Global positioning system (GPS) time series in Guerrero (Mexico) reveal the existence of large slow slip events (SSEs) at the boundary between the Cocos and North American plates. In this study, we examined the last three SSEs that occurred in 2001/2002, 2006 and 2009/2010, and their impact on the strain accumulation along the Guerrero subduction margin. GPS displacements were inverted to retrieve the slip distribution during each SSE and the inter‐SSE coupling of the subduction interface. The three analyzed SSEs have equivalent moment magnitudes of between 7.50 and 7.65, their lateral extents are variable, and they all show significant slip in the Guerrero seismic gap. During the inter‐SSE epochs, the interplate coupling is high in the area where slow slip subsequently occurs. In the Guerrero gap, the shallow portion of the plate interface from the trench to the coast is weakly coupled. The average slip deficit accumulated in the Guerrero gap over a period of 12 years, which corresponds to three cycles of SSE, is only 1/4 of the slip deficit accumulated on both sides of the gap. Moreover, the regions of large slip deficit coincide with the rupture areas of recent large earthquakes. We conclude that the SSEs in the Guerrero gap release a significant part of the strain accumulated during the inter‐SSE period. If large subduction thrust earthquakes occur in the Guerrero gap, their recurrence time is probably increased compared to adjacent regions.

No abstract

International audienceThe Guerrero 2006 Slow Slip Event (SSE), Mexico, one of the world's largest observed SSEs, was recorded at 15 continuous GPS stations. This event provides the opportunity to analyse in detail the spatial and temporal evolution of slip at depth, and to constrain the characteristics of a large SSE. We perform an inversion in two steps. First, we invert the cumulative GPS displacements to retrieve the total slip amplitude. Second, we invert for the initiation time and duration of the slip, using a linearized least-squares inversion procedure and assuming a functional form for the slip function. Our results show that the slip is located on a patch of 300 km × 150 km (parallel and perpendicular to the coast, respectively), and extends from the bottom of the seismogenic zone to the transition zone. This slow slip event has an equivalent moment magnitude of 7.5. The maximum slip over a length scale of 25 km is 15 cm and the mean slip is 5.5 cm. Its lateral extension coincides with the segmentation of the subduction. Our inversion scheme allows us to analyse the spatial variability of the rise time, rupture velocity and slip function. We obtain a continuous image of the spatial and temporal variations of slip on the fault plane. The rupture initiated at a depth of 40 km (transition zone), in the western part of the Guerrero gap. The rupture then propagated from the western to the eastern part of the Guerrero segment with an average velocity of 0.8 km d−1. Our results show that a slip dislocation pulse, characterized by a symmetric ramp function, can model the 2006 SSE. The rise time (local duration of slip) does not show large spatial variations and is equal to about 185 d. The local slip duration is compared to the total duration (11-12 months) of the event, suggesting a large interaction of a large part of the fault during the dynamic process. We find that our inverted slip model is well resolved on the shallow part of the fault and in the central section of the fault

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.

5p.International audienceRepeated 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

When a frictional interface is subject to a localized shear load, it is often (experimentally) observed that local slip events initiate at the stress concentration and propagate over parts of the interface by arresting naturally before reaching the edge. 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 non-linearity 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 are found to be the dominant factors. Finally, we apply our model to a different, symmetric set-up and provide a prediction of the propagation distance of frictional slip for future experiments.

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 finiteelement 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.nonsteady rupture dynamics | acoustic radiation | friction | earthquake dynamics | seismic radiation T he onset of motion along a frictional interface entails rupture-front propagation. These rupture fronts have long been considered to have much in common with propagating cracks (1-3). Recent friction experiments (4) have shown that the stresses and material motion surrounding the tip of a propagating rupture are indeed quantitatively described by singular linear elastic fracture mechanics (LEFM) solutions originally developed for brittle shear fracture. These singular fields are only regularized by dissipative and nonlinear processes in the vicinity of the rupture tip.Nonsteady processes such as rapid rupture velocity variation during the nucleation or arrest phases result in the generation of stress-wave radiation (2, 5). In the study of earthquakes, understanding the source mechanism of those waves is of primary importance. Long-wavelength radiation is usually described by simple dislocation models (3, 6). High-frequency radiation, however, was proposed (2) to be controlled by the strong slip velocity concentrations at the rupture tip predicted by fracture mechanics. Descriptions that go beyond singular contributions to fracture involve significant analytical complications; full solutions of nonsteady dynamic crack problems are generally extremely difficult to obtain. Of the few full-field analytic solutions available, self-similar solutions of suddenly expanding shear cracks have provided much intuition (2,5,7,8). These solutions, under shear loading (mode II), predic...

No abstract