Christophe Voisin

Characterization of Fault Roughness at Various Scales: Implications of Three-Dimensional High Resolution Topography Measurements

Accurate description of the topography of active fault surfaces represents an important geophysical issue because this topography is strongly related to the stress distribution along fault planes, and therefore to processes implicated in earthquake nucleation, propagation, and arrest. To date, due to technical limitations, studies of natural fault roughness either performed using laboratory or field profilometers, were obtained mainly from 1-D profiles. With the recent development of Light Detection And Ranging (LIDAR) apparatus, it is now possible to measure accurately the 3-D topography of rough surfaces with a comparable resolution in all directions, both at field and laboratory scales. In the present study, we have investigated the scaling properties including possible anisotropy properties of several outcrops of two natural fault surfaces (Vuache strike-slip fault, France, and Magnola normal fault, Italy) in limestones. At the field scale, digital elevation models of the fault roughness were obtained over surfaces of 0.25xa0m2 to 600xa0m2 with a height resolution ranging from 0.5xa0mm to 20xa0mm. At the laboratory scale, the 3-D geometry was measured on two slip planes, using a laser profilometer with a spatial resolution of 20xa0µm and a height resolution less than 1xa0µm.Several signal processing tools exist for analyzing the statistical properties of rough surfaces with self-affine properties. Among them we used six signal processing techniques: (i) the root-mean-squares correlation (RMS), (ii) the maximum-minimum height difference (MM), (iii) the correlation function (COR), (iv) the RMS correlation function (RMS-COR), (v) the Fourier power spectrum (FPS), and (vi) the wavelet power spectrum (WPS). To investigate quantitatively the reliability and accuracy of the different statistical methods, synthetic self-affine surfaces were generated with azimuthal variation of the scaling exponent, similar to that which is observed for natural fault surfaces. The accuracy of the signal processing techniques is assessed in terms of the difference between the “input” self-affine exponent used for the synthetic construction and the “output” exponent recovered by those different methods. Two kinds of biases have been identified: Artifacts inherent to data acquisition and intrinsic errors of the methods themselves. In the latter case, the statistical results of our parametric study provide a quantitative estimate of the dependence of the accuracy with system size and directional morphological anisotropy. Finally, based on this parametric study, we used the most reliable techniques (RMS-COR, FPS, WPS) to analyze field data. These three methods provide complementary results. The FPS and WPS methods determine a robust characterization of the fault surface roughness in the direction of striations and perpendicular to them. The RMS-COR method allows investigation of the azimuth dependence of the scaling exponent. For both field and laboratory data, the topography perpendicular to the slip direction displays a similar scaling exponent H⊥xa0=xa00.8. However, our analysis indicates that for the Magnola fault surface the scaling roughness exponent parallel to the mechanical striation is identical at large and small scales H//xa0=xa00.6–0.7, whereas for the Vuache fault surface it is characterized by two different self-affine regimes at small and large scales. We interpret this cross-over length scale as a witness of different mechanical processes responsible for the creation of fault topography at different spatial scales.

High resolution 3D laser scanner measurements of a strike‐slip fault quantify its morphological anisotropy at all scales

[1]xa0The surface roughness of a recently exhumed strike-slip fault plane has been measured by three independent 3D portable laser scanners. Digital elevation models of several fault surface areas, from 1 m2 to 600 m2, have been measured at a resolution ranging from 5 mm to 80 mm. Out of plane height fluctuations are described by non-Gaussian distribution with exponential long range tails. Statistical scaling analyses show that the striated fault surface exhibits self-affine scaling invariance with a small but significant directional morphological anisotropy that can be described by two scaling roughness exponents, H1 = 0.7 in the direction of slip and H2 = 0.8 perpendicular to the direction of slip.

Triggering of tremors and slow slip event in Guerrero, Mexico, by the 2010 Mw 8.8 Maule, Chile, earthquake

[1]xa0We investigate the triggering of seismic tremor and slow slip event in Guerrero (Mexico) by the February 27, 2010 Maule earthquake (Mw 8.8). Triggered tremors start with the arrival of S wave generated by the Maule earthquake, and keep occurring during the passing of ScS, SS, Love and Rayleigh waves. The Rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor, indicating a strong modulation of the source of tremors by the passing surface wave. This correlation and modulation by the passing waves is progressively lost with time over a few hours. The tremor activity continues during the weeks/months after the earthquake. GPS time series suggest that the second sub-event of the 2009–2010 SSE in Guerrero is actually triggered by the Maule earthquake. The southward displacement of the GPS stations starts coincidently with the earthquake and tremors. The long duration of tremors indicate a continuing deformation process at depth, which we propose to be the second sub-event of the 2009–2010 SSE. We show a quasi-systematic correlation between surface displacement rate measured by GPS and tremor activity, suggesting that the NVT are controlled by the variations in the slip history of the SSE. This study shows that two types of tremors emerge: (1) Those directly triggered by the passing waves and (2) those triggered by the stress variations associated with slow slip. This indicates the prominent role of aseismic creep in the Mexican subduction zone response to a large teleseismic earthquake, possibly leading to large-scale stress redistribution.

Dynamic versus static stress triggering and friction parameters: Inferences from the November 23, 1980, Irpinia earthquake

This paper concentrates on the problem of fault interaction and earthquake triggering through the 1980 Irpinia, Italy, sequence. More specifically, this paper deals with the problem of the triggering of the second subevent by the mainshock. The interaction between the two segments is modeled through a dynamic Coulomb failure function. The aims of this paper are, first, to discriminate between the dynamic and the static stress effects on the triggering, if these effects exist, second, to estimate the fault strength relative to the initial state of stress, third, to determine the parameters of a slip-dependent friction law that lead to the observed delay of 20 s. Numerical simulations show that the critical slip Dc may range from 0.03 m up to 1.7 m, and that the initial slope of the friction law μ′(0) must be lower than 0.04 m−1. We show that the relative magnitude of the fault strength and the initial state of stress govern the existence and value of a Dc lower threshold under which the fault always ruptures before 13 s. A close to failure fault is not consistent with a critical slip Dc less than 0.8 m, whereas small values of Dc , typically 0.03 m, imply a far from failure fault. General results concern the effect of a dynamic stress pulse. We show that an event can be triggered by a transient stress pulse and that in this case the event can have an initiation duration much longer than the pulse duration. We show that it is possible to explain both the triggering and the time delay only with the effect of the transient stress pulse. This may explain aftershock triggering even in regions of negative Coulomb failure function or long distance triggering of earthquakes by propagating waves.

On the effective friction law of a heterogeneous fault

Numerical simulation of the rupture process is usually performed under an assumption of scale invariance of the friction process although heterogeneous fault properties are shown by both direct observations of surface crack geometry and slip inversion results. We investigate if it is possible to define an effective friction law for a finite fault with a small-scale heterogeneity, that is, with a distribution of narrow segments with a resistance to rupture higher than the rest of the fault. We consider a model where the local boundary condition corresponds to a linear slip-dependent friction law. We define the effective slip-dependent friction law by analogy with the theoretical spectral solution for the initiation phase in the case of a homogeneous infinite fault. We use finite difference simulations to test the validity of this approach. The results show a surprisingly good agreement between the calculations for the complete heterogeneous fault model and for a homogeneous fault with an effective friction law. The time of initiation and the average of the slip velocity on the fault are well predicted by the effective model. The effective friction law exhibits a nonlinear slip dependence with an initial weakening rate different from the one of the local laws. This initial weakening rate is related to the geometry of the heterogeneity and can be obtained by an eigenvalue analysis. The effective law shows a kink at a slip that corresponds to the average slip on the fault for which the stress concentration of the strong segments is sufficient to trigger their rupture. While based on a rather simple model of a fault, these results indicate that an effective friction can be defined and used for practical purposes. The heterogeneity of a fault tends to decrease the initial weakening rate of the local weak patches. Since the initial weakening rate controls the initiation duration, this last point indicates that the duration of initiation expected from actual heterogeneous faults is much larger than the one deduced from small-scale laboratory measurements. The actual fracture energy is not conservative in the rescaling of the friction law.

Long term friction: From stick‐slip to stable sliding

[1]xa0We have devised an original laboratory experiment where we investigate the frictional behaviour of a single crystal salt slider over a large number of deformation cycles. Because of its physical properties, salt, an analogue for natural faults, allows for frictional processes plastic deformation and pressure solution creep to operate on the same timescale. During the same experiment, we observe a continuous change of the frictional behaviour of the slider under constant conditions of stiffness, temperature and loading velocity. The stick-slip regime is progressively vanishing, eventually reaching the stable sliding regime. Concomitantly, the contact interface, observed under the microscope, develops a striated morphology with contact asperities increase in length and width, arguing for an increase in the critical slip distance dc. Complementary experiments including velocity jumps show that the frictional parameters of the rate and state friction law, a and b, progressively vanish with accumulated slip. The ultimate stage of friction is therefore rate and state independent under our experimental conditions.

A unified model for dynamic and static stress triggering of aftershocks, antishocks, remote seismicity, creep events, and multisegmented rupture

[1]xa0Aftershocks have been thought to be triggered by static stress transfer. The 1992 Landers and 1999 Hector Mine, California, earthquakes have demonstrated the possibility of remote triggering of seismicity, highlighting the role of dynamic seismic waves. Static stress changes and transient deformations have different timescales. The mechanisms by which they trigger earthquakes are thought to be different, leading to an unwieldy vision of aftershock triggering. We propose a model that encompasses both static and dynamic triggering of aftershocks and antishocks as well as the occurrence of stable slip (creep events) or multisegmented rupture. The framework of this model is based on the stability/instability transition of faults. We propose that dynamic and static stress triggering pertain to the same physical process. We study the effect of the complete Coulomb failure function. We demonstrate the possibility of the triggering of seismicity by a dynamic stress pulse in a stress shadow zone characterized by a negative static stress field. The limited time efficiency of dynamic triggering shown by the data is explained by the model. We suggest that dynamic waves trigger only the most unstable faults associated with a relatively short characteristic time, while the static stress field triggers also faults that are stable, associated with a larger characteristic time.

Evolution of seismic signals and slip patterns along subduction zones: Insights from a friction lab scale experiment

[1]xa0We investigate the influence of the cumulative slip on the frictional and acoustic patterns of a lab scale subduction zone. Shallow loud earthquakes, medium depth slow, deeper silent quakes and deepest steady-state creep are reproduced by the ageing of contact interface with cumulative displacement. The Acoustic Emission evolves with cumulative displacement and interface ageing, following a trend from strong impulsive events similar to earthquake seismic signals, to a collection of smaller amplitude and longer duration signals similar to NVT. The latter emerge as the local recollection of the unstable behaviour of the contact interface globally evolving towards the stable sliding regime.

Slip acceleration generates seismic tremor like signals in friction experiments

[1]xa0Since their discovery nearly a decade ago, the origin of seismic tremor remains unclear. Recent studies indicate that various driving phenomena such as Earth and ocean tides, regional and teleseismic earthquakes enhance tremor activity. Observations of the coincidence with slow-slip events and of fast migrations of tremors have led frictional slip to be considered as the possible source of tremors. Indeed, laboratory friction experiments succeeded in generating and recording tremor like signals (TLS). Here we show a systematic correlation between the onset of slip acceleration and the emission of TLS in a laboratory friction experiment. TLS are generated when the shear stress reaches the peak static resistance and the dilatancy meets its maximum that is when the mature interface is close to failure. This robust result provides a comprehensive image of how natural seismic tremors might be generated and/or triggered by passing seismic waves, tides or even slow slip events.

Dynamic triggering of earthquakes: The linear slip‐dependent friction case

This paper is devoted to the modeling of dynamic triggering of earthquakes by a sinusoidal plane wave. I consider a 2D antiplane finite fault of length L under a linear slip‐dependent friction law, characterized by a nondimensional weakening parameter β. The finite fault is perturbed by an incident sinusoidal stress wave of wavelength λ and amplitude a. The positive pulse of the wave loads the fault and promotes the instability, while the negative pulse tends to heal the initiation process that may lead to the rupture. The occurrence of triggering depends on the balance between the loading terms and the intrinsic mechanics of the fault. Both amplitude and frequency exert a clock advance effect on the triggering. A threshold in frequency for the incident wave to trigger the rupture is revealed, which depends on the non‐dimensional weakening parameter β and separates a nontriggering domain with stable slip from a triggering domain with unstable slip.