) that are otherwise mapped accurately by hand. We also compare selected areas of the optically photographed grains (white frames in Fig. 10a) with SEM images of the same areas (Fig. 11). The fragments observed under reflected light do not appear to be more fragmented in the SEM images. Particles that are not observed under reflected light (with the optical limit for X100 magnification at about 5 microns) appear to be restricted mostly to fractures, floating in the epoxy. Some zones in the epoxy are densely populated with such particles while others contain few or no particles. We mapped and calculated the total perimeter length of the small particles in Figure 11b (white lines in lower panel) and estimated the corresponding value for the entire grain. When we add the total perimeter length of these small fragments to the autodigitized, noise-free total perimeter length (Fig. 10c), the FIPL rises from 7.2 to 9.6. When we add the total perimeter length of the small fragments to the manually-digitized, noise-free total perimeter length (Fig. 10e), the FIPL rises from 7.7 to 10.1. The origin of the SEM observed particles in the epoxy-filled fractures is uncertain. During the impregnation process, epoxy flows through the fractures and may carry and deposit grain fragments from the same grain, bring fragments in from neighboring grains, or redistribute matrix material from the host-rock. The original amount of small particles inside the fractures could have been larger or smaller than the current amount. Calculating adjustments associated with the small (<5 lm) particles in the fractures, with noise removal and with corrections due to manual digitization for other grains may 1766 O. Dor et al. Pure appl. geophys., Figure 10 a. A gray scale image of a quartz grain from sample 8E-a. b, c. An automatically digitized fragment map of the grain in (a). (b) includes noise and (c) is manually cleaned from the noise. The image analysis program does not measure holes. d, e. A manually digitized fragment map of the grain in (a). (d) includes noise and (e) cleaned from the noise. FIPL values appear in parenthesis. result in different values of increase (or decrease) of the FIPL values, but is unlikely to be different by more than a factor of two. 4.2. Width of the Damage Zone For the portion of the SAF fault that displaces the JHF at the surface, the reduction in FIPL values between the samples in the first 115 m suggests that the fault at this location may be characterized by a microfractured damage zone of about 100–200 m. This scale is commonly observed in geological studies of fault zones (e.g., SCHULZ and EVANS, 2000), and is consistent with the zone of pulverized crystalline rocks along the same section of the fault (DOR et al., 2006a) and the width of seismic fault zone trapping structures (e.g., LI et al., 1990; PENG et al., 2003). The JHF data are consistent with a linear decrease in damage intensity with the logarithm of distance from the SAF (Fig. 12) as was found for faults with comparable displacements (e.g., CHESTER et al., 2004b) and smaller magnitudes of displacement (e.g., VERMILYE and SCHOLZ 1998). The damage zone in the HVF appears to be wider than about 100 m, possibly because of its association with the Big Bend in the SAF, however, our width constraints at this location are relatively poor. Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1767 Figure 11 SEM (Backscatter electron mode) images corresponding to frames in Figure 10a. Both images are rotated clockwise with respect to their orientation in Figure 10a. The images show many fragments that were not detected by the optical microscope analysis, floating in an epoxy matrix. The small fragments are restricted to certain zones within open fractures between larger fragments. They are mapped (white lines) in (b), lower panel. The damage zones of ancestral, exhumed strands of the SAF system show similar distribution, mode and fabric of microfractures (WILSON et al., 2003; CHESTER et al., 2004b), lending support to our inferences about the width of the SAF damage zone. 4.3. Depth of Damage Generation Fluid inclusion planes (healed fractures) that form a small minority in the fracture population are cut by open fractures, have no preferred orientation, and are likely inherited from the source rock. The fact that most of the remaining fractures are open is consistent with a near surface environment in which temperatures are low and fracture sealing is not favored (e.g., SPRUNT and NUR, 1979). Although some grains can survive transportation while they are partially fractured, the extensive open-mode fractures that transect entire grains indicate that these latter fractures occurred in-situ and were not inherited from the source rock. This inference is supported by the presence of a preferred orientation in our data and by the overall poor consolidation state of the JHF. 1768 O. Dor et al. Pure appl. geophys., FIPL vs. Distance from SAF 10 9 8 7 6 FIPL 5 4 3 2 10 100 1000 Distance from SAF Figure 12 Area-weighted averaged FIPL values plotted against log-distance from the SAF. With the exception of the sample at 115 m from the SAF, the data seem to have a linear correlation in this space. These observations imply that the damage zone within the JHF extends to the ground surface or very close to it. This conclusion combined with the inference about the shallow exhumation depth of the JHF suggests that the observed damage was generated close to the ground surface. Similar conclusions apply to the HVF based on the mode of fractures and assuming that it was never deeply buried as suggested by the state of the rocks (BARROWS et al., 1985). The similarity between the fracture fabric and the width of the damage zone between the SAF and its exhumed ancestral faults (Section 4.2) brings up the possibility that the observed damage near the SAF may extend to a depth of a few km. 4.4. Mechanical Interpretation of Damage Fabric Surface exposures of the sedimentary rocks we studied adjacent to the SAF, away from mesoscale shears, are pervasively and penetratively microfractured at the grain scale. These fractures are dominantly opening-mode features that display nominal shear displacements and indicate that tensile stresses existed locally (i.e., at the grain- or cracktip-scale) during their formation. This type of damage was observed also in crystalline rocks spatially associated with the SAF (ROCKWELL et al., 2009). Fractures associated Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1769 with Hertzian-type contacts along the SAF were, however, observed only in the sandstones, and are not expected in tight crystalline rocks. Overall, the microstructures morphologies suggest that they formed under a state of macroscopic compression, i.e., under a compressive mean stress, consistent with conical, axial and radial fracture geometries emanating from grain-grain contacts (Fig. 3) and with the JHF behaving as a porous, poorly consolidated granular aggregate during deformation (CHESTER et al., 2004a). However, the fact that microfracture orientation measurements suggest a preferred fabric within the fault damage zone further suggests that these fractures formed under a directed macroscopic stress that was not isotropic, i.e., not lithostatic or uniform tension. The transform plate boundary of the SAF system has been at its current location on the north side of the San Gabriel Mountains since about 4–5 Ma (e.g., POWELL and WELDON, 1992). If the currently active trace of the SAF participated in accommodating the slip since its onset in this area, which is likely the case (POWELL and WELDON, 1992), then the JHF, being only late Pliocene-Pleistocene in age, was deposited over an existing structure. In this case the damage that we see is associated primarily with slip events on the SAF and not with the inelastic deformation associated with the propagation of the fault process zone during its formation (e.g., SCHOLZ et al., 1993). 4.5. Implications of Results for Dynamic Rock Failure The JHF was deposited in the vicinity, and displaced by strands of the SAF system and therefore its entire deformation history is likely associated with the activity of those fault strands. We assume that displacement of the JHF occurred by seismic slip (Section 1.2). If this is correct, concentrated damage within * 100– 200 m of the active trace of the SAF is most likely the product of SAF earthquake ruptures. The inference for the depth of damage generation (Section 4.3) implies that dynamic generation of rock damage along this section of the SAF occurs very close to the surface of the earth. Fault-related microfracture fabrics may have been modified and weakened by local tilting and folding of the JHF layers. Nevertheless, orientation data at 10 m from the fault suggest that the fractures have a moderate preferred orientation consistent with extensive fracture under conditions close to a fault-normal compressive stress state in the near surface environment. CHESTER and CHESTER (2000) presented a mechanical model of stress and deformation in the vicinity of a wavy frictional surface and showed that stress is heterogeneous near the fault due to the juxtaposition of geometric irregularities. With a far-field stress oriented at high angles to a low friction master fault, fault-normal compression is generated locally during cyclic slip events and inelastic deformation is expected. This deformation is partially consistent with our data at 10 m from the fault in the JHF, and with previous studies on faults of the same system (WILSON et al., 2003). 1770 O. Dor et al. Pure appl. geophys., Fault opening and significant reduction of normal stress associated with the tip zone of the propagating earthquake rupture are expected to produce a change in the orientation of stresses as a function of distance from the fault. This may lead to a variability in the preferred orientation of microfractures within the damage zone that reflect changes from the background stress to near-zero transient shear stress on the fault. Our limited orientation data show changes in the preferred orientation of microfractures throughout the damage zone that is therefore compatible with dynamic fault opening produced by the rupture tip (e.g., BRUNE et al., 1993; BEN-ZION and HUANG, 2002). 5. Conclusions Our observations show that the JHF sandstone contains a considerable amount of damage that is spatially associated with the trace of the SAF, demonstrating that significant fracture damage can be produced in the near surface portion of a seismic fault zone. These inferences are supported by additional microscale observations on the damage content of the HVF along the northwest Mojave section of the SAF. The width of the damage zone within the JHF is similar to the 100–200 m width of the crystalline pulverized rock layer (DOR et al., 2006a). In both rock units the grains are shattered in situ, and distortion of original fabrics is minimal in all observational scales. A distinctive feature of the porous sandstones described here, however, is the abundance of fractures associated with Hertzian-type contacts. The damage fabric and its spatial association with the fault are consistent with overall formation under directed compressional tectonic stressing and not burial-induced stressing, and cannot be used to distinguish between proposed off-fault damage mechanisms. The distribution of microfracture fabric and density in the active SAF damage zone is consistent with the microfracture fabrics and density distribution of the exhumed North-branch San Gabriel fault and the Punchbowl fault (CHESTER et al., 2004b). Acknowledgments We thank two anonymous referees for constructive comments that improved the quality of this manuscript. This study was funded by the Southern California Earthquake Center (based on NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008), and by the National Science grant EAR0510892 to JSC. Vol. 166, 2009 Damaged Sandstones along the San Andreas Fault 1771 Appendix 1 Table 5 Shapiro-Wilk and Shapiro-Francia W tests for normality of the distribution of FIPL results in Table 2, 3 Shapiro-Wilk W’ test for normal data Variable 8E-b_104 set_10 set_9 set_8 set_7 set_6 set_5 set_4 set_3 set_2 set_1 8E-a 8E-c 31 28 27 6a Obs 104 40 40 40 40 40 40 40 40 40 40 43 43 47 47 46 41 W V 0.82 0.88 0.87 0.83 0.85 0.83 0.76 0.87 0.86 0.75 0.82 0.94 0.74 0.88 0.83 0.75 0.92 15.08 4.55 5.03 6.54 5.94 6.54 9.47 5.09 5.52 9.69 7.00 2.64 10.89 5.58 7.63 10.87 3.18 Shapiro-Francia W’ test for normal data Variable 8E-b_104 set_10 set_9 set_8 set_7 set_6 set_5 set_4 set_3 set_2 set_1 8E-a 8E-c 31 28 27 6a Obs 104 40 40 40 40 40 40 40 40 40 40 43 43 47 47 46 41 W’ 0.82 0.88 0.88 0.83 0.85 0.83 0.76 0.87 0.86 0.75 0.82 0.94 0.73 0.88 0.83 0.75 0.92 z Prob > z 6.03 3.19 3.40 3.95 3.75 3.95 4.73 3.42 3.60 4.78 4.10 2.05 5.05 3.65 4.32 5.06 2.44 0 0.00071 0.00034 0.00004 0.00009 0.00004 0 0.00031 0.00016 0 0.00002 0.02006 0 0.00013 0.00001 0 0.00737 z Prob > z 5.35 3.01 3.10 3.63 3.43 3.61 4.27 3.11 3.34 4.32 3.76 1.80 4.51 3.35 3.87 4.49 2.38 0.00001 0.00132 0.00098 0.00014 0.0003 0.00015 0.00001 0.00093 0.00042 0.00001 0.00008 0.0359 0.00001 0.00041 0.00005 0.00001 0.00871 V’ 16.60 5.16 5.43 7.36 6.58 7.27 10.69 5.47 6.23 10.99 7.94 2.62 12.23 6.15 8.30 11.94 3.61 REFERENCES BARROWS, A.G., KAHLE, J.E., and BEEBY, D.J. 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