Mathilde Radiguet

Seismic evidence of nonlinear crustal deformation during a large slow slip event in Mexico

Repeated cross-correlations of ambient seismic noise indicate a long-term seismic velocity change associated with the 2006 M7.5 slow-slip event (SSE) in the Guerrero region, Mexico. Because the SSE does not radiate seismic waves, the measured velocity change cannot be associated with the response of superficial soil layers to strong shaking as observed for regular earthquakes. The perturbation observed maximized at periods between 7 s and 17 s, which correspond to surface waves with sensitivity to the upper and middle crust. The amplitude of the relative velocity change (∼10−3) was much larger than the volumetric deformation (∼10−6) at the depths probed (∼5-20 km). Moreover, the time dependence of the velocity perturbation indicated that it was related to the strain rate rather than the strain itself. This suggests that during strong slow-slip events, the deformation of the overlying crust shows significant nonlinear elastic behavior.

Slow slip events and strain accumulation in the Guerrero gap, Mexico

Note: Best student presentation award Reference EPFL-TALK-183540 Record created on 2013-02-01, modified on 2016-08-09

Uncovering the geodetic signature of silent slip through repeating earthquakes

Slow transient slip that releases stress along the deep roots of plate interfaces is most often observed on regional GPS networks installed at the surface. The detection of slow slip is not trivial if the dislocation along the fault at depth does not generate a geodetic signal greater than the observational noise level. Instead of the typical workflow of comparing independently gathered seismic and geodetic observations to study slow slip, we use repeating low-frequency earthquakes to reveal a previously unobserved slow slip event. By aligning GPS time series with episodes of low-frequency earthquake activity and stacking, we identify a repeating transient slip event that generates a displacement at the surface that is hidden under noise prior to stacking. Our results suggest that the geodetic investigation of transient slip guided by seismological information is essential in exploring the spectrum of fault slip.

Linear elastic fracture mechanics predicts the propagation distance of frictional slip

AbstractWhen a frictional interface is subject to a localized shear load, it is often (experimentally) observed that local slip events propagate until they arrest naturally before reaching the edge of the interface. We develop a theoretical model based on linear elastic fracture mechanics to describe the propagation of such precursory slip. The model’s prediction of precursor lengths as a function of external load is in good quantitative agreement with laboratory experiments as well as with dynamic simulations, and provides thereby evidence to recognize frictional slip as a fracture phenomenon. We show that predicted precursor lengths depend, within given uncertainty ranges, mainly on the kinetic friction coefficient, and only weakly on other interface and material parameters. By simplifying the fracture mechanics model, we also reveal sources for the observed nonlinearity in the growth of precursor lengths as a function of the applied force. The discrete nature of precursors as well as the shear tractions caused by frustrated Poisson’s expansion is found to be the dominant factors. Finally, we apply our model to a different, symmetric setup and provide a prediction of the propagation distance of frictional slip for future experiments.

Dependency of Near-Field Ground Motions on the Structural Maturity of the Ruptured Faults

Littlework has been undertaken to examine the role of specific long-term fault properties on earthquake ground motions. Here, we empirically examine the in- fluence of the structural maturity of faults on the strong ground motions generated by the rupture of these faults, and we compare the influence of fault maturity to that of other source properties (slip mode, and blind versus surface rupturing). We analyze the near-field ground motions recorded at rock sites for 28 large (Mw 5.6-7.8) crustal earthquakes of various slip modes. The structural maturity of the faults broken by those earthquakes is classified into three classes (mature, intermediate, and immature) based on the combined knowledge of the age, slip rate, cumulative slip, and length of the faults. We compare the recorded ground motions to the empirical prediction equa- tion of Boore et al. (1997). At all frequencies, earthquakes on immature faults produce ground motions 1.5 times larger than those generated by earthquakes on mature faults. The fault maturity appears to be associated with larger differences in ground-motion amplitude than the style of faulting (factor of 1.35 between reverse and strike-slip earthquakes) and the surface rupture occurrence (factor of 1.2 between blind and surface-rupturing earthquakes). However, the slip mode and the fault maturity are dependent parameters, and we suggest that the effect of slip mode may only be apparent, actually resulting from the maturity control. We conclude that the structural maturity of faults is an important parameter that should be considered in seismic hazard assessment.

Two successive slow slip events evidenced in 2009–2010 by a dense GPS network in Guerrero, Mexico

A large slow slip event (SSE) had been expected for the Guerrero gap for 2010. It was actually observed with an onset in July 2009. Comparison with the preceding large SSEs, which occurred in 2002 and 2006, highlights both persistent characteristics of the Guerrero SSEs (e.g. the localization of slip in the seismogenic part of the subduction interface), and also particularities of the 2009/2010 event (namely two distinct slip patches on the fault interface moving consecutively). The long GPS time series and the density of the GPS network provide evidence that the Guerrero SSEs, like classical earthquakes, have complex features. Despite having very short and relatively regular repeat times (∼4 yr), Guerrero SSEs appear aperiodic. A shorter loading time before the 2009/2010 event than before the 2006 SSE seems to produce consistently reduced surface displacements for a group of stations in a core zone.

Properties of the shear stress peak radiated ahead of rapidly accelerating rupture fronts that mediate frictional slip

Significance The transition from stick to slip is governed by rupture fronts propagating along a frictional interface. It has long been suggested that rapid acceleration of these ruptures generates a shear stress peak that propagates ahead of the front. These peaked waves are strong; they can reach amplitudes that are large enough to trigger secondary supershear ruptures. We provide the first extensive quantitative experimental study to our knowledge of these highly directed waves and their relation to the rupture fronts driving them. Combining our experiments with finite-element simulations, we observe how these waves scale. This study provides insight into how rupture fronts accelerate beyond the shear-wave speed and may offer a possibility to obtain illusive information about propagating earthquakes. We study rapidly accelerating rupture fronts at the onset of frictional motion by performing high-temporal-resolution measurements of both the real contact area and the strain fields surrounding the propagating rupture tip. We observe large-amplitude and localized shear stress peaks that precede rupture fronts and propagate at the shear-wave speed. These localized stress waves, which retain a well-defined form, are initiated during the rapid rupture acceleration phase. They transport considerable energy and are capable of nucleating a secondary supershear rupture. The amplitude of these localized waves roughly scales with the dynamic stress drop and does not decrease as long as the rupture front driving it continues to propagate. Only upon rupture arrest does decay initiate, although the stress wave both continues to propagate and retains its characteristic form. These experimental results are qualitatively described by a self-similar model: a simplified analytical solution of a suddenly expanding shear crack. Quantitative agreement with experiment is provided by realistic finite-element simulations that demonstrate that the radiated stress waves are strongly focused in the direction of the rupture front propagation and describe both their amplitude growth and spatial scaling. Our results demonstrate the extensive applicability of brittle fracture theory to fundamental understanding of friction. Implications for earthquake dynamics are discussed.

Lateral Variations of Interplate Coupling along the Mexican Subduction Interface: Relationships with Long-Term Morphology and Fault Zone Mechanical Properties

Although patterns of interseismic strain accumulation above subduction zones are now routinely characterised using geodetic measurements, their physical origin, persistency through time, and relationships to seismic hazard and long-term deformation are still debated. Here, we use GPS and morphological observations from southern Mexico to explore potential mechanical links between variations in inter-SSE (in between slow slip events) coupling along the Mexico subduction zone and the long-term topography of the coastal regions from Guerrero to Oaxaca. Inter-SSE coupling solutions for two different geometries of the subduction interface are derived from an inversion of continuous GPS time series corrected from slow slip events. They reveal strong along-strike variations in the shallow coupling (i.e. at depths down to 25 km), with high-coupling zones (coupling >0.7) alternating with low-coupling zones (coupling <0.3). Coupling below the continent is typically strong (>0.7) and transitions to uncoupled, steady slip at a relatively uniform

GPS deformation related to the Mw 7.3, 2014, Papanoa earthquake (Mexico) reveals the aseismic behavior of the Guerrero seismic gap

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