Fault dip vs shear stress gradient

In the brittle regime, faults tend to be oriented along an angle of about 30 relative to the principal stress direction. This empirical Andersonian observation is usually explained by the orientation of the stress tensor and the slope of the yield envelope defined by the Mohr-Coulomb criterion, often called critical-stress theory, assuming frictional properties of the crustal rocks (friction coefficient = 0.6-0.8). However, why the slope has a given value? We suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along that inclination. High homogeneous shear stress, i.e., without gradients, may generate aseismic creep as for example in flat decollements, both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine greater energy accumulation and stick-slip behaviour with larger sudden seismic energy release. Further variability of the angle is due to variations of the internal friction and of the Poisson ratio, being related to different lithologies, anisotropies and pre-existing fractures and faults. Misaligned faults are justified to occur due to the local weaknesses in the crustal volume; however, having lower stress gradients along dip than the optimally-oriented ones, they have higher probability of being associated with lower seismogenic potential or even aseismic behavior

Tidal modulation of plate motions

While mantle convection is a fundamental ingredient of geodynamics, the driving mechanism of plate tectonics remains elusive. Are plates driven only from the thermal cooling of the mantle or are there further astronomical forces acting on them? GPS measurements are now accurate enough that, on long baselines, both secular plate motions and periodic tidal displacements are visible. The now >20 year-long space geodesy record of plate motions allows a more accurate analysis of the contribution of the horizontal component of the body tide in shifting the lithosphere. We review the data and show that lithospheric plates retain a non-zero horizontal component of the solid Earth tidal waves and their speed correlates with tidal harmonics. High-frequency semidiurnal Earth's tides are likely contributing to plate motions, but their residuals are still within the error of the present accuracy of GNSS data. The low-frequency body tides rather show horizontal residuals equal to the relative motion among plates, proving the astronomical input on plate dynamics. Plates move faster with nutation cyclicities of 8.8 and 18.6 years that correlate to lunar apsides migration and nodal precession. The highfrequency body tides are mostly buffered by the high viscosity of the lithosphere and the underlying mantle, whereas low-frequency horizontal tidal oscillations are compatible with the relaxation time of the low-velocity zone and can westerly drag the lithosphere over the asthenospheric mantle. Variable angular velocities among plates are controlled by the viscosity anisotropies in the decoupling layer within the low-velocity zone. Tidal oscillations also correlate with the seismic release

Global versus local clustering of seismicity. Implications with earthquake prediction

The estimation of the maximum expected magnitude is crucial for seismic hazard assessment. It is usually inferred via Bayesian analysis; alternatively, the size of the largest possible event can be roughly obtained from the extent of the seismogenic source and the depth of the brittle–ductile transition. However, the effectiveness of the first approach is strongly limited by catalog completeness and the intensity of recorded seismicity, so that it can be of practical use only for aftershocks, while the second is affected by extremely large uncertainties. In this article, we investigate whether it may be possible to assess the magnitude of the largest event using some statistical properties of seismic activity. Our analysis shows that, while local features are not appropriate for modeling the emergence of peaks of seismicity, some global properties (e.g., the global coefficient of variation of interevent times and the fractal dimension of epicenters) seem correlated with the largest magnitude. Unlike several scientific articles suggest, the b-value of the Gutenberg–Richter law is not observed to have a predictive power in this case, which can be explained in the light of heterogeneous tectonic settings hosting fault systems with different extension

Variable seismic responsiveness to stress perturbations along the shallow section of subduction zones. The role of different slip modes and implications for the stability of fault segments

Assessing the stability state of fault interfaces is a task of primary interest not only for seismic hazards, but also for understanding how the earthquake machine works. Nowadays it is well known that a relationship exists between slow and fast earthquakes;moreover, it is more and more evident that such a connection is quite diffuse all over the Earth. In this paper, we perform a spatial and temporal analysis of both geodetic and seismic—non-volcanic tremors, low-frequency events (LFEs), and regular earthquakes—time series. We focus on the relationship between the clustering of properties of the different kinds of seismicity and their response to stress perturbations. Earth tides and large earthquakes are used as a source of additional stress. Seismic activity hosted in the Cascadia subduction zone, Manawatu region in New Zealand, and Japan during the last two decades is considered. Our analysis suggests that tremors become more and more sensitive to Earthtide perturbations as the fault interface is seismically locked. Therefore, tremors and regular events show a similar response to tidal stress perturbations. This feature is also accompanied by relatively lower spatial and temporal coefficients of variation. A series of recordings by several GNSS stations along the Hikurangi Trench, North Island, New Zealand, and along the Nankai coasts in Japan is taken into account for studying how large thrust-faulting earthquakes affect silent events and geodetic signals and vice-versa. In the last section, a simple model for grasping a glimpse of the local stability condition of the Earth’s crust and for explaining previous observations is provided

Tidal drag and westward drift of the lithosphere

Tidal forces are generally neglected in the discussion about the mechanisms driving plate tectonics despite a worldwide geodynamic asymmetry also observed at subduction and rift zones. The tidal drag could theoretically explain the westerly shift of the lithosphere relative to the underlying mantle. Notwithstanding, viscosity in the asthenosphere is apparently too high to allow mechanical decoupling produced by tidal forces. Here, we propose a model for global scale geodynamics accompanied by numerical simulations of the tidal interaction of the Earth with the Moon and the Sun. We provide for the first time a theorem that the tidal drag can produce a westerly motion of the lithosphere, also compatible with the slowing of the Earth’s rotational spin. Our results suggest a westerly rotation of the lithosphere with a lower bound of 0.1-0.2°/Myr in the presence of a basal effective shear viscosity 10^16 Pa s, but it may rise to >1°/Myr with a viscosity <3x10^14 Pa s within the Low-Velocity Zone (LVZ) atop the asthenosphere. This faster velocity would be more compatible with the mainstream of plate motion and the global asymmetry at plate boundaries. Based on these computations, we suggest that the super-adiabatic asthenosphere, being vigorously convecting, may further reduce the viscous coupling within the LVZ. Therefore, the combination of solid Earth tides, ultra-low viscosity LVZ and asthenospheric polarized small-scale convection may mechanically satisfy the large-scale decoupling of the lithosphere relative to the underlying mantle. Relative plate motions are explained because of lateral viscosity variations at the base of the lithosphere, which determine variable lithosphere-asthenosphere decoupling and plate interactions, hence plate tectonics

From physical modeling to seismic precursors

In classical models, an earthquake is a sudden slip event taking place along a smmoth interface as a consequence of stress accumulation. The strain is accommodated as long as the failure stress is reached with a complex spatial and temporal evolution mediated by fault rheology and pore-fluid pressure which can now be reconstructed in detail thanks to current computational and observational power. However, while this approach is ale to provide more and more advanced knowledge of the different physical contributions to fault dynamics, it cannot be useful for predicting future seismic activity, except for peculiar cases such as induced events. In fact, the prediction time horizon of the coseismic dynamic is noise-embedded because of the chaotic behavior of the system and poorly constrained physical parameters. Since the number of degrees of freedom is extremely large in fault systems, it is unlikely data will ever be enough for the prediction of the incoming evolution of seismicity; therefore, we need to set up models to capture the dependence on observables with effective prediction power. A possible answer consists in considering collective properties of seismicity within extended crustal volumes associated with clustering and memory effects

Double-couple troubles

A planar seismic source approximates a real fault. In nature, several faults are activated in the volume during a quake and are rather undulated. In the shallow cold crustal layer, rocks are disaggregated in a breccia, whose thickness varies with tectonic style, being larger in extensional settings

Fault dip vs shear stress gradient

In the brittle regime, faults tend to be oriented along an angle of about 30° relative to the principal stress direction. This empirical Andersonian observation is usually explained by the orientation of the stress tensor and the slope of the yield envelope defined by the Mohr-Coulomb criterion, often called critical-stress theory, assuming frictional properties of the crustal rocks (μ≈ 0.6−0.8). However, why the slope has a given value? We suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along that inclination. High homogeneous shear stress, i.e., without gradients, may generate aseismic creep as for example in flat decollements, both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine higher energy accumulation and stick-slip behaviour with larger sudden seismic energy release. Further variability of the angle is due to variations of the internal friction and of the Poisson ratio, being related to different lithologies, anisotropies and pre-existing fractures and faults. Misaligned faults are justified to occur due to the local weaknesses in the crustal volume; however, having lower stress gradients along dip than the optimally-oriented ones, they have higher probability of being associated with lower seismogenic potential or even aseismic behavior

Not so planar faults: on the impact of faulting complexity and type on earthquake rupture dynamics

The increasing quality and completeness of global moment tensor solutions allow to advance our comprehension of seismicity delving into the connection between earthquake occurrence, tectonics, and faulting. Since, at least theoretically, each seismic event can be described using a time-dependent moment tensor averaging over different spatial orientations of various fault patches; then, if the rupture is roughly nonplanar, shear sliding may not be appropriate to modelling the whole seismic event. This implies a drop of the double-couple components. Therefore, we focus our attention on the different compositions of the moment tensors of moderate and large seismic events of global and regional catalogues as a function of the tectonic setting looking for the effect predicted by theory. We have found that thrusts host earthquakes with more elevated double-couple percentages with respect to strike-slip and normal faults. The compensated-linear-vector-dipole component decreases as the size of earthquakes increases in reverse faulting, while this trend is weaker or absent in other classes of seismicity. We have also noticed that the double-couple component positively correlates with the b-value and it is negatively related to the corner magnitude of the frequency-size distribution, which is compatible with a systematic magnitude underestimation in low double-couple earthquakes. Our results suggest that, at least for large seismic events featured by suspiciously high non-double-couple components (above 30%, e.g., 30/10/2016 Mw 6.5 Norcia and 13/11/2016 Mw 7.8 Kaikoura events) should be considered to better assess their size accurately also because of possible impact on seismic forecasting

A simpler explanation for fault dip angles distribution?

Thrust faulting events, usually featured by dip angles ranging in between 5°-30°, mostly take place along convergent plate boundaries and are caused by the elastic strain accumulation, which is released by multifaceted fault slip dynamics, varying from almost periodic silent events to mega-quakes. Strike-slip-faulting earthquakes localize along steeply dipping faults (70°-90°) or transcurrent plate boundaries and transfer zones. Finally, normal faults develop along rift zones in extensional regimes having intermediate dip (45°-65°), with a dominant gravitational contribution to their energy budget. Structural, morphological, and geophysical differences have been highlighted among the three main tectonic settings (Anderson, 1905). This empirical observation is usually explained by the orientation of the stress tensor and the slope of the yield envelope defined by the Mohr-Coulomb criterion, often called critical-stress theory, assuming frictional properties of the crustal rocks (Sibson, 1974). However, why the slope has a given value? In a recent paper (Zaccagnino and Doglioni, 2023) we suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along that inclination. High homogeneous shear stress, i.e., without gradients, may generate aseismic creep as for example in flat decollements, both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine greater energy accumulation and stick-slip behaviour with larger sudden seismic energy release. Therefore, we set up a simple model and test it using about three hundred dip angles of non-volcanic shallow (depth less than 30 km) global large (Mw > 7.0) natural seismic events from 1990 to 2021. Our model correctly reproduces observations. Our idea complements previous knowledge on friction and faulting. References Anderson, E.M., 1905. The dynamics of faulting. Transactions of the Edinburgh Geological Society 8, 387–402. Sibson, R.H., 1974. Frictional constraints on thrust, wrench and normal faults. Nature 249, 542–544. Zaccagnino, D., & Doglioni, C. (2023). Fault dip vs shear stress gradient. Geosystems and Geoenvironment, 100211