Estimating effective normal stress during slow slip events from slip velocities and shear stress variations

Abstract We propose a normal-stress-dependent Nagata law. Nagata et al. (J Geophys Res 117:B02314, 2012) revised the rate- and state-dependent friction law by introducing the shear stress dependence. We further extended the Nagata law by incorporating the normal stress dependence obtained by Linker and Dieterich (J Geophys Res 97:4923–4940, 1992). We performed numerical simulations of earthquake triggering by assuming the extended Nagata law. In the case of repeated earthquakes, we applied dynamic Coulomb failure function (CFF) perturbation due to normal or shear stress changes. CFF perturbation increased the slip velocity after the cessation of perturbation, relative to that of the repeated events without triggering. This leads to dynamic earthquake triggering for certain perturbation amplitudes with time to instability of 0 to several tens of days. In addition, triggering potential of the static CFF jump (ΔCFFs) was investigated. Static stress perturbation has a higher triggering potential than dynamic stress perturbation for the same magnitude of CFF. The equivalent ΔCFFeq is evaluated for dynamic perturbation that results in a triggering potential approximately the same as in the case of static stress perturbation if ΔCFFs = ΔCFFeq. We calculated ΔCFFeq on the interface of the Philippine Sea plate for the Mie offshore earthquake, which occurred around the Nankai Trough on April 1, 2016, using OpenSWPC. The results shows that ΔCFFeq is large around the trough, where slow slip events followed the Mie earthquake, suggesting that a large ΔCFFeq may have triggered slow slip events.

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Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

Journal of Glaciology ◽

10.3189/s0022143000032184 ◽

1988 ◽

Vol 34 (117) ◽

pp. 154-169 ◽

Cited By ~ 5

Author(s):

C. F. Raymond ◽

W. D. Harrison

Keyword(s):

Shear Stress ◽

Seasonal Variation ◽

Normal Stress ◽

Deformation Rate ◽

Surface Slope ◽

Thickness Increase ◽

Effective Normal Stress ◽

Inverse Effect ◽

Basal Sliding ◽

Basal Shear

AbstractDuring the decade prior to its 1982–83 surge, Variegated Glacier experienced progressive changes in geometry and velocity. It thickened in the upper 60% and thinned in the lower 40% of its 20 km length. Thickness changes were up to 20%. Annual velocity increased by up to 500%, reaching a maximum of 0.7 m d−1 in the year before surge onset. Amplitude of seasonal variation in velocity increased up to 0.3 m d−1 by 1978, but did not increase markedly after that. The changes in velocity were larger than predicted from changes in deformation rate caused by changes in shear stress and depth. This anomalous velocity was especially large after 1978 in the zone of thickening on the upper glacier. If it is assumed to arise from basal sliding, the inferred pattern of sliding shows qualitative features consistent with a direct effect from basal shear stress and an inverse effect from effective normal stress. A drop in effective normal stress in a zone of decreasing surface slope up-glacier from the largest thickness increase may have been significant in the initiation of surge motion in 1982.

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Stress triggering and the mechanics of fault slip behavior

10.5194/egusphere-egu2020-21610 ◽

2020 ◽

Author(s):

Marco Maria Scuderi ◽

Cristiano Collettini

Keyword(s):

Stress Field ◽

Normal Stress ◽

Stability Boundary ◽

Fault Slip ◽

Fault Gouge ◽

Recurrence Time ◽

Slow Slip ◽

Stick Slip ◽

Slow Slip Events ◽

Stress Changes

Dynamic changes in the stress field during the seismic cycle of tectonic faults can control frictional stability and the mode of fault slip. Small perturbation in the stress field, like those produced by tidal stresses can influence the evolution of frictional strength and fault stability with the potential of triggering a variety of slip behaviors. However, an open question that remains still poorly understood is how amplitude and frequency of stress changes influence the triggering of an instability and the associated slip behavior, i.e. slow or fast slip.Here we reproduce in the laboratory the spectrum of fault slip behaviors, from slow-slip to dynamic stick-slip, by matching the critical fault rheologic stiffness (kc) with the surrounding stiffness (k). We investigate the influence of normal stress variations on the slip style of a quartz rich fault gouge at the stability boundary, i.e. k/kc slightly less than one, by adopting two techniques: 1) instantaneous step-like changes and 2) sinusoidal variations in normal stress. For the latter case, modulations of normal stress were chosen to have amplitudes greater, less or equal to the typical stress drop observed during unperturbed experiments. Also, the period was varied to be greater, less or equal to the typical recurrence time of laboratory slow-slip events. During the experiments, we continuously record ultrasonic wave velocity to monitor the microphysical state of the fault. We find that frictional stability is profoundly affected by variation in normal stress giving rise to a variety of slip behaviors. Furthermore, during strain accumulation and fabric development, changes in normal stress permanently influence the microphysical state of the fault gouge increasing kc and producing a switch from slow to fast stick-slip. Our results indicate that perturbations in the stress state can trigger a variety of slip behaviors along the same fault patch. These results have important implications for the formulation of constitutive laws in the framework of rate- and state- friction, highlighting the necessity to account for the microphysical state of the fault in order to improve our understanding of frictional stability.

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Evolution of Variegated Glacier, Alaska, U.S.A., Prior to its Surge

Journal of Glaciology ◽

10.1017/s0022143000032184 ◽

1988 ◽

Vol 34 (117) ◽

pp. 154-169 ◽

Cited By ~ 56

Author(s):

C. F. Raymond ◽

W. D. Harrison

Keyword(s):

Shear Stress ◽

Seasonal Variation ◽

Normal Stress ◽

Deformation Rate ◽

Surface Slope ◽

Thickness Increase ◽

Effective Normal Stress ◽

Inverse Effect ◽

Basal Sliding ◽

Basal Shear

Abstract During the decade prior to its 1982–83 surge, Variegated Glacier experienced progressive changes in geometry and velocity. It thickened in the upper 60% and thinned in the lower 40% of its 20 km length. Thickness changes were up to 20%. Annual velocity increased by up to 500%, reaching a maximum of 0.7 m d−1 in the year before surge onset. Amplitude of seasonal variation in velocity increased up to 0.3 m d−1 by 1978, but did not increase markedly after that. The changes in velocity were larger than predicted from changes in deformation rate caused by changes in shear stress and depth. This anomalous velocity was especially large after 1978 in the zone of thickening on the upper glacier. If it is assumed to arise from basal sliding, the inferred pattern of sliding shows qualitative features consistent with a direct effect from basal shear stress and an inverse effect from effective normal stress. A drop in effective normal stress in a zone of decreasing surface slope up-glacier from the largest thickness increase may have been significant in the initiation of surge motion in 1982.

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The Role of Thermal Pressurization and Dilatancy in Controlling the Rate of Fault Slip

Journal of Applied Mechanics ◽

10.1115/1.4005896 ◽

2012 ◽

Vol 79 (3) ◽

Cited By ~ 29

Author(s):

Paul Segall ◽

Andrew M. Bradley

Keyword(s):

Pore Pressure ◽

Subduction Zones ◽

Slow Slip ◽

Dynamic Rupture ◽

Dynamic Simulations ◽

Slow Slip Events ◽

Temperature Changes ◽

Dynamic Events ◽

Effective Normal Stress ◽

Thermal Pressurization

Geophysical observations have shown that transient slow slip events, with average slip speeds v on the order of 10−8 to 10−7 m/s, occur in some subduction zones. These slip events occur on the same faults but at greater depth than large earthquakes (with slip speeds of order ∼ 1 m/s). We explore the hypothesis that whether slip is slow or fast depends on the competition between dilatancy, which decreases fault zone pore pressure p, and thermal pressurization, which increases p. Shear resistance to slip is assumed to follow an effective stress law τ=f(σ-p)≡ fσ¯. We present two-dimensional quasi-dynamic simulations that include rate-state friction, dilatancy, and heat and pore fluid flow normal to the fault. We find that at lower background effective normal stress (σ¯), slow slip events occur spontaneously, whereas at higher σ¯, slip is inertially limited. At intermediate σ¯, dynamic events are followed by quiescent periods, and then long durations of repeating slow slip events. In these cases, accelerating slow events ultimately nucleate dynamic rupture. Zero-width shear zone approximations are adequate for slow slip events but substantially overestimate the pore pressure and temperature changes during fast slip when dilatancy is included.

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Dilation and compaction accompanying changes in slip velocity in clay-bearing fault gouges

10.5194/egusphere-egu21-8679 ◽

2021 ◽

Author(s):

Isabel Ashman ◽

Daniel Faulkner

Keyword(s):

Normal Stress ◽

Pore Pressure ◽

Volume Change ◽

Slip Velocity ◽

Fault Slip ◽

Slow Slip ◽

Volume Changes ◽

Effective Normal Stress ◽

Earthquake Nucleation ◽

Fault Gouges

Many natural fault cores comprise volumes of extremely fine, low permeability, clay-bearing fault rocks. Should these fault rocks undergo transient volume changes in response to changes in fault slip velocity, the subsequent pore pressure transients would produce significant fault weakening or strengthening, strongly affecting earthquake nucleation and possibly leading to episodic slow slip events. Dilatancy at slow slip velocity has previously been measured in quartz-rich gouges but little is known about gouge containing clay. In this work, the mechanical behaviour of synthetic quartz-kaolinite fault gouges and their volume response to velocity step changes were investigated in a suite of triaxial deformation experiments at effective normal stresses of 60MPa, 25MPa and 10MPa. Kaolinite content was varied from 0 to 100wt% and slip velocity was varied between 0.3 and 3 microns/s.Upon a 10-fold velocity increase or decrease, gouges of all kaolinite-quartz contents displayed measurable volume change transients. The results show the volume change transients are independent of effective normal stress but are sensitive to gouge kaolinite content. Peak dilation values did not occur in the pure quartz gouges, but rather in gouges containing 10wt% to 20wt% kaolinite. Above a kaolinite content of 10wt% to 20wt%, both dilation and compaction decreased with increasing gouge kaolinite content. At 25MPa effective normal stress, the normalised volume changes decreased from 0.1% to 0.06% at 10wt% to 100wt% kaolinite.&#160; The gouge mechanical behaviour shows that increasing the gouge kaolinite content decreases the gouge frictional strength and promotes more stable sliding, rather than earthquake slip. Increasing the effective normal stress slightly decreases the frictional strength, enhances the chance of earthquake nucleation, and has no discernible effect on the magnitude of the pore volume changes during slip velocity changes.Low permeabilities of clay-rich fault gouges, coupled with the observed volume change transients, could produce pore pressure fluctuations up to 10MPa in response to fault slip. This assumes no fluid escape from an isolated fault core. Where the permeability is finite, any pore pressure changes will be mediated by fluid influx into the gouge. Volume change transients could therefore be a significant factor in determining whether fault slip leads to earthquake nucleation or a dampened response, possibly resulting in episodic slow slip in low permeability fault rock volumes.

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Fault zone heterogeneities explain depth-dependent pattern and evolution of slow earthquakes in Cascadia

Nature Communications ◽

10.1038/s41467-021-22232-x ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Yingdi Luo ◽

Zhen Liu

Keyword(s):

Pore Pressure ◽

Fault Zone ◽

Cascadia Subduction Zone ◽

Slow Slip ◽

Slow Slip Events ◽

Close Link ◽

Megathrust Earthquakes ◽

Effective Normal Stress ◽

Slow Earthquakes ◽

Rate And State Friction

AbstractSlow earthquakes including tremor and slow-slip events are recent additions to the conventional earthquake family and have a close link to megathrust earthquakes. Slow earthquakes along the Cascadia subduction zone display a diverse behavior at different spatiotemporal scales and an intriguing increase of events frequency with depth. However, what causes such variability, especially the depth-dependent behavior is not well understood. Here we build on a heterogeneous asperities-in-matrix fault model that incorporates differential pore pressure in a rate-and-state friction framework to investigate the underlying processes of the observed episodic tremor and slow-slip (ETS) variability. We find that the variations of effective normal stress (pore pressure) is one important factor in controlling ETS behavior. Our model reproduces the full complexity of ETS patterns and the depth-frequency scaling that agree quantitatively well with observations, suggesting that fault zone heterogeneities can be one viable mechanism to explain a broad spectrum of transient fault behaviors.

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