The physical mechanisms governing slow earthquakes remain unknown, as does the relationship between slow and regular earthquakes. To investigate the mechanism(s) of slow earthquakes and related ...quasi‐dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration. We reproduced the full spectrum of slip behavior, from slow to fast stick slip, by altering the elastic stiffness of the loading apparatus (k) to match the critical rheologic stiffness of fault gouge (kc). Our experiments show an evolution from stable sliding, when k > kc, to quasi‐dynamic transients when k ~ kc, to dynamic instabilities when k < kc. To evaluate the microphysical processes of fault weakening we monitored variations of elastic properties. We find systematic changes in P wave velocity (Vp) for laboratory seismic cycles. During the coseismic stress drop, seismic velocity drops abruptly, consistent with observations on natural faults. In the preparatory phase preceding failure, we find that accelerated fault creep causes a Vp reduction for the complete spectrum of slip behaviors. Our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults, depending on frictional properties. In agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by Vp changes during the seismic cycle. The observed reduction in Vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high‐resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors.
Key Points
The full spectrum of fault slip behavior from slow‐ to fast‐slip events is observed in laboratory fault zones
We find clear changes in seismic velocity precursory to the stick‐slip failure events
The evolution of shear stress on a fault can be monitored by P‐wave velocity changes during seismic cycle
We report on frictional properties of rocks within the 3‐D crustal volume surrounding the San Andreas Fault Observatory at Depth (SAFOD). Samples include lithologies adjacent to the San Andreas Fault ...(SAF) in the subsurface, SAFOD borehole rocks, and synthetic fault gouge composed of talc, serpentinite, and quartz. Granodiorite, arkosic sandstone, and siltstone samples from the SAFOD borehole are frictionally strong (μ = 0.56 − 0.66). Sand and clay‐rich lithologies from outcrop exhibit friction in the range μ = 0.56 − 0.68. Natural serpentinite thought to abut the SAF at depth exhibits low friction (μ = 0.18 − 0.26). Our results indicate that 1) serpentinite exhibits low strength, but is not weak enough to completely satisfy weak fault models, 2) all other samples are consistent with a strong fault and crust and, 3) if the SAF is weak (μ ≤ 0.2) due to the presence of serpentinite or talc, these minerals would likely need to constitute over 50% by weight of the shear zone.
We investigate dynamic wave‐triggered slip under laboratory shear conditions. The experiment is composed of a three‐block system containing two gouge layers composed of glass beads and held in place ...by a fixed load in a biaxial configuration. When the system is sheared under steady state conditions at a normal load of 4 MPa, we find that shear failure may be instantaneously triggered by a dynamic wave, corresponding to material weakening and softening if the system is in a critical shear stress state (near failure). Following triggering, the gouge material remains in a perturbed state over multiple slip cycles as evidenced by the recovery of the material strength, shear modulus, and slip recurrence time. This work suggests that faults must be critically stressed to trigger under dynamic conditions and that the recovery process following a dynamically triggered event differs from the recovery following a spontaneous event.
Key Points
In the lab instantaneous dynamic triggering requires fault gouge and dynamic strains exceeding 10−6
Following triggering there is a recovery process that takes place over multiple stick‐slip cycles
We posit that similar processes may take place in situ
We investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low‐angle normal fault in Panamint Valley, California. Gouge samples were collected from ...the fault zone at five locations along a north–south transect of the range‐bounding fault system, spanning a variety of bedrock lithologies. Samples were powdered and sheared in the double‐direct shear configuration at room temperature and humidity. The coefficient of friction, μ, was measured at a range of normal stresses from 5 to 150 MPa for all samples. Our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous. Samples from a single location exhibit dramatic differences in behavior, yet all three were collected within a meter of the fault core. For most of the samples, friction varies from μ = 0.6 to μ = 0.7, consistent with Byerlee's law. However, samples with greater than 50 wt % total clay content were much weaker (μ = 0.2–0.4). Expandable clay content of the samples ranged from 10 to 40 wt %. Frictional weakness did not correlate with expandable clays. Our results indicate that friction decreases with increasing total clay content, rather than with the abundance of expandable clays. The combination of field relations, analytical results, and friction measurements indicates a positive correlation between clay content, fabric intensity, and localization of strain in the fault core. A mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material. We provide broad constraints of 1–5 km on the depth of gouge generation and the depth at which fault weakness initiates. We show that slip on the Panamint Valley fault and similar low‐angle normal faults is mechanically feasible in the mid‐upper crust if the strength of the fault is limited by weak, clay‐rich fault gouge.
We investigate the stick‐slip behavior of a granular system confined and sheared by deformable solid blocks using three‐dimensional discrete element method simulations. Our modeling results show that ...large slip events are preceded by a sequence of small slip events—microslips—whose occurrence accelerates exponentially before the large slip event onset. Microslips exhibit energy release several orders of magnitude smaller than the large slip events. The microslip event rate is proposed as a measure of slip activity in the granular gouge layer. A statistical analysis shows that microslip event rate correlates well with large slip event onset and that variations in it can be used to predict large slip events. The emergence of microslips and their duration are found to be controlled by the value of the slipping contact ratio and are therefore related to the jamming/unjamming transition of frictional granular packings.
Key Points
Microslip event rate is proposed as a measure of activity in granular layer
Microslip event rate correlates well with large slip events
Microslips emergence is controlled by the value of the slipping contact ratio
Faulting and brittle deformation of mantle rocks occurs in many tectonic settings such as oceanic transform faults, oceanic detachment faults, subduction zones, and continental rifts. However, few ...data exist that directly explore the frictional properties of peridotite rocks. Improved constraints on the brittle deformation of peridotite is important for a more complete understanding of the rheological properties of the lithosphere. Furthermore, our comparatively detailed understanding of plastic deformation in olivine allows us to explore the possible role of thermally activated intracrystalline deformation mechanisms in macroscopically brittle processes. It has been hypothesized, and some experimental data indicate, that plastic yielding by dislocation glide (low temperature plasticity) determines the direct effect in the rate and state frictional constitutive formulation. Plastic flow may also have important implications for the blunting or necking at asperity contacts that influences the time and/or displacement dependent friction evolution effect and frictional healing. We present results from saw cut experiments on fine grained synthetic olivine fault gouge conducted in a gas‐medium deformation apparatus in the temperature range of 400–1000°C with 100 MPa confining pressure. We conducted velocity stepping tests to explore the rate and temperature dependence of sliding stability. We also conducted slide‐hold‐slide experiments to investigate the time and temperature dependence of fault zone restrengthening (frictional healing). The mechanical data and microstructural observations allow us to explore the role of thermally activated processes in frictional sliding. The data indicate systematic temperature dependenceof rate and state variables that can be attributed to plastic yielding at grain to grain contacts. We explore the implications of such temperature dependent behavior for controlling the base of the seismogenic zone in the oceanic lithosphere, and we seek insight into possible mechanistic models for the interactions between fracture and flow that could lead to improved constraints on the strength of the lithosphere.
Key Points
Two transitions in sliding stability as a function of temperature are found
The transitions are linked to plastic deformation at asperity contacts
Mechanical transitions may explain observations from oceanic transform faults
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.
Key Points
Experiments motivated by field observations of heterogeneous fault structureLaboratory investigation into the implications of fault zone heterogeneityFault zone structure influences friction constitutive behavior and mode of slip
Subduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been ...studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well‐studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present‐day megathrust at depths of ∼5–>25 km, and we shear these at their peak in situ pressure‐temperature (P‐T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105°C to 470°C. We used pore fluid pressures of 45–240 MPa, corresponding to fluid overpressure ratios λ of 0.65 and 0.90. Slip velocities of 0.1–100 μm/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity‐strengthening (inherently stable) behavior under all conditions for λ = 0.65. For λ = 0.90, velocity‐weakening behavior was observed at 350°C, with velocity‐strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a‐b) increases with slip velocity at temperatures up to ∼200°C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts.
Plain Language Summary
The largest earthquakes on Earth occur along fault zones at subduction plate boundaries. These earthquakes are controlled, in large part, by the frictional properties of the fault zone materials. However, those properties are not fully understood, and laboratory experiments are key to advancing our knowledge. One example of a subduction zone is the Nankai Trough in southwest Japan. This subduction zone has been the subject of several drilling expeditions, recovering samples from the subsurface, and providing materials used in laboratory experiments. However, the experiments to date focus on materials recovered from depths shallower than those where earthquakes nucleate, and have been performed at temperatures lower than those in earthquake source regions. In this study, we used samples known to come from depths where earthquakes nucleate, and we performed friction experiments at temperatures and pressures relevant for these depths. We find that the frictional properties are primarily dependent on temperature and on the velocity of sliding.
Key Points
Friction experiments on samples exhumed in the Shimanto Belt and Sanbagawa Belt at sample‐specific in situ peak PT conditions
Behavior predominantly velocity‐strengthening with velocity‐weakening at 350°C
Frictional slip stability shows temperature‐dependent trend with slip velocity
The computation of fluid balances (FBs) by subtracting fluid outputs from inputs is a common critical care practice. Limited information exists about the accuracy and consistency of nurse-registered ...cumulative FBs and regarding the value of suggested corrections for non-measurable losses.
From 147 ICU patients, we prospectively evaluated the cumulative FBs and their relationship to changes in body weight (BW). Standardised measurements of BW were performed on admission and discharge. FB charts were accurately reviewed and arithmetic errors corrected. Net cumulative FBs and adjusted cumulative FBs (considering sensible or insensible fluid losses/fever/liquid faeces) were analysed for all patients and 3 subgroups (cardiac-cerebral, septic, and others). Agreement between FBs and BW changes was calculated according to the defined subgroups and confounding variables.
Cumulative FBs were inaccurate in 49 cases (33%) with errors ranging from -3606 mL to +2020 mL. The total (average daily) difference between measured BW and FBs (mean ± SD) was 0.185±1.874 kg (0.101±1.020 kg). Correlation (r(2)) and Bland-Altman agreement was poor between BW changes and net cumulative FBs (0.552 and -1.26±5.41 kg) and slightly better between BW changes and adjusted cumulative FBs (0.714 and +0.18±3.68 kg). Standard deviations of the average daily differences between BW changes and FBs were always >1 L. Correction of the net FBs as suggested in the literature were not useful. New multiple regression models only modestly improved correlation.
For a large portion of patients nurse-registered cumulative FBs are neither accurate nor do they agree with standardised BW measurements. Patient care and clinical decision-making should be based on more objective techniques.
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