Thermal Pressurization Research Articles

Core surprise: What's inside a plate boundary?

Despite the fact that 90% of global seismicity occurs at plate boundary faults, our understanding of their internal structure is lacking. It’s not easy to see inside a plate boundary fault – typically composed of a high-strain fault core surrounded by a fractured damage zone – and when we can, it often requires expensive drilling projects that yield limited information on the internal structure of the whole fault. Understanding the internal structure of large faults is crucial, because their chemical and mechanical properties control how and where earthquakes rupture, nucleate and propagate. This in turn limits the size of the earthquake or the amount of radiated seismic energy, and consequently the severity of surface damage. The 1999 magnitude 7.7 earthquake along the Chelungpu plate boundary fault, for example – the second deadliest earthquake in Taiwan’s recorded history – saw significant variations in slip and ground motion at different locations along the fault which resulted in large local variations in casualties and damage. Subsequent field investigations related these variations to changes in the fault’s structure (i.e., clay width, geometry), which in turn controlled how the fault moved.

Modeling of thermal pressurization in tight claystone using sequential THM coupling: Benchmarking and validation against in-situ heating experiments in COx claystone

We apply thermoporoelasticity and a sequentially coupling technique for modeling thermally-driven coupled Thermo-Hydro-Mechanical (THM) processes in tight claystone. A THM benchmark case with a corresponding analytic solution for thermoporoelasticity under a constant heat loading verifies the model. Thereafter, two in situ heating experiments are simulated for model validation: a smaller-scale heating experiment (TED experiment) and a larger-scale experiment (ALC experiment) in Callovo-Oxfordian (COx) claystone at the Meuse/Haute-Marne underground research laboratory in France. The model exhibits good performance to match the observed temperature and pore pressure evolution for the smaller-scale TED experiment. For the larger-scale ALC experiment, general trends of thermal-pressurization are captured in the modeling, but pressure is underestimated at some monitoring points during cool-down. This indicates that the THM response in the field may be affected by the variability of rock’s properties or irreversible or time-dependent mechanical processes that are not included in the current thermoporoelastic model. The main contributions of this work are as follows: (1) we verify and validate the numerical simulator, TOUGH-FLAC, to be a valuable coupled THM modeling tool; (2) prove that the laboratory determined material parameters can be used as reference values for upscaling experiments. However, to better identify and quantify THM processes with modeling of in situ tests, more emphasize should be dedicated to obtaining high-quality mechanical deformation data.

INFLUENCE OF VISCOSITY ON THE THERMAL ENERGY PRODUCED IN SEISMIC FAULT: ONE-DIMENSIONAL MODELING

This study is focused on thermal energy due to slip-weakening friction, but also on the temperature distribution. In this paper, we considered a one-body spring-slider model in the presence of slip-weakening friction and viscosity. We examine the effect of slip-weakening friction law and viscosity on thermal energy. We established the expression of thermal energy produced by slip-weakening friction in the case that we considered a simplified linearly slip-weakening friction law. Numerical simulation with Euler method are made for determining the thermal energy in the case of three others slip-weakening friction law, i.e., the thermal-pressurization (TP) friction law, the softening-hardening (SH) friction law and a simple slip-weakening (SW) friction law. The effect of the viscosity strongly modifies the results of the evolution of the temperature and the thermal energy. We establish that the TP friction law is better to evaluate the thermal energy in seismic fault. We solve the 1-D heat conduction equation by numerical simulation. We showed that, contrary to others frictions forces the slip-weakening friction law better illustrate the accumulation of thermal energy produced in seismic fault and temperature distribution.

Thermal Pressurization Weakening in Laboratory Experiments

AbstractThermal pressurization (TP) is expected to be a dominant frictional weakening mechanism during earthquakes. However, most of our understanding of TP relies on theoretical studies. Our experimental setup allows us to test the mechanical response of experimental faults under elevated confining and pore pressures in the laboratory using a rotary‐shear apparatus. We observe dynamic weakening by TP of water‐saturated faults in Frederick diabase sliding at subseismic slip rates of 2.5–5 mm/s under an effective normal stress of 25–50 MPa and effective confining pressure of 20–49 MPa. No other weakening mechanism is activated at these conditions in dry samples. The permeability of the samples is varied by heat treatment before each experiment, and the frictional behavior is tested by velocity‐stepping experiments. Dynamic weakening was investigated after both low and high total fault displacements. For low‐displacement samples: (1) the magnitude and rate of weakening increase as the sample permeability decreases and (2) frictional heating is limited during sliding. These observations are consistent with the expected behavior of TP from theory. Weakening of high‐displacement samples is similar in magnitude to low‐displacement faults, but with slower rates of weakening and less dilation. The difference in weakening of the high‐displacement samples may reflect changes in pore compressibility in the fault zone, which would have a significant effect on the frictional response during TP. The activation of TP at modest slip rates in these experiments supports the hypothesis that TP is an active, and likely dominant, dynamic weakening mechanism during earthquakes in the early stages of slip.

Variation of Hydraulic Properties Due to Dynamic Fracture Damage: Implications for Fault Zones

AbstractHigh strain rate loading causes pervasive dynamic microfracturing in crystalline materials, with dynamic pulverization being the extreme end‐member. Hydraulic properties (permeability, porosity, and storage capacity) are primarily controlled by fracture damage and will therefore change significantly by intense dynamic fracturing—by how much is currently unknown. Dynamic fracture damage observed in the damage zones of seismic faults is thought to originate from dynamic stresses near the earthquake rupture tip. This implies that during an earthquake, hydraulic properties in the damage zone change early. The immediate effect this has on fluid‐driven coseismic slip processes following the rupture, and on postseismic and interseismic fault zone processes, is not yet clear. Here, we present hydraulic properties measured on the full range of dynamic fracture damage up to dynamic pulverization. Dynamic damage was induced in quartz‐monzonite samples by performing uniaxial high strain rate (>100 s−1) experiments in compression using a split‐Hopkinson pressure bar. Hydraulic properties were measured on samples subjected to single and successive loadings, the latter to simulate cumulative damage from repeated rupture events. We show that permeability increases by 6 orders of magnitude and porosity by 15% with dissipated energy up to dynamic pulverization, for both single and successive loadings. We present damage zone permeability profiles induced by earthquake rupture and how it evolves with repeated ruptures. We propose that the enhanced hydraulic properties measured for pulverized rock decrease the efficiency of thermal pressurization, when emplaced adjacent to the principal slip zone.

Dilatancy-induced fluid pressure drop during dynamic rupture: Direct experimental evidence and consequences for earthquake dynamics

Fluid pressure and flow in the crust are key parameters controlling earthquake physics. Since earthquake slip is linked to spatio-temporal localisation of deformation, it is expected that the local fluid pressure around the fault plane could potentially impact the dynamic strength of the slipping fault zone. Coseismic fluid pressure drops have been inferred from field studies, notably in gold deposits which are thought to be formed by this process, but reliable quantitative predictions are still lacking. Here, experimental results are presented where local on- and off-fault fluid pressure variations were measured in situ during dynamic rock fracture and frictional slip under upper crustal stress conditions. During the main rupture, the on-fault fluid pressure dropped rapidly to zero, indicating partial vaporisation and/or degassing. Further deformation produced stick-slip events systematically associated with near-instantaneous drops in fluid pressure, providing direct experimental support of the concept of “seismic suction pump”. In situ fluid volume and wave speed measurements together with microstructural investigations show that dilatancy is the process driving fluid pressure drops during rupture and slip. Extrapolation of the laboratory results indicate that dilatancy-induced fluid pressure drops might be a widespread phenomenon in the crust, counteracting thermal pressurisation as a weakening mechanisms in freshly fractured rock.

A dynamic model for rapid startup of high-speed landslides based on the mechanism of friction-induced thermal pressurization considering vaporization

The friction-induced thermal pressurization mechanism in the shear band is an important reason for the transition of motion state of landslides from creeping to high-speed sliding during their initial startup phase. In this paper, a dynamic model for the startup phase of high-speed landslides with thermo-hydro-vapor-mechanical (THVM) coupling in shear band is established and applied to the Vaiont landslide for case study. The model consists of three governing equations: the equation of motion controls the motion of the overlying slide mass, the heat equation controls the heat or temperature change in the shear band at the bottom, and the pore pressure equation controls the change of excess pore pressure in the shear band. The main variables of these equations interact with each other. In addition to the thermal expansion pressurization of pore water, vaporizing pressurization is systematically considered for the first time in this model. The analysis results of the model applied to the Vaiont landslide show that, at 10 s, the excess pore pressure in the shear band is up to 4.5 MPa, the maximum temperature is 295 °C, and the velocity of each block is in the range of 24~31 m/s. The evolution of excess pore pressure in the shear band is divided into four stages which have been discussed in detail. The THVM model established in this paper can reflect the physical mechanism of thermal pressurization inside the shear band of high-speed landslides during their startup process to a certain extent, and it could also explain why some landslides quickly turn to catastrophic high-speed sliding after they start.

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.

Structural records and mechanical characteristics of seismic slip along an active fault crosscutting unconsolidated Quaternary sediments: Suryum fault, SE Korea

The Suryum fault is an active fault in southeast Korea crosscutting the Quaternary sediment deposit. We conducted outcrop-scale to microscale observations of the fault slip zone and performed shear experiments on the fault gouge at subseismic (3–30 µm/s) and seismic slip rates (0.53 m/s). The gouge is rich in clay minerals (> 58%), particularly in expandable clay minerals (40%). In the gouge zone, a very narrow (20–150 µm thick) principal slip zone (PSZ), identified by the strong alignment of clay minerals, was developed. At several places along the fault, the gouge is observed to have been injected into the Quaternary wallrock sediments. The development of the narrow PSZ is incompatible with the velocity-strengthening behavior of the Suryum fault gouge observed in the shear tests, and it may indicate that seismic rupture could be propagated along the PSZ in the clay-rich gouge. Slip localization into the PSZ was presumably possible because of significant dynamic fault weakening due to thermal pressurization (or buildup of pore pressure caused by frictional heating) in the low-permeability clay gouge during fast slip. The gouge injections along the Suryum fault may be the geological record of gouge fluidization caused by thermal pressurization. This idea is supported by observations of very low friction of the Suryum fault gouge when sheared experimentally at the seismic slip rate and of the gouge injections in the sheared gouge. The dynamic weakness of the Suryum fault implies that large displacement and low-frequency ground motion at near-surface depths would be possible when it is reactivated.

Temperature response and brine availability to heated boreholes in bedded salt

AbstractThere is a growing need for disposal of high‐level nuclear waste. To reduce uncertainty associated with brine availability to repository excavations in salt formations, a collaboration between Sandia, Los Alamos, and Lawrence Berkeley National Laboratories is performing a series of borehole‐scale coupled process tests. Here, we report on the first round of the Brine Availability Test in Salt (BATS) project, a “shakedown” experiment called Phase 1s. Experimental testing included placing a resistive heater, a 260‐W radiative heater, and a 750‐W radiative heater within previously drilled horizontal boreholes at the Waste Isolation Pilot Plant (WIPP) while monitoring temperature and water inflow. The experiments successfully achieved the targeted temperature of 120 °C when using the 750‐W radiative heater. Simulations using FEHM (Finite Element Heat and Mass transfer code) and TOUGH‐FLAC (Transport Of Unsaturated Groundwater and Heat–Fast Lagrangian Analysis of Continua) were able to accurately predict the coupled thermo–hydro–mechanical–chemical response of salt, matching the observed temperature and brine production. Due to the extremely low permeability of salt, these systems take many years to reach steady state when perturbed by mining activities. Long‐term numerical simulations are used to develop the initial pressure and saturation conditions. The inclusion of a damaged rock zone with higher permeability around the borehole also affects the saturation and pressure distributions and plays an important role in dissipating the potential for thermal pressurization. Knowledge gained from this round of experimentation and simulation will be used to conduct the next BATS project experiment in new boreholes at WIPP.

Water Availability and Deformation Processes in Smectite‐Rich Gouges During Seismic Slip

AbstractSmectite clays occur in subduction zone fault cores at shallow depth (approximately 1 km; e.g., Japan Trench) and landslide décollements (e.g., Vajont, Italy, 1963). The availability of pore fluids affects the likelihood that seismic slip propagates from deeper to shallow fault depths or that a landslide accelerates to its final collapse. To investigate the deformation processes active during seismic faulting we performed friction experiments with a rotary machine on 2‐mm‐thick smectite‐rich gouge layers (70/30 wt % Ca‐montmorillonite/opal) sheared at 5‐MPa normal stress, at slip rates of 0.001, 0.01, 0.1, and 1.3 m/s, and total displacement of 3 m. Experiments were performed on predried gouges under vacuum, under room humidity and under partly saturated conditions. The fault shear strength measured in the experiments was included in a one‐dimensional numerical model incorporating frictional heating, thermal, and thermochemical pressurization. Quantitative X‐ray powder diffraction and scanning electron microscopy investigations were performed on pristine and deformed smectite‐rich gouges. Under dry conditions, cataclasis and amorphization dominated at slip rates of 0.001–0.1 m/s, whereas grain size sensitive flow and, under vacuum, frictional melting occurred at fast slip rates (1.3 m/s). Under partly saturated conditions, frictional slip in a smectite foliation occurred in combination with pressurization of water by shear‐enhanced compaction and, for V = 0.01–1.3 m/s, with thermal pressurization. Pseudotachylytes, the only reliable microstructural markers for seismic slip, formed only with large frictional power (>2 MW/m2), which could be achieved at shallow depth with high slip rates, or, at depth, with high shear stress in dehydrated smectites.

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.

Landslide motion assessment including rate effects and thermal interactions: revisiting the Canelles landslide

The re-activation of a large (40 Mm3) landslide on the valley slopes of a reservoir motivated a research initiative to estimate the risk of a fast-sliding mass moving into the reservoir. A previous simplified analysis had suggested that a joint consideration of strain rate effects on friction and thermal pressurization phenomena in the sliding surface could provide a rational approach to answer the question raised. The paper describes first the capability of strain rate effects on friction to reproduce long-term creeping records of two real cases. The joint and coupled phenomena of creeping motion and thermal pressurization in shearing bands was incorporated into a material point method computational technique for hydromechanical analysis of porous materials. A representative cross section of the Canelles landslide was then analysed, profiting from previous finite element investigations of the landslide. It was found that a rapid rate of landslide acceleration could be a possibility under extreme external actions. However, it was also found that a moderate strain rate effect on the basal residual friction angle could create conditions that avoid the triggering of a fast motion.

Fault Stability Perturbation by Thermal Pressurization and Stress Transfer Around a Deep Geological Repository in a Clay Formation

AbstractThe increase of temperature of a low‐permeability, fluid‐saturated media may trigger significant thermal pressurization, where the expansion of pore fluid cannot be accommodated by the thermal expansion of the pore space. With the aid of a coupled thermohydromechanical numerical simulator, we investigate the possible impact of thermal pressurization during the life of a deep geological repository (DGR) for high‐level radioactive waste, characterized by the emplacement of radioactive material‐filled canisters in a series of parallel tunnels, excavated in a low‐permeability clay formation. We represent the fault as a planar structure embedded in a thermoporoelastic material and shear activation evaluated by a strain‐softening Mohr‐Coulomb failure criterion. The results show that stress changes caused by temperature and thermal pressurization of a rock mass around the emplacement tunnels may trigger a slip event on a fault plane in proximity of the geological disposal site: Rupture nucleates at depth, hundreds of meters below the DGR. Stress transfer plays a key role, while a direct hydraulic connection between the repository and the nucleation zone is not necessary in order to trigger rupture. A low stress ratio may favor the occurrence of slip up to a distance of 600 m of the fault from the outermost tunnel. These results highlight the need of investigating hydromechanical properties and local stress conditions at depth to characterize the geomechanical response of weak planes located in the surroundings of a high‐level radioactive waste repository and to provide sufficient knowledge for the safe development of the DGR site.

Stability of Pulse‐Like Earthquake Ruptures

AbstractPulse‐like ruptures arise spontaneously in many elastodynamic rupture simulations and seem to be the dominant rupture mode along crustal faults. Pulse‐like ruptures propagating under steady state conditions can be efficiently analyzed theoretically, but it remains unclear how they can arise and how they evolve if perturbed. Using thermal pressurization as a representative constitutive law, we conduct elastodynamic simulations of pulse‐like ruptures and determine the spatiotemporal evolution of slip, slip rate, and pulse width perturbations induced by infinitesimal perturbations in background stress. These simulations indicate that steady state pulses driven by thermal pressurization are unstable. If the initial stress perturbation is negative, ruptures stop; conversely, if the perturbation is positive, ruptures grow and transition to either self‐similar pulses (at low background stress) or expanding cracks (at elevated background stress). Based on a dynamic dislocation model, we develop an elastodynamic equation of motion for slip pulses and demonstrate that steady state slip pulses are unstable if their accrued slip b is a decreasing function of the uniform background stress τb. This condition is satisfied by slip pulses driven by thermal pressurization. The equation of motion also predicts quantitatively the growth rate of perturbations and provides a generic tool to analyze the propagation of slip pulses. The unstable character of steady state slip pulses implies that this rupture mode is a key one determining the minimum stress conditions for sustainable ruptures along faults, that is, their “strength.” Furthermore, slip pulse instabilities can produce a remarkable complexity of rupture dynamics, even under uniform background stress conditions and material properties.

Role of Weak Materials in Earthquake Rupture Dynamics

Weak materials in seismic slip zones are important in studies of earthquake mechanics. For instance, the exceptionally large slip during the 2011 Tohoku-Oki earthquake has been attributed to the presence of smectite in the fault zone. However, weak fault rocks cannot accumulate large amounts of elastic strain, which is thought to counter their ability to enhance seismic rupture. It is well known that if the permeability of a weak fault is low enough to allow friction-induced thermal pressurization of interstitial fluid, the fault strength decreases dramatically. However, whether intrinsic weakness of fault material or thermal pressurization more efficiently produces large slip on faults bearing weak materials has not been determined. To investigate the role of weak materials in earthquake rupture dynamics, we conducted friction experiments and dynamic rupture simulations using pure smectite and pure graphite to represent weak fault materials. Even when we assumed no thermal pressurization, simulated faults in both media were able to trigger large slip because their extremely low friction was insufficient to arrest the inertial motion of rupture propagating along the fault. We used similar rupture simulations to investigate the cause of the huge slip near the trench during the 2011 Tohoku-Oki earthquake and demonstrated that it can be attributed to thermal pressurization, although our findings suggest that the presence of smectite in the plate-boundary fault may also be required.

Quasi-Dynamic 3D Modeling of the Generation and Afterslip of a Tohoku-oki Earthquake Considering Thermal Pressurization and Frictional Properties of the Shallow Plate Boundary

The generation of the 2011 Tohoku-oki earthquake has been modeled by many authors by considering a dynamic weakening mechanism such as thermal pressurization (TP). Because the effects of TP on afterslip have not been investigated, this study develops a 3D quasi-dynamic model of the earthquake cycle to investigate afterslip of the Tohoku-oki earthquake, considering TP and the geometry of the plate boundary. We employ several velocity-weakening (VW) patches for Mw 7 class events, and two large shallow VW patches. The frictional properties are set as velocity-strengthening (VS) outside the VW patches. The results show that, during megathrust earthquakes, fast slip propagates to the surrounding VS regions near the VW patches owing to weakening by TP. Following Mw 9 events, large afterslips occur in regions below the northern shallow rupture area in the off-Fukushima region close to the Japan Trench, which is consistent with observations. In the VS region near the VW patches, during the early afterslip period, frictional behavior exhibits less VS with increasing slip velocity due to pore pressure reduction. We also consider the frictional properties of the shallow plate boundary fault off Tohoku, which exhibits a transition from VW to VS from low to high slip velocities. The results show the occurrence of slow slip events (SSEs) at intervals of a few decades at the shallow plate boundary. During megathrust events, the VW property at low slip velocity promotes slip along the shallow SSE region more than the case with VS property throughout the entire velocity range.

Paleo-earthquakes revealed by rock magnetic evidence from the Anxian-Guanxian Fault, Sichuan Province, China

To determine the rock magnetic characteristics of this co-seismic rupture generated by the 2008 Wenchuan earthquake in Sichuan Province, China, we excavated a trench on the Anxian-Guanxian fault. High-resolution magnetic susceptibility and other rock magnetic measurements were conducted on the field section and on samples from the trench. We divided the fault rocks into 8 units according to lithology and measurements of magnetic susceptibility were made on fault rocks within different units, both in the field and in the laboratory. The results demonstrate that the highest magnetic susceptibility occurs in the red gouge (64.08 × 10−6 SI for surface magnetic susceptibility and 30.61 × 10−8 m3/kg for mass magnetic susceptibility), while relatively high magnetic susceptibility occurs in the black gouge (14.31× 10−6 SI for surface magnetic susceptibility and 7.15 × 10−8 m3/kg for mass magnetic susceptibility) compared to the protolith. More detailed magnetic measurements were used to determine the magnetic mineralogy of each unit. The principal mechanism responsible for the high magnetic susceptibility values of the red gouge is the neoformation of magnetite from siderite or pyrrhotite caused by thermal pressurization during the latest earthquakes, which reveals the location of the latest seismic slip zone. By contrast, the magnetic susceptibility values of the black gouge have been reduced by the formation of goethite during weathering. We conclude that although the occurrence of goethite reveals paleo-earthquake, factors such as the oxidation or dissolution of fluids that effect magnetic minerals need further investigation.

Simulation of landslide run-out by considering frictional heating and thermal pressurization

Some of the remarkable characteristics of natural landslides, such as surprisingly long travel distances and high velocities, have been attributed to the mechanisms of frictional heating and thermal pressurization. In this work, this mechanism is combined with a depth-averaged model to simulate the long runout of landslides in the condition of deformation. Some important factors that influence frictional heating and thermal pressurization within the shear zone are further considered, including velocity profile and pressurization coefficient. In order to solve the coupled equations, a combined computational method based on the finite volume method and quadratic upwind interpolation for convective kinematics scheme is proposed. Several numerical tests are performed to demonstrate the feasibility of the computational scheme, the influence of thermal pressurization on landslide run-out, and the potential of the model to simulate an actual landslide.

Autofluidization of collapsing bed of fine particles: Implications for the emplacement of pyroclastic flows

Long runout distances of fines-rich pyroclastic flows may be partly attributed to high gas pore pressure that reduces interparticle friction. Here we examine the generation of pore pressure caused by collapse of granular materials into ambient air, which may occur at the impact zone of a fountain collapse or when pyroclastic flows bypass topographic breaks or propagate over a rough substrate. We carried out laboratory experiments, which consisted of collapse of beds of glass beads in a vertical column where pressure sensors measured the air pore pressure at different levels. The collapse height, the bed thickness and temperature as well as the particle size were systematically changed. Measurements revealed a first overpressure phase, both at different heights in the initial bed and at column base, when the particles began to fall and forced the air beneath to ascend through the bed. Even at low collapse height of 20 cm, the maximum pore pressure compensated almost completely the bed lithostatic pressure provided the particle size was <337 μm, meaning that the material was fully fluidized. Particles then accumulated at column base and formed and aggrading deposit, in which a second peak of overpressure was measured before pore pressure diffused slowly. The overpressure was high when the particles were small enough (<100–200 μm) but it was poorly developed or even absent when the particle size was increased. The duration of pressure diffusion at end of this second phase increased with the square of the thickness of the aggrading deposit and decreased strongly with increasing particle size. Additional experiments with ignimbrite material showed similar behavior but with a much longer duration of final pressure diffusion. At high temperature (up to 200 °C), overpressure during the first phase could overpass the bed lithostatic pressure. This suggested thermal pressurization of the ambient cold air, which expanded rapidly when percolating through the hot material. We conclude that fluidization of hot fines-rich pyroclastic flows can be easily achieved when the granular material collapses into the ambient air.