Search

While sliding at seismic slip-rates of ca. 1 m/s, a natural fault undergoes an abrupt decrease of its strength called enhanced dynamic weakening. Asperity-scale (<< mm) processes related to flash heating & weakening and meso-scale (mm-cm) processes involving shear across the bulk slipping zone related to frictional melting or viscous flow of minerals, have been invoked to explain pronounced velocity-dependent weakening. Here we present a compilation of ca. 100 experiments performed with two rotary shear apparatuses, i.e. SHIVA installed in INGV (Rome, Italy) and HVR installed, at the time of experiments, in Kyoto University (Japan). Cohesive rock cylinders of basalt, gabbro, tonalite, granite and calcitic marble were sheared under a range of effective normal stresses (sn=5-40 MPa), target slip-rates (Vt=0.1-6.5 m/s) and fluid pressures (from room humidity conditions RH or Pf=0, to Pf =15 MPa). We fit the measured shear stress evolution with slip with two dynamic weakening mechanisms models, which include, depending on rock type: (1) flash heating and bulk melting (granitoid, gabbro and basalt), (2) flash heating and diffusion creep (calcitic marble), (3) flash heating and dislocation creep (calcitic marble). We provide a set of optimized parameters, specific for each mechanism, that control the dynamic weakening.Lastly, the modelling procedure allow us to estimate the slip-switch distance d0, i.e. the slip necessary for the complete transition from the asperity-scale to bulk slipping zone dynamic weakening mechanism. Our analysis shows that (1) the d0 decreases with increasing effective normal stress acting on the fault and, (2) for the same type of transition between dynamic weakening mechanisms (e.g., from flash heating to bulk melt lubrication) the d0 is a function of rock composition. The decrease of d0 with normal stress indicates that during earthquakes, bulk mechanisms dominate over asperity scale weakening mechanisms with increasing crustal depths. This study provides constitutive law parameters to be included in physically- and geologically-based dynamic earthquake rupture simulations.

Pseudotachylytes originate from the solidification of frictional melt, which transiently forms and lubricates the fault plane during an earthquake. Here we observe how the pseudotachylyte thickness a scales with the relative displacement D both at the laboratory and field scales, for measured slip varying from microns to meters, over six orders of magnitude. Considering all the data jointly, a bend appears in the scaling relationship when slip and thickness reach ∼1 mm and 100 µm, respectively, i.e. MW > 1. This bend can be attributed to the melt thickness reaching a steady‐state value due to melting dynamics under shear heating, as is suggested by the solution of a Stefan problem with a migrating boundary. Each increment of fault is heating up due to fast shearing near the rupture tip and starting cooling by thermal diffusion upon rupture. The building and sustainability of a connected melt layer depends on this energy balance. For plurimillimetric thicknesses (a > 1 mm), melt thickness growth reflects in first approximation the rate of shear heating which appears to decay in D−1/2 to D−1, likely due to melt lubrication controlled by melt + solid suspension viscosity and mobility. The pseudotachylyte thickness scales with moment M0 and magnitude MW; therefore, thickness alone may be used to estimate magnitude on fossil faults in the field in the absence of displacement markers within a reasonable error margin.

Several earthquake source parameters cannot be estimated from the analysis of seismic waves, instead, they may be derived from field surveys and experimental studies. Among these parameters, the fault strength evolution (tf (t) in MPa) and the frictional power dissipation ( Q'= tf (t) V(t) in MW m-2, with V being the slip rate) during seismic slip control the moment release rate, the temperature increase in the slip zone and therefore the activation of coseismic fault dynamic weakening mechanisms. Frictional melts (preserved as pseudotachylytes) along the slip zone can be the result of relatively high Q'. In fact, shear heating is proportional to Q': the higher Q', the higher the heat production rate and, consequently, the faster the temperature increase in the slip zone and the steeper the temperature gradient in the boundary rocks (Nielsen et al., 2010). [PR1] The tonalite rocks used in this study come from the Gole Larghe Fault zone (Southern Alps, Italy), and they are made of minerals with different individual melting temperatures. The presence of a steep temperature gradient (high Q') with closely-spaced isotherms at the boundary walls, will cause the minerals to melt uniformly near the sliding surface (i.e. independently of their melting points), resulting in a relatively smooth pseudotachylyte-wall rock boundary. On the other hand, a gentle temperature gradient (low Q') with widely-spaced isotherms will mainly melt those minerals with low melting points, generating higher micro-roughness.To consider these different scenarios, we collected samples of natural pseudotachylytes belonging to ‘wavy’ faults, together with samples of injection veins (tensile cracks with Q' -> 0). A ‘wavy’ fault presents shear cracks from compressional (high Q'), neutral, and extensional (low Q') domains along strike. We performed a series of experiments using a rotary shear apparatus (i.e., SHIVA, Di Toro et al., 2010) to produce artificial pseudotachylytes at increasing slip rates and normal stresses corresponding to values of increasing Q', ranging from 5 to 25 MW m-2. The micro-roughness is then measured from optical and scanning electron microscope images obtained both from natural and artificial samples for comparison. We found that in the experimental samples, the micro-roughness is inversely proportional to Q', as predicted by the theoretical model. Natural samples show similar trends with the higher micro-roughness present in the injection veins where Q' -> 0. This study demonstrates the robustness of the relation between and fault micro-roughness in both natural and experimental samples. However, further investigations are required to calibrate this methodology to estimate quantitatively the frictional power dissipated during natural earthquakes.