In order to clarify the main controlling factors influencing fluid pressure changes in fault zones during the seismic cycle, we conducted laboratory rock friction experiments where fluid pressure was monitored in situ during sequences of quasi-static loading followed by dynamic slip events. The simulated fault was a 30$^\circ$ saw-cut in a Westerly granite cylinder, saturated with water, tested under triaxial conditions. Pore pressure was held constant at the boundaries of the block, but the low hydraulic diffusivity of Westerly granite made the fault hydraulically disconnected from the boundaries. During quasi-static loading while the fault was locked, we observed pore pressure increases which we interpret as poroelastic closure of the fault. During dynamic slip events, pore pressure systematically dropped by amplitudes commensurate to the normal stress drop. A large contribution to the pore pressure drop is interpreted as poroelastic opening of the fault. Deviations from the poroelastic effects are observed: in small events, pore pressure dropped further than anticipated, indicating inelastic dilation. In a few large events, pore pressure dropped less than anticipated, which could be the sign of compaction or thermal pressurisation. Prior to macroscopic slip events, we detect systematic pore pressure decreases by up to around 1 MPa, correlated to the occurrence of inhomogeneous preslip along the fault. Slip nucleation, inferred by kinematic inversion of local strain gauge data, is linked to local slip magnitudes of the order of 1 to 10 $\mu$m, and appears to lead to inelastic dilation. A stability analysis of fault slip including dilatant and poroelastic effects shows that poroelastic coupling tends to compensate normal stress variations, leading to faults operating under mostly constant effective normal stress if conditions are undrained.
Elastic flexure of tectonic plates shapes large-scale topographic features and can induce notable slip on lithosphere-scale faults. At mid-ocean ridges, recent evidence shows that up to 50% of extensional fault offsets formed within axial valleys can be reversed by compressional slip across valley flanks. While standard models predict the location of this faulting, they consistently underpredict the depth-extent of compression and related surface offsets. Here, using numerical models, we show that elevated pore fluid pressures in compression allow increased slip on flexure-driven reverse faults. Quantitative agreement between modeled and observationally inferred reverse fault–bending strain is achieved only when pore-fluid pressures in reverse faults approach lithostatic levels. These findings highlight the critical role of inherited structural heterogeneities and high pore-fluid pressures in facilitating deep-seated reverse faulting during lithospheric unbending. This mechanism may also amplify flexure-induced seismicity in continental forebulges, such as the 2001 Bhuj earthquake (moment magnitude, 7.6), and underscores pore-pressure modulation as a fundamental control on global tectonic hazards.
Zhonglan Liu, W. R. Buck, J. Olive· Science Advances· 0 citations
Fluid injection in deep reservoirs can induce fault reactivation and seismicity, posing challenges for geothermal and subsurface energy development. This study investigates the mechanical interaction between two adjacent non-intersecting faults under fluid injection using a pseudo-three-dimensional thermo-hydro-mechanical (THM)-coupled numerical model. The results show that injection first triggers slip on F2, which then redistributes stress onto F1. The response of F1 is strongly heterogeneous: some segments are stabilized due to a decrease in Coulomb failure stress, whereas other segments are destabilized due to an increase in Coulomb failure stress. Stress-path analysis indicates that the immediate response of F1 to F2 slip is mainly governed by changes in effective normal stress and shear stress, rather than abrupt pore pressure changes on F1. These findings demonstrate that fault slip can act as a mechanical stress source that either promotes or inhibits adjacent fault reactivation. Therefore, slip-induced stress transfer should be explicitly considered when assessing fault stability in reservoirs containing multiple closely spaced faults.
Fracture mechanics models suggest that aseismic slip may either lag behind or outpace the fluid pressure front, depending on injection conditions and the fault’s initial stress state. However, direct experimental validation of these predictions has been lacking. Here, we report laboratory experiments on granite samples under upper crustal stress conditions, where we simultaneously tracked the propagation of fluid pressure and aseismic slip during fluid injection. We show that aseismic slip lags the pressure front at low injection rates and low initial stress but rapidly outpaces it under high-rate injection or near-failure initial stress conditions. The transition is governed by a dimensionless loading parameter that integrates injection rate, fault strength, and initial shear stress. Our results provide direct support for fracture mechanics models of fluid-induced aseismic slip and suggest that in critically stressed crustal faults, rupture propagation may commonly outpace fluid diffusion.
F. Passelègue, N. Brantut, H. Chauris et al.· Science Advances· 0 citations
The 2024 Noto Peninsula earthquake (M7.6), which occurred in central Japan on January 1, is thought to have ruptured multiple fault segments with different strike and dip angles. Seismicity along these segments remained active for over one year after the mainshock. To understand the fault zone in terms of fluid and frictional properties, this study estimates the stress drop of small- to moderate-sized earthquakes. We used seismic waveform data from onshore and offshore stations for approximately one month, beginning in mid-January 2024. The estimated stress drop was nearly scale-independent and gradually decreased from southwest to northeast across the offshore source area. The possible causes of this lateral variation were fault orientation, seismic velocity, pore pressure, low initial stress and frictional properties. Among these, fault orientation, seismic velocity and low initial stress were considered unlikely, indicating that frictional properties, potentially influenced by pore pressure, were the likely cause. This study showed that the lateral variation in stress drop is not due to differences in focal depth. Within the 4–10 km depth range, earthquakes on the northeastern segments exhibited significantly smaller stress drops than those on the southern segments. We also found a statistically significant increase in stress drop with increasing depth between 2 and 10 km, whereas no clear depth dependence was observed at depths greater than 10 km. The results of this study may contribute to investigations of frictional properties and fluid effects in the offshore source area.
Tsutomu Takahashi, Gou Fujie, M. Shinohara et al.· Earth, Planets and Space· 0 citations
Understanding how fault creep evolves in space and time is essential for identifying zones of aseismic stress release and stress accumulation. Creeping faults typically exhibit rate-strengthening friction, in which frictional resistance increases with sliding velocity and promotes stable slip. The northern Aceh segment of the Sumatran Fault Zone is a notable exception, hosting active creep yet rupturing in a Mw 6.1 strike-slip earthquake in 2013. Here we show, using satellite observations, that creep rates decreased by ~60% from 2007–2010 to 2017–2023. Numerical models constrained by these observations reveal two creep events: one triggered by stress transfer from the distant 2004 Mw 9.2 megathrust earthquake, and a second re-acceleration driven by nearby continental earthquakes. These results suggest that the Aceh fault exhibits emergent, nearly velocity-neutral behaviour at the segment scale. In this regime, the fault behaves like a brittle Coulomb surface lacking the self-stabilizing influence of velocity- and slip-history-dependent friction, enabling long-range triggering and the coexistence of creep and earthquakes. The fault can thus act as a natural stress gauge in a region of significant seismic hazard.
Unknown authors· Communications Earth & E...· 0 citations
