Brett M. Carpenter

Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.

Frictional properties and sliding stability of the San Andreas fault from deep drill core

The strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We find that the fault is extremely weak (friction coefficient, μ = ∼ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of μ > ∼0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ∼10 MPa in the upper 5–8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes.

Frictional strength and healing behavior of phyllosilicate‐rich faults

[1] We study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals. We report laboratory and microstructural work from fault rocks associated with a regional, low-angle normal fault in Central Italy. Experiments were conducted in a biaxial deformation apparatus at room temperature and humidity, nominally dry, under constant normal stresses of 20 and 50 MPa, and at a sliding velocity of 10 mm/s. Our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure. The phyllosilicate contents of our samples, which include clay, talc and chlorite range from 0 to 52 weight %. We study both intact rock samples, sheared in their in situ geometry, and powders made from the same rocks to address the role of fabric in fault healing. We measured frictional healing, Dm, using slide-hold-slide tests with hold periods ranging from 3 to 3000 s. Phyllosilicate-free materials show friction values of m ≈ 0.6 and healing rates that are larger in powdered samples, b ≈ 0.006 (Dm per decade in time, s) compared to intact wafers of fault rock, b ≈ 0.004. For phyllosilicate-bearing materials, healing rates are low, b < 0.002, and independent of fabric, phyllosilicate content and normal stress. We observe that frictional strength decreases systematically with increasing phyllosilicate content. Intact, phyllosilicate-bearing fault rock is consistently weaker than its powdered equivalent (0.2 < m < 0.3 versus 0.4 < m < 0.5, respectively). We compare our data to results from experiments conducted on a wide range of materials and conditions. Deformation microstructures show localized slipping along sub-parallel shear planes. We suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate-bearing faults and promote stable, aseismic creep.

Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling

The first phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer- to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20?30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifically the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.

Frictional heterogeneities on carbonate‐bearing normal faults: Insights from the Monte Maggio Fault, Italy

Observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and/or spatial heterogeneity of fault frictional behavior. Such complex slip patterns have been observed for earthquakes on normal faults throughout central Italy, where many of the Mw 6 to 7 earthquakes in the Apennines nucleate at depths where the lithology is dominated by carbonate rocks. To explore the relationship between fault structure and heterogeneous frictional properties, we studied the exhumed Monte Maggio Fault, located in the northern Apennines. We collected intact specimens of the fault zone, including the principal slip surface and hanging wall cataclasite, and performed experiments at a normal stress of 10 MPa under saturated conditions. Experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 MPa stress drop as the fault surface fails, then velocity-neutral frictional behavior and significant frictional healing. Overall, our results suggest that (1) earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface; (2) postseismic slip is more likely to occur in areas of the fault where gouge is present; and (3) high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop.

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

We present results from a comprehensive laboratory study of the frictional strength and constitutive properties for all three active strands of the San Andreas Fault penetrated in the San Andreas Observatory at Depth (SAFOD). The SAFOD borehole penetrated the Southwest Deforming Zone (SDZ), the Central Deforming Zone (CDZ), both of which are actively creeping, and the Northeast Boundary Fault (NBF). Our results include measurements of the frictional properties of cuttings and core samples recovered at depths of ~2.7 km. We find that materials from the two actively creeping faults exhibit low frictional strengths (μ = ~0.1), velocity-strengthening friction behavior, and near-zero or negative rates of frictional healing. Our experimental data set shows that the center of the CDZ is the weakest section of the San Andreas Fault, with μ = ~0.10. Fault weakness is highly localized and likely caused by abundant magnesium-rich clays. In contrast, serpentine from within the SDZ, and wall rock of both the SDZ and CDZ, exhibits velocity-weakening friction behavior and positive healing rates, consistent with nearby repeating microearthquakes. Finally, we document higher friction coefficients (μ > 0.4) and complex rate-dependent behavior for samples recovered across the NBF. In total, our data provide an integrated view of fault behavior for the three active fault strands encountered at SAFOD and offer a consistent explanation for observations of creep and microearthquakes along weak fault zones within a strong crust.

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

Laboratory observations of time-dependent frictional strengthening and stress relaxation in natural and synthetic fault gouges

Interseismic recovery of fault strength (healing) following earthquake failure is a fundamental requirement of the seismic cycle and likely plays a key role in determining the stability and slip behavior of tectonic faults. We report on laboratory measurements of time- and slip-dependent frictional strengthening for natural and synthetic gouges to evaluate the role of mineralogy in frictional strengthening. We performed slide-hold-slide (SHS) shearing experiments on nine natural fault gouges and eight synthetic gouges at conditions of 20 MPa normal stress, 100% relative humidity (RH), large shear strain (~15), and room temperature. Phyllosilicate-rich rocks show the lowest rates of frictional strengthening. Samples rich in quartz and feldspar exhibit intermediate rates of frictional strengthening, and calcite-rich gouges show the largest values. Our results show that (1) the rates of frictional strengthening and creep relaxation scale with frictional strength, (2) phyllosilicate-rich fault gouges have low strength and healing characteristics that promote stable, aseismic creep, (3) most natural fault gouges exhibit intermediate rates of frictional strengthening, consistent with a broad range of fault slip behaviors, and (4) calcite-rich fault rocks show the highest rates of frictional strengthening, low values of dilation upon reshear, and high frictional strengths, all of which would promote seismogenic behavior.

Physicochemical processes of frictional healing: Effects of water on stick‐slip stress drop and friction of granular fault gouge

Understanding the micromechanical processes that dictate the evolution of fault strength during the seismic cycle is a fundamental problem in earthquake physics. We report on laboratory experiments that investigate the role of water during repetitive stick-slip frictional sliding, with particular emphasis on the grain-scale and atomic-scale mechanisms of frictional restrengthening (healing). Our experiments are designed to test underlying concepts of rate and state friction laws. We sheared layers of soda-lime glass beads in a double direct shear configuration at a constant normal stress of 5 MPa. Shear stress was applied via a constant displacement rate from 0.3 to 300 µm/s. During each experiment, relative air humidity (RH) was kept constant at values of 5, 50, or 100%. Our data show a systematic increase in maximum friction (μmax), stick-slip friction drop (Δμ), and frictional healing rate, with increasing RH. The highest values of interevent dilation occur at 100% RH. Postexperiment scanning electron microscope observations reveal details of contact junction processes, showing a larger grain-to-grain contact area at higher RH. We find that the evolution of contact area depends inversely on slip velocity and directly on RH. Our results illuminate the fundamental processes that dictate stick-slip frictional sliding and provide important constraints on the mechanisms of rate and state friction.

A microphysical interpretation of rate‐ and state‐dependent friction for fault gouge

The evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip, nature of earthquake stress drop, and earthquake nucleation. Laboratory-based rate- and state-dependent friction (RSF) laws can describe changes in fault strength during slip, but the connections between fault strength and the mechanisms that dictate the mode of failure, from aseismic creep to earthquake rupture, remain poorly understood. The empirical nature of RSF laws remains a drawback to their application in nature. Here we analyze an extensive data set of friction constitutive parameters with the goal of illuminating the microphysical processes controlling RSF. We document robust relationships between: (1) the initial value of sliding (or kinetic) friction, (2) RSF parameters, and (3) the time rates of frictional strengthening (aging). We derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by: (1) an activation energy that controls the rate of asperity growth by plastic creep, and (2) an inverse relationship between material hardness and the activation volume of plastic deformation. Collectively, our results illuminate the physics expressed by the RSF parameters, and which describe the absolute value of frictional strength and its dependence on time and slip rate. Moreover, we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain-scale asperity contacts.