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Open access Aug 2026

Slip-Weakening and Rate-Strengthening Friction Law for Shallow Layer Above the Seismogenic Zone

Accurate modeling of slip-rate time functions in the shallow portion above the seismogenic zone on a fault is essential for ground-motion prediction. Recent observations, such as those from the 2016 Kumamoto, central Kyushu, Japan, earthquake (MJMA 7.3), have revealed long-period ground motions and permanent displacements with long rise times near the surface fault, indicating that contributions of the shallow fault portion must be considered in addition to strong-motion generation areas within the deep seismogenic zone. In this study, we propose a friction law applicable to fault surfaces in an elastic medium, in which a slip-rate strengthening mechanism operates below a characteristic slip rate Vc, supplementing the standard slip-weakening law. Pure slip weakening is applied within the seismogenic zone, whereas the slip-rate strengthening branch is applied to the shallow fault portion above the seismogenic zone. Dynamic rupture simulations using a 3D elastic model with a shallow stress drop of 0 MPa demonstrate that the model incorporating slip-rate strengthening suppresses slip rates on the shallow layer and reduces peak ground velocity amplitudes within approximately 1 km of the surface fault compared with a purely slip-weakening model, despite producing nearly identical overall rupture propagation and seismic moments. Sensitivity analyses further show that δ, which is the incremental parameter of the rate-strengthening friction law, primarily controls peak slip rate through its influence on the effective fracture energy, whereas Vc mainly affects rupture duration, particularly the slip-termination phase. Although long Dc or negative stress drop has traditionally been used to mimic suppression of shallow slip rates, the proposed law more efficiently reproduces a gradual slip-rate onset, prolonged slip duration, and peak suppression, compared to friction laws without slip-rate dependence that have a similar effective fracture energy. The proposed friction formulation is simple to implement, computationally efficient, and useful for large-scale source-model exploration and applications in near-fault ground-motion prediction.

Y. Kase, K. Irie, Ken Miyakoshi et al. · 0 citations
Open access Jul 2026

Shear Fault Formation in Granite Under High and Transient Co‐Seismic Strain Rates Unveiled by Ultra‐Fast X‐Ray Imaging

During an earthquake a dynamic rupture propagates, causing fractures and permanent damage that modify the mechanical properties of the fault and its surroundings. Reciprocally, the energy dissipated by the mechanical transformation of the fault rock has a direct feedback on the dynamics of the seismic rupture. We reproduce and image these co‐seismic damage processes with sub‐microsecond time resolution through dynamic loading experiments on confined Westerly granite samples. Using synchrotron X‐ray imaging and ultra‐high‐speed cameras, we observed that dynamic compression with confinement produces shear faults and a localized granulation of the rock, known as gouge, before significant slip occurs. This fragmentation process begins with volumetric damage, followed by shear deformation less than 5 microseconds later. Our results indicate that gouge formation is initiated by the dynamic rupture itself, which reduces the strength of a fault prior to other weakening mechanisms that may occur during the earthquake slip.

A. Vishnu, M. Doan, J. Hollingsworth et al. · 0 citations
Open access Jul 2026

Spatial variation of stress drop in the offshore source area of the 2024 Noto Peninsula earthquake

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. · 0 citations
Open access Aug 2026

Coulomb-like creeping segment acts as a stress sensor in Northern Sumatra

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 · 0 citations
Aug 2026

Kinematics and Dynamics of Normal Faults in Southern Tibet: Insights From the 2025 Mw 7.1 Dingri Earthquake

Normal faults in southern Tibet have long suffered from limited ground‐based seismic and geodetic observations, constraining our understanding of both interseismic and coseismic processes and their interactions. The 2025 Mw 7.1 Dingri earthquake provides an opportunity to address these gaps. Here, we integrate interseismic slip deficit rate inversion, finite‐fault slip analyses, and 3D dynamic rupture simulations constrained by geodetic and seismic data to investigate fault behavior. The earthquake ruptured two asperities, with a shallow slip gap located above the hypocentral region, and exhibited a slow initiation followed by rapid strain release. A fault bend may have arrested the southward rupture, while the northward termination appears to have been controlled by a low slip deficit rate barrier. The shallow slip gap above the hypocenter may represent a high‐strength segment requiring elevated fracture energy for rupture initiation, resulting in the relatively slow rupture onset during the event. These results indicate that geometric complexity and heterogeneity of on‐fault stress and strength jointly governed the normal faulting. In addition, we quantified seismic moment accumulation rates on 132 normal faults in southern Tibet, finding that the accumulated moments over 500 years correspond to earthquakes of Mw 5.3–7.2. However, geometric and stress‐strength heterogeneities may reduce the likelihood of large cascading ruptures compared to large block‐bounding strike‐slip and thrust faults. Overall, our findings highlight rupture complexities of the Dingri earthquake, while underscoring substantial seismic hazards posed by rift systems in southern Tibet.

Haicheng Xiong, Yanchuan Li, Xinjan Shan et al. · 0 citations
Open access Jul 2026

Pore pressure change during nucleation and slip along experimental faults

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.

N. Brantut, F. Passelegue, P. Dublanchet · 0 citations