Laboratory experiments suggest that the evolution of in-plane shear rupture along an interface separating two elastic blocks typically shows a transition from slow to fast slip. In contrast to the commonly used continuum mechanics-based approaches, here we study the shear rupture process along a weak interface using the discontinuous deformation analysis (DDA) method. We incorporate a slip-weakening constitutive friction law to simulate the initiation and propagation of shear rupture under external conditions of a constant normal load and a steadily increased shear load. As the shear load increases, our modeling results reveal a sharp transition from episodic expansion and arrest to unstable runaway rupture, consistent with previous experimental results. In the stage of dynamic runaway, rupture velocity is limited by the Rayleigh wave velocity. We further investigate the effects of external loading conditions including load point velocity and normal stress on rupture behavior. We find that the dynamic rupture velocity increases with load point velocity and normal stress, also consistent with previous studies. Our results indicate that the DDA method can well capture some of the general characteristics of shear rupture process and, hence, can be applied to study other aspects of dynamic shear ruptures.

Recent large-scale seismological observations have shown that off-fault strain localization and foreshock migration could serve as an early warning of an impending earthquake. However, this process is still largely unknown. In this study, state-of-the-art friction experiments were conducted in a oil-confined biaxial shear apparatus to investigate the link between stick-slip nucleation and off-fault deformation. Our findings indicate that there is a direct link between stick-slip nucleation and off-fault deformation, provided that the fault is conditionally unstable (a - b < 0). Inelastic off-fault deformation may trigger unstable slip by decreasing the stiffness of the surrounding rock volume, which favors earthquake nucleation. Additionally, the study presents laboratory observation of precursory strain localization around a fault during stick-slip cycles. These findings suggest that volumetric deformation processes could be a main factor in the nucleation of large ruptures and strain localization could be a reliable harbinger of large earthquakes.

Surface roughness ubiquitously prevails in natural faults across various length scales. Despite extensive studies highlighting the important role of fault geometry in the dynamics of tectonic earthquakes, whether and how fault roughness affects fluid-induced seismicity remains elusive. Here, we investigate the effects of fault geometry and stress heterogeneity on fluid-induced fault slip and associated seismicity characteristics using laboratory experiments and numerical modeling. We perform fluid injection experiments on quartz-rich sandstone samples containing either a smooth or a rough fault. We find that geometrical roughness slows down injection-induced fault slip and reduces macroscopic slip velocities and fault slip-weakening rates. Stress heterogeneity and roughness control hypocenter distribution, frequency-magnitude characteristics, and source mechanisms of injection-induced acoustic emissions (AEs) (analogous to natural seismicity). In contrast to smooth faults where injection-induced AEs are uniformly distributed, slip on rough faults produces spatially localized AEs with pronounced non-double-couple source mechanisms. We demonstrate that these clustered AEs occur around highly stressed asperities where induced local slip rates are higher, accompanied by lower Gutenberg-Richter b-values. Our findings suggest that real-time monitoring of induced microseismicity during fluid injection may allow identifying progressive localization of seismic activity and improve forecasting of runaway events

Aseismic slip may occur during a long preparatory phase preceding earthquakes, and what controls it remains poorly understood. In this study, we explored the role of load point velocity and surface roughness on slow slip during the preparatory stage prior to stick‐slip events. To that end, we conducted displacement‐rate controlled friction experiments by imposing varying load point velocities on sawcut granite samples with different surface roughness at a confining pressure of 35 MPa. We measured the average slip along the fault with the recorded far‐field displacements and strain changes, while acoustic emission sensors and local strain gages were used to capture local slip variations. We found that the average amount of aseismic slip during the preparatory stage increases with roughness, whereas precursory slip duration decreases with increased load point velocity. These results reveal a complex slip pattern on rough faults which leads to dynamic ruptures at high load point velocities.

Natural faults are heterogeneous features, with complex geometries and material properties. Understanding how the geometrical complexities of a fault affects the dynamics and preparatory phase of earthquakes is of crucial importance for seismic hazard assessment. In laboratory samples, frictional sliding along prefabricated faults may produce so called stick-slips comparable to dynamic ruptures observed during earthquakes. While the effect of roughness has been shown to influence significantly the frictional behavior of laboratory faults, there are only a few studies investigating more complex types of fault heterogeneities. In this study, we conduct friction experiments on granite with inclined sawcut faults, under a constant confining pressure of 35MPa. Samples are loaded using an axial displacement rate of 0.5 µm/s. At similar boundary conditions we compare the slip behavior of (1) a smooth fault, (2) a smooth fault with a single asperity, a 7 mm diameter vertical pin traversing the contact interface, and (3) a rough fault prepared by sandblasting the surface with silicon carbide. A key result of this study is that slip behavior depends on fault roughness and is influenced in a non-trivial way by asperities. The smooth fault displays unstable stick-slip as opposed to the rough fault showing predominantly creep. The smooth fault with the pin exhibits a slip behavior in-between, with very regular stress oscillations that seem to be attenuated by the presence of the pin (asperity). Only after failure of the pin, we observe the stress drop during instabilities to increase regularly with cumulative slip. We also show that in the case of a fault with a single asperity, the slip velocity is less than an order of magnitude lower compared to a similar smooth fault without this asperity.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Preparatory Slip in Laboratory Faults: Effects of Roughness and Loading RateAuthorsSimonGuérin-MartheiDGrzegorzKwiatekiDLeiWangiDAudreyBonnelyePatriciaMartínez-GarzónGeorgDresenSee all authors Simon Guérin-MartheiDCorresponding Author• Submitting AuthorHelmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,iDhttps://orcid.org/0000-0003-3578-037Xview email addressThe email was not providedcopy email addressGrzegorz KwiatekiDHelmholtz Centre Potsdam GFZ German Research Centre for GeosciencesiDhttps://orcid.org/0000-0003-1076-615Xview email addressThe email was not providedcopy email addressLei WangiDHelmholtz-Zentrum Potsdam - Deutsches GeoforschungszentrumiDhttps://orcid.org/0000-0001-6784-4572view email addressThe email was not providedcopy email addressAudrey BonnelyeGFZview email addressThe email was not providedcopy email addressPatricia Martínez-GarzónHelmholtz Centre Potsdam GFZ German Research for Geosciencesview email addressThe email was not providedcopy email addressGeorg DresenGFZ Potsdamview email addressThe email was not providedcopy email address

Rapid estimation of the intensity of seismic ground motions is crucial for an effective rapid response when an earthquake occurs. To this end, maps of updated grond-motion fields (or shakemaps) are produced by using observations or measurements in near real-time to better constrain initial estimates. In this work, two types of observations are integrated to generate shakemaps right after an earthquake: the common type of data recorded by physical sensors (seismic stations) and the data extracted from social sensors (Twitter), or the combination of both. We investigate an approach to extract an approximation of the macroseismic intensity from social sensors 10 min after the earthquake; the approach relies on Twitter feeds to define the "felt area" where the earthquake was felt by the population, and the "unfelt locations" where the earthquake was not reported. Two recent earthquakes in France of moderate magnitude are studied and the results are compared to the official macroseismic intensity maps for validation. For the two studied cases, we note that Peak Ground Acceleration recordings far from the epicenter tend to underestimate the entire macroseismic field, and that the tweets from "felt areas" are complementary for a better estimation of the intensity shakemap. We highlight the importance and the limits of each type of observations when generating the seismic shakemaps.

Systems for Rapid Response to Earthquakes (RRE) aim at providing reliable and near-real time ground-motion and loss estimates following an earthquake, in order to help authorities taking appropriate actions when needed. The purpose of this study is to review the state-of-the-art for RRE systems, focusing first on the tools and methods that have been developed for shake-map computation (i.e., taking into account the seismic recordings as well as macroseismic observations when available), in order to provide a map of the ground shaking intensity rapidly after an earthquake event. The second part focuses on the different approaches taken for rapid loss assessment, the ones based on shake-maps and the others. We give an overview of the current operating systems with emphasis on European ones, and we highlight their differences and identify the current gaps and pending issues. Finally, we call attention for the need to treat carefully uncertainties propagated at each calculation step, which added up are non-negligible, and are an important part of the result itself. Considering the full statistical distribution of loss and damage estimates provides more information than average or median values, and such statistics should therefore be provided by RRE systems and taken into account by decision-makers in order to take informed actions following an earthquake.

The transpressional Alpine Fault in New Zealand has created a thick shear zone with associated highly anisotropic rocks. Low seismic velocity zones and high seismic reflectivity are recorded in the Alpine Fault Zone, but no study has explored the underlying physical rock parameters of the shallow crust that control these observations. Protomylonites are the volumetrically dominant lithology of the fault zone. Here we combine experimental measurements of P-wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged. Numerical models that account for the porosity-free real samples' fabric elastic tensors from electron backscatter diffraction (EBSD) are calculated by MTEX and a finite element model (FEM), while microfractures are modeled with differential effective medium (DEM) theory. At effective pressures representative of the Alpine Fault brittle zone, experimental wave speeds are lower than those predicted by MTEX/FEM. A possible DEM model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures <70MPa. Such microporosity in the form of foliation- and mica basal plane-parallel microfractures and grain boundaries is validated with synchrotron X-ray microtomography and transmission electron microscopy (TEM) images. Finally, by modeling anisotropy of seismic reflection coefficients with angle of incidence, we demonstrate that the high reflectivity and low-velocity zone (LVZ) observed at the Alpine Fault can only be explained if this microporosity is accounted for throughout the brittle fault zone, even at depths of 7-10km.

Earthquakes represent one of the most important natural risks facing human populations in urban areas. Understanding the processes at the origin of these destructive events requires seismological observations, but also the use of laboratory analogues and numerical models for earthquake rupture. They allow for controlled conditions under which we can investigate the relative importance of different physical quantities involved in the system. The main points investigated in this thesis are the influence of loading rate on the nucleation of earthquakes, and the evolution of friction during dynamic ruptures. I conduct photoelastic experiments using polycarbonate plates, but also direct-shear experiments of precut granite blocks in a pressure vessel. I use finite-difference numerical models to reproduce and understand the dynamic laboratory ruptures, and I developed static finite element codes in order to reproduce the loading conditions induced by the experimental setup. The main results are that under certain conditions, increasing the loading rate makes the nucleation length shrink, and affects the nucleation position, which in this case is consistently situated on high coulomb stress areas. This is not necessarily the case for low loading rates. The shrinking of nucleation length may explain partly why some asperities in subduction zones can behave seismically or aseismically depending on the local tectonic loading velocity. Finally, I propose a method to estimate the dependence of friction on slip and slip velocity from strain gauge data during friction experiments. When conducted under realistic pressure conditions, this can provide useful constitutive laws to implement in numerical models simulating earthquakes. Eventually, the results presented in this thesis can be used in order to improve rupture scenarios, and short-term earthquake forecast.

Recent Global Positioning System observations of major earthquakes such as the 2014 Chile megathrust show a slow preslip phase releasing a significant portion of the total moment (Ruiz et al., 2014, ). Despite advances from theoretical stability analysis (Rubin & Ampuero, 2005, ; Ruina, 1983, ) and modeling (Kaneko et al., 2017, ), it is not fully understood what controls the prevalence and the amount of slip in the nucleation process. Here we present laboratory observations of slow slip preceding dynamic rupture, where we observe a dependence of nucleation size and position on the loading rate (laboratory equivalent of tectonic loading rate). The setup is composed of two polycarbonate plates under direct shear with a 30-cm long slip interface. The results of our laboratory experiments are in agreement with the preslip model outlined by Ellsworth and Beroza (1995, ) and observed in laboratory experiments (Latour et al., 2013, ; Nielsen et al., 2010, ; Ohnaka & Kuwahara, 1990, ), which show a slow slip followed by an acceleration up to dynamic rupture velocity. However, further complexity arises from the effect of (1) rate of shear loading and (2) inhomogeneities on the fault surface. In particular, we show that when the loading rate is increased from 10(-2) to 6MPa/s, the nucleation length can shrink by a factor of 3, and the rupture nucleates consistently on higher shear stress areas. The nucleation lengths measured fall within the range of the theoretical limits L-b and L derived by Rubin and Ampuero (2005, ) for rate-and-state friction laws.