Thermal Pressurization Of Pore Fluid Research Articles

Absolute stress levels in models of low-heat faults: Links to geophysical observables and differences for crack-like ruptures and self-healing pulses

Absolute levels of stress on faults have profound implications for earthquake physics and fault mechanics. A number of observations suggest that well-developed, mature faults such as the San Andreas Fault are generally “weak,” i.e. operate at much lower levels of shear stress compared to the higher expected shear resistance ∼100 MPa at seismogenic depths. In particular, low heat flow measurements suggest shear stress levels of ∼10 MPa or less on highly localized faults. Geodynamic constraints based on topography and similar considerations also support “weak” fault operation, and are comparable with heat-based constraints for some mature faults, but potentially higher for regions with substantial topography. Here, we investigate measures of average fault shear stress and their relationship to geophysically inferable quantities using numerical simulations of earthquake sequences on rate-and-state faults with low heat production, due to chronic fluid overpressure and/or enhanced dynamic weakening from the thermal pressurization of pore fluids. We review the earthquake energy balance, focusing on energy-based definitions of average shear stress and how the average fault prestress (a measure of fault strength plausibly relevant to geodynamic constraints) can be expressed as the sum of the dissipation-based average rupture stress (which can, in principle, be inferred from shear-heating constraints), and seismologically inferable source properties, such as the static stress drop and apparent stress. Our modeling demonstrates that rapid dynamic weakening and healing of shear resistance during ruptures, as exhibited in self-healing pulses, allows faults to maintain higher average interseismic stress levels despite low dynamic resistance and realistic static stress drops, providing a physical explanation for potential differences between topography-based and heat-based constraints on fault shear stress. In our models, the difference is related to stress undershoot and apparent stress, which can be as large as 1-3 times the static stress drop based on our simulations. Yet suitably large values of apparent stress (and hence radiated energy) are rarely inferred for natural earthquakes, either because radiated energy is underestimated, or suggesting that most large earthquakes do not propagate as sharp enough self-healing pulses with sufficiently large undershoot. Our results emphasize the distinction between dynamic versus static stress changes when relating earthquake source observations to absolute levels of fault stress and suggest that reviewing estimates of radiated energy and static stress drop from large earthquakes, with input from finite-fault numerical modeling, may improve constraints on absolute fault stress levels.

How Does Thermal Pressurization of Pore Fluids Affect 3D Strike-Slip Earthquake Dynamics and Ground Motions?

ABSTRACT Frictional heating during earthquake rupture raises the fault-zone fluid pressure, which affects dynamic rupture and seismic radiation. Here, we investigate two key parameters governing thermal pressurization of pore fluids – hydraulic diffusivity and shear-zone half-width – and their effects on earthquake rupture dynamics, kinematic source properties, and ground motions. We conduct 3D strike-slip dynamic rupture simulations assuming a rate-and-state dependent friction law with strong velocity weakening coupled to thermal-pressurization of pore fluids. Dynamic rupture evolution and ground shaking are densely evaluated across the fault and Earth’s surface to analyze the variations of rupture parameters (slip, peak slip rate, rupture speed, and rise time), correlations among rupture parameters, and variability of peak ground velocity. Our simulations reveal how variations in thermal-pressurization affect earthquake rupture properties. We find that the mean slip and rise time decrease with increasing hydraulic diffusivity, whereas mean rupture speed and peak slip-rate remain almost constant. Mean slip, peak slip-rate, and rupture speed decrease with increasing shear-zone half-width, whereas mean rise time increases. Shear-zone half-width distinctly affects the correlation between rupture parameters, especially for parameter pairs (slip, rupture speed), (peak slip-rate, rupture speed), and (rupture speed, rise time). Hydraulic diffusivity has negligible effects on these correlations. Variations in shear-zone half-width primarily impact rupture speed, which then may affect other rupture parameters. We find a negative correlation between slip and peak slip-rate, unlike simpler dynamic rupture models. Mean peak ground velocities decrease faster with increasing shear-zone half-width than with increasing hydraulic diffusivity, whereas ground-motion variability is similarly affected by both the parameters. Our results show that shear-zone half-width affects rupture dynamics, kinematic rupture properties, and ground shaking more strongly than hydraulic diffusivity. We interpret the importance of shear-zone half-width based on the characteristic time of diffusion. Our findings may inform pseudodynamic rupture generators and guide future studies on how to account for thermal-pressurization effects.

Scale Dependence of Earthquake Rupture Prestress in Models With Enhanced Weakening: Implications for Event Statistics and Inferences of Fault Stress

AbstractDetermining conditions for earthquake slip on faults is a key goal of fault mechanics highly relevant to seismic hazard. Previous studies have demonstrated that enhanced dynamic weakening (EDW) can lead to dynamic rupture of faults with much lower shear stress than required for rupture nucleation. We study the stress conditions before earthquake ruptures of different sizes that spontaneously evolve in numerical simulations of earthquake sequences on rate‐and‐state faults with EDW due to thermal pressurization of pore fluids. We find that average shear stress right before dynamic rupture (aka shear prestress) systematically varies with the rupture size. The smallest ruptures have prestress comparable to the local shear stress required for nucleation. Larger ruptures weaken the fault more, propagate over increasingly under‐stressed areas due to dynamic stress concentration, and result in progressively lower average prestress over the entire rupture. The effect is more significant in fault models with more efficient EDW. We find that, as a result, fault models with more efficient weakening produce fewer small events and result in systematically lower b‐values of the frequency‐magnitude event distributions. The findings (a) illustrate that large earthquakes can occur on faults that appear not to be critically stressed compared to stresses required for slip nucleation; (b) highlight the importance of finite‐fault modeling in relating the local friction behavior determined in the lab to the field scale; and (c) suggest that paucity of small events or seismic quiescence may be the observational indication of mature faults that operate under low shear stress due to EDW.

Rupture-dependent breakdown energy in fault models with thermo-hydro-mechanical processes

Abstract. Substantial insight into earthquake source processes has resulted from considering frictional ruptures analogous to cohesive-zone shear cracks from fracture mechanics. This analogy holds for slip-weakening representations of fault friction that encapsulate the resistance to rupture propagation in the form of breakdown energy, analogous to fracture energy, prescribed in advance as if it were a material property of the fault interface. Here, we use numerical models of earthquake sequences with enhanced weakening due to thermal pressurization of pore fluids to show how accounting for thermo-hydro-mechanical processes during dynamic shear ruptures makes breakdown energy rupture-dependent. We find that local breakdown energy is neither a constant material property nor uniquely defined by the amount of slip attained during rupture, but depends on how that slip is achieved through the history of slip rate and dynamic stress changes during the rupture process. As a consequence, the frictional breakdown energy of the same location along the fault can vary significantly in different earthquake ruptures that pass through. These results suggest the need to reexamine the assumption of predetermined frictional breakdown energy common in dynamic rupture modeling and to better understand the factors that control rupture dynamics in the presence of thermo-hydro-mechanical processes.

Nearly Magnitude‐Invariant Stress Drops in Simulated Crack‐Like Earthquake Sequences on Rate‐and‐State Faults with Thermal Pressurization of Pore Fluids

AbstractStress drops, inferred to be magnitude‐invariant, are a key characteristic used to describe natural earthquakes. Theoretical studies and laboratory experiments indicate that enhanced dynamic weakening, such as thermal pressurization of pore fluids, may be present on natural faults. At first glance, magnitude invariance of stress drops and enhanced dynamic weakening seem incompatible since larger events may experience greater weakening and should thus have lower final stresses and higher stress drops. We hypothesize that enhanced dynamic weakening can be reconciled with magnitude‐invariant stress drops due to larger events having lower average prestress when compared to smaller events. We conduct numerical simulations of long‐term earthquake sequences in fault models with rate‐and‐state friction and thermal pressurization, and in the parameter regime that results mostly in crack‐like ruptures, we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude. Smaller events indeed have larger average initial stresses than medium‐sized events, and we find nearly constant stress drops for events spanning up to two orders of magnitude in average slip, comparable to approximately six orders of magnitude in seismic moment. Segment‐spanning events have more complex behavior, which depends on the properties of the arresting velocity‐strengthening region at the edges of the faults.

A fully coupled thermo-poro-mechanical finite element analysis to predict the thermal pressurization and thermally induced pore fluid flow in soil media

Temperature gradient and heat transfer in saturated porous media may affect pore fluid pressure and/or pore fluid flow depending on thermal, mechanical, and hydraulic properties of the media and the saturating fluid. Therefore, several Thermo-hydro-mechanical (THM) models have been developed to theoretically analyze the thermal behavior of soil media. In this study a coupled thermo-poro-mechanical model is adopted to investigate the heat and mass transfer in deformable porous media in a transient and quasi-steady state conditions. The Effects and importance of different properties of porous media are carefully taken into account to simulate two different cases with different scenarios. The results confirm that in order to accurately predict the thermal pressurization and changes in total stress, temperature dependency of properties of the soil and saturating fluid must be considered carefully. It is found that even a slight perturbation in porosity variation and temperature dependency of the thermal expansion coefficient of the fluid can greatly influence the thermal pressurization of pore fluid in very low permeable soils (e.g. clays), while variations of pore fluid density governs thermally-induced pore fluid flow in high permeable soils (e.g. sands and silty sands). Moreover, the feasibility of heat convection and heat-induced pore fluid flow is discussed in a parametric study with different temperature and permeability values. The results demonstrate that the Boussinesq approximation is a key assumption when dealing with heat-induced pore fluid flow in quasi-steady state condition.

Ubiquitous weakening of faults due to thermal pressurization

Faults weaken during earthquakes. Analysis of the amount of energy released during earthquakes globally suggests that heat-induced pressurization of pore fluids can weaken faults during earthquakes of all sizes. Laboratory simulations of earthquakes show that at high slip rates, faults can weaken significantly, aiding rupture1,2,3. Various mechanisms, such as thermal pressurization and flash heating, have been proposed to cause this weakening during laboratory experiments1,4,5,6, yet the processes that aid fault slip in nature remain unknown. Measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening, known as an event’s fracture energy7,8,9. Here we compile new and existing8,9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes. We observe a distinct transition in how fracture energy scales with event size, which implies that faults weaken differently during small and large earthquakes, and earthquakes are not self-similar. We use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms. We find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes, over seven orders of fault slip magnitude. We conclude that thermal pressurization is a widespread and prominent process for fault weakening.

Jointing around magmatic dikes as a precursor to the development of volcanic plugs

Basaltic volcanic eruptions typically initiate as fissures fed by dikes. In some cases, eruptions become localized through discrete vents, until the flow is sustained through a single vent underlain by a volcanic plug, at which point, the eruption is capable of delivering much greater volumes of magma to the surface. Existing theories to explain flow localization focus on the relationship between non-uniform dike thickness and variations in magma flow rates. Here, we propose a mechanism for conduit widening that precedes flow localization, and we present geologic evidence from an Oligocene-aged dike-plug system at Ship Rock, New Mexico. We use techniques of structure from motion to map 621 joints in the sedimentary rock adjacent to the intrusions and find that these joints are part of a systematic set oriented perpendicular to the dikes and localized to within a meter of the dike contacts. This joint set, along with a set of dike-parallel joints and the bedding planes, divides the strata into rectangular blocks that subsequently are entrained in the flowing magma to widen the dikes. Using field and lab data and a stress analysis, we show that the dike-perpendicular joints are intimately associated with the emplacement of magma and thermal pressurization of pore fluids in the host rock. This fracturing process is a precursor to entrainment of the host rock and sustained flow through discrete vents.

Dynamic weakening of smectite‐bearing faults at intermediate velocities: Implications for subduction zone earthquakes

AbstractThe hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water‐saturated smectite‐quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite‐bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly <30 wt %). Drastic slip weakening occurred at smectite contents of 10–30 wt % at slip rates of ~10 mm/s, which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear‐enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

A scenario for the generation process of the 2011 Tohoku earthquake based on dynamic rupture simulation: Role of stress concentration and thermal fluid pressurization

We perform dynamic rupture simulations to improve the understanding of the generation process of the 2011 Tohoku earthquake. We assume a dynamic weakening mechanism (dynamic thermal pressurization of pore fluid, hereinafter called TP) on the fault plane to represent nonlinear weakening friction, and take into account the shear stress changes before the Tohoku earthquake, due to the four M 7-class earthquakes that occurred during 2003–2011. To constrain the dynamic rupture simulation, the moment release rate obtained by seismic slip inversions is referred. The simulation result implies the following about the 2011 Tohoku earthquake: (1) The rupture around the hypocenter was enhanced by the stress accumulation due to the preceding M 7-class earthquakes. (2) The enhanced rupture triggered the TP mechanism in the near-trench area causing large slip, which promoted propagation of the rupture over a wide region including the source areas of the M 7-class earthquakes and surrounding conditionally-stable areas. (3) Without sufficient stress accumulation, the moment release of the Tohoku earthquake ended as an M 8-class earthquake. (4) TP in the near-trench area should be effective but moderate. The occurrence time of the next megaquake would be strongly affected by the nonlinear effects of TP and the stress conditions. Thus, our model may contradict the concept of the (quasi-)cyclic occurrence of M 9 earthquakes.

Seismic and aseismic slip pulses driven by thermal pressurization of pore fluid

There are several lines of evidence that suggest that thermal pressurization (TP) of pore fluid within a low‐permeability fault core may play the key role in the development of earthquake slip. To elucidate effects of TP on spontaneous fault slip, we consider solutions for a steadily propagating slip pulse on a fault with a constant sliding friction, the level of which may reflect other thermally‐activated processes at the rupture front (such as the flash heating on asperities). Upon arrival of the pulse front, essentially undrained‐adiabatic TP takes place during the initial slip acceleration from the locked state with a corresponding reduction of the fault strength. With passage of time, the diminishing rate of heating (due to the reduced fault strength) and increasing rate of hydrothermal diffusion from the shear zone offset TP and result in partial recovery of the strength, slip deceleration and eventual locking and healing of the slip. We show that the rupture speedvr decreases with thickness hof the principal shear zone. For lab‐constrained values of fault‐gouge parameters, the TP‐pulse solution predicts seismic (vr∼ km/s) slip on a millimeter‐to‐cm thin principal shear zone; and aseismic slip withvr ∼ 10 km/day and slip rates 1–2 orders above the plate rate on a relatively thick (h∼ 1 m) shear zone. These and other predictions of the TP‐pulse model are consistent with the independent sets of observational constraints for large crustal and subduction interplate earthquakes, and slow slip transients (North Cascadia), respectively. Locking of the slip soon after the diffusive transport of the heat and pore fluid becomes efficient significantly limits the maximum co‐seismic temperature rise to values well below previous theoretical estimates. As a result, the onset of macroscopic melting and some of thermal decomposition reactions, recently suggested to explain strong co‐seismic fault weakening, are precluded over much of the seismogenic zone.

Megaquake cycle at the Tohoku subduction zone with thermal fluid pressurization near the surface

For the 2011 Tohoku earthquake, we propose a mechanical model to explain rare giant (M9-class) and frequent large (M7-class) earthquakes on a thrust fault in the subduction zone. Observations implied, in the M9 Tohoku earthquake, that extremely large slip on the order of tens of meters occurs in a shallower part to release a slip deficit, as well as substantial slip about ten meters or so in a deeper part including the source area of the M7-class earthquakes. Here, we present a model in which the extremely large slip is caused by hydrothermal weakening (dynamic thermal pressurization of pore fluid) on the fault plane, not by contrast of frictional properties in terms of rate- and state-dependent friction. The model explains that the Tohoku earthquake followed a M7-class earthquake in two days, but M7-class earthquakes are not always followed by a giant earthquake. In a giant event, large coseismic slip can occur over an area where quasistatic slip, namely, afterslip of M7-class earthquakes or spontaneous slow slip events, takes place. Slight differences of stress state in the shallow part can result in drastically different coseismic slips. We further perform numerical experiments varying hydraulic parameters and the length of effective hydrothermal weakening area. The experiments imply that observations for monitoring the effective hydrothermal weakening area need spatial resolution on the order of 10km or finer.

Evidence of thermal pressurization in high-velocity friction experiments on smectite-rich gouges

Terra Nova, 22, 347–353, 2010 Abstract Thermal pressurization of pore fluid is one of the possible mechanisms responsible for dynamic weakening in landslides and earthquakes, but, to date, has not been reproduced in the laboratory. Here, we report high-velocity experiments performed in a rotary shear friction apparatus on smectite-rich gouges from the 1963 Vaiont landslide (Italy). The gouges were slid under 1 MPa normal stress, for displacements up to 30 m and a slip rate of 1.31 m s−1 under room-humidity and water-saturated conditions. Sample dilatancy was observed in room-humidity runs after ∼3–4 m of slip, concomitant with an increase in normal stress and a decrease in shear stress. Mineralogical and microstructural investigations suggest that dilatancy resulted from expansion of the H2O released by the collapse of the smectite structure due to frictional heating of the slipping zone at T >200 °C. We conclude that sample dilatancy is due to thermal pressurization of the clay-rich gouge.

Earthquake ruptures with thermal weakening and the operation of major faults at low overall stress levels

We model ruptures on faults that weaken in response to flash heating of microscopic asperity contacts (within a rate‐and‐state framework) and thermal pressurization of pore fluid. These are arguably the primary weakening mechanisms on mature faults at coseismic slip rates, at least prior to large slip accumulation. Ruptures on strongly rate‐weakening faults take the form of slip pulses or cracks, depending on the background stress. Self‐sustaining slip pulses exist within a narrow range of stresses: below this range, artificially nucleated ruptures arrest; above this range, ruptures are crack‐like. Natural earthquakes will occur as slip pulses if faults operate at the minimum stress required for propagation. Using laboratory‐based flash heating parameters, propagation is permitted when the ratio of shear to effective normal stress on the fault is 0.2–0.3; this is mildly influenced by reasonable choices of hydrothermal properties. The San Andreas and other major faults are thought to operate at such stress levels. While the overall stress level is quite small, the peak stress at the rupture front is consistent with static friction coefficients of 0.6–0.9. Growing slip pulses have stress drops of ∼3 MPa; slip and the length of the slip pulse increase linearly with propagation distance at ∼0.14 and ∼30 m/km, respectively. These values are consistent with seismic and geologic observations. In contrast, cracks on faults of the same rheology have stress drops exceeding 20 MPa, and slip at the hypocenter increases with distance at ∼1 m/km.

What Does Control Earthquake Ruptures and Dynamic Faulting? A Review of Different Competing Mechanisms

The fault weakening occurring during an earthquake and the temporal evolution of the traction on a seismogenic fault depend on several physical mechanisms, potentially concurrent and interacting. Recent laboratory experiments and geological field observations of natural faults revealed the presence, and sometime the coexistence, of thermally activated processes (such as thermal pressurization of pore fluids, melting of gouge and rocks, material property changes, thermally-induced chemical environment evolution), elasto-dynamic lubrication, porosity and permeability evolution, gouge fragmentation and wear, etc. In this paper, by reviewing in a unifying sketch all possible chemico-physical mechanisms that can affect the traction evolution, we suggest how they can be incorporated in a realistic fault governing equation. We will also show that simplified theoretical models that idealistically neglect these phenomena appear to be inadequate to describe as realistically as possible the details of breakdown process (i.e., the stress release) and the consequent high frequency seismic wave radiation. Quantitative estimates show that in most cases the incorporation of such nonlinear phenomena has significant, often dramatic, effects on the fault weakening and on the dynamic rupture propagation. The range of variability of the value of some parameters, the uncertainties in the relative weight of the various competing mechanisms, and the difference in their characteristic length and time scales sometime indicate that the formulation of a realistic governing law still requires joint efforts from theoretical models, laboratory experiments and field observations.

Thermal decomposition of carbonates in fault zones: Slip‐weakening and temperature‐limiting effects

During an earthquake, the heat generated by fault friction may be large enough to activate the devolatilization of minerals forming the fault rocks. In this paper, we model the mechanical effects of calcite thermal decomposition on the slip behavior of a fault zone during an earthquake. To do so, we introduce the coupled effects of calcite volume loss, heat consumption, and CO2 production in the theoretical analysis of shear heating and thermal pressurization of pore fluids. We consider a rapidly deforming shear band consisting of a fluid‐saturated carbonate rock. The equations that govern the evolution of pore pressure and temperature inside the band and the mass of emitted CO2 are deduced from the mass and energy balance of the multiphase‐saturated medium and from the kinetics of the chemical decomposition of calcite. Numerical simulation of seismic slip at depths of 5 to 8 km show that decarbonation has two critical consequences on fault slip. First, the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than ∼800°C (corresponding to the initiation of the chemical reaction) inside the shear band. Second, the rapid emission of CO2 by decarbonation significantly increases the slip‐weakening effect of thermal pressurization. The pore pressure reaches a maximum and then decreases due to the reduction of solid volume, causing a restrengthening of shear stress. Our theoretical study shows, on the example of decarbonation, that the thermal decomposition of minerals is an important slip‐weakening process and that a large part of the frictional heat of earthquakes may go into endothermic devolatilization reactions.

Effects of supershear rupture speed on the high‐frequency content of S waves investigated using spontaneous dynamic rupture models and isochrone theory

In this paper we achieve three goals: (1) We demonstrate that crack tips governed by friction laws, including slip weakening, rate‐ and state‐dependent laws, and thermal pressurization of pore fluids, propagating at supershear speed have slip velocity functions with reduced high‐frequency content compared to crack tips traveling at subshear speeds. This is demonstrated using a fully dynamic, spontaneous, three‐dimensional earthquake model, in which we calculate fault slip velocity at nine points (locations) distributed along a quarter circle on the fault where the rupture is traveling at supershear speed in the in‐plane direction and subshear speed in the antiplane direction. This holds for a fault governed by the linear slip‐weakening constitutive equation, by slip weakening with thermal pressurization of pore fluid, and by rate‐ and state‐dependent laws with thermal pressurization. The same is also true even assuming a highly heterogeneous initial shear stress field on the fault. (2) Using isochrone theory, we derive a general expression for the spectral characteristics and geometric spreading of two pulses arising from supershear rupture, the well‐known Mach wave, and a second lesser known pulse caused by rupture acceleration. (3) We demonstrate that the Mach cone amplification of high frequencies overwhelms the de‐amplification of high‐frequency content in the slip velocity functions in supershear ruptures. Consequently, when earthquake ruptures travel at supershear speed, a net enhancement of high‐frequency radiation is expected, and the alleged “low” peak accelerations observed for the 2002 Denali and other large earthquakes are probably not caused by diminished high‐frequency content in the slip velocity function, as has been speculated.

Heating and weakening of faults during earthquake slip

Field observations of mature crustal faults suggest that slip in individual events occurs primarily within a thin shear zone, <1–5 mm, within a finely granulated, ultracataclastic fault core. Relevant weakening processes in large crustal events are therefore suggested to be thermal, and to involve the following: (1) thermal pressurization of pore fluid within and adjacent to the deforming fault core, which reduces the effective normal stress and hence also the shear strength for a given friction coefficient and (2) flash heating at highly stressed frictional microcontacts during rapid slip, which reduces the friction coefficient. (Macroscopic melting, or possibly gel formation in silica‐rich lithologies, may become important too at large enough slip.) Theoretical modeling of mechanisms 1 and 2 is constrained with lab‐determined hydrologic and poroelastic properties of fault core materials and lab friction studies at high slip rates. Predictions are that strength drop should often be nearly complete at large slip and that the onset of melting should be precluded over much (and, for small enough slip, all) of the seismogenic zone. A testable prediction is of the shear fracture energies that would be implied if actual earthquake ruptures were controlled by those thermal mechanisms. Seismic data have been compiled on the fracture energy of crustal events, including its variation with slip in an event. It is plausibly described by theoretical predictions based on the above mechanisms, within a considerable range of uncertainty of parameter choices, thus allowing the possibility that such thermal weakening prevails in the Earth.

A fault constitutive relation accounting for thermal pressurization of pore fluid

The heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip. The amount of fluid pressure rise depends on the associated fluid flow. The heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time. If the slip velocity function is crack‐like, there is a prompt fluid pressure rise at the onset of slip, followed by a slower increase. The stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance. The threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity. If hydraulic diffusivity is 0.02 m2/s, estimated from borehole samples of fault zone material, the threshold propagation distance is 300 m. The stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity, both of which will lessen the fluid pressure effect. Nevertheless, if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value, then stress drop is complete in large earthquakes.

Dike intrusion as a trigger for large earthquakes and the failure of volcano flanks

Pore fluid pressures induced through dike intrusion have the capability to trigger large earthquakes and to initiate and sustain massive and catastrophic failure of volcano flanks. Suprahydrostatic pore pressures are generated as a result of both mechanical and thermal straining of the rock‐fluid medium. Mechanical strains and resulting pore pressures are described using an analogy to a moving volumetric dilation within a porous elastic medium. Thermal pore fluid pressures are simply represented by a static diffusive model subjected to uniform temperature rise at the dike interface. Resulting excess pore pressure distributions acting along the base of a wedge‐shaped slide block are used to define the stability of the flanks when subjected to the magma pressures that accompany intrusion. The destabilizing influence of mechanically induced pore pressures is maximized as the intruded width, or corresponding overpressure, of the dike is increased. For realistic parameter magnitudes in volcaniclastic materials, induced pore pressures are capable of initiating failure; however, the mechanical influence is restricted to the proximity of the intrusion, and by itself may not be capable of sustaining sliding motion once it is initiated. The destabilizing influence of thermally induced pore pressures is conditioned by the severity of thermal forcing, ratios of thermal and hydraulic diffusivities, and the time available for the combined thermal and pressure disturbance to propagate outward from the dike. Thermal pressurization of pore fluid extends more slowly than mechanical straining but is shown to be capable of developing massive uplift forces that could initiate and possibly sustain failure along downslope‐dipping failure planes. Deviatoric stress‐induced generation of pore pressures and frictional heating of pore fluids may act following slide initiation to maintain the impetus of flank failure and enable long runout over compressible marine sediments. In addition to the development of shallow failure, limited deep‐seated sliding may also result from the mechanical or thermal pore pressure loading mechanisms. In addition, pore pressures generated through intrusion or frictional heating may trigger, or amplify, large earthquakes, and these in turn may aid shallower flank slip through seismic ground accelerations and vibration‐induced pore pressure generation. These phenomenological models of pore pressure rise and stability control may aid comprehension of the cyclic growth, lateral expansion, and subsequent destruction of shield and stratovolcano flanks. Examples in this work refer particularly to oceanic volcanoes typified by the Hawaiian Islands and Réunion Island.