Fault Mechanics Webinar: the role of fault asperity in the generation of laboratory earthquake


The role of fault asperity in the generation of laboratory earthquake
Lifeng Wang, State Key Laboratory of Earthquake Dynamics, China Earthquake Administration

Fault asperity is often deemed to be responsible for occurrences of seismic clusters, repeating earthquakes, or concentrated slip during large events. However, because of the inaccessible fault surface, its possible presence and relevant role in mechanical controls are still based on indirect inferences from fault activities, and a direct measure of the roles of fault asperity is still missing. Here, we take the advantage of a rock experiment about two half-meter granite plates with apparent asperity structures. Acoustic emission and fault slip measurements, assisted by numerical simulation, unveil asperity acting as a mechanical attractor to small events during the interseismic phase, while as a barrier to slow slip during the mainshock nucleation. The slip deficit long hosted by asperity is finally filled up, producing the largest coseismic slip there. The full scenario unfolds how fault asperity partitions stress in both temporal and spatial domains, constituting a physical link about the background seismicity, foreshocks, preseismic slow slip and the characteristic mainshock. [more info] [register]

Next Week

Unravelling complex deformation and localization of brittle failure in triaxial tests on crystalline rock
Paul Selvadurai, ETH

In triaxial tests on crystalline rock, the deformation and failure behavior can be complex and highly localized. Brittle failure in rock typically involves the formation and propagation of cracks or fractures, which can occur at various scales depending on the rock properties and loading conditions. In some cases, the deformation and failure behavior may be highly localized, with cracks or fractures forming in specific regions of the rock sample. This can be due to heterogeneities in the rock properties, stress concentrations, or the presence of pre-existing fractures that act as preferred pathways for crack propagation. To better understand the complex deformation researchers may use various experimental techniques, such as acoustic emission monitoring, digital image correlation, and computed tomography. These techniques can provide insights into the evolution of cracks and fractures within the rock sample, as well as the distribution of stress and strain. At ETH Zurich, in the Rock Physics and Mechanics laboratory, we have deployed distributed strain sensing (DSS) techniques with fiber optic technologies to continue to unravel the complex strain field – a rarely measured and difficult metric to constrain in triaxial rock deformation tests. This technique is of particular interest when studying the accumulation of damage and generation of shear fractures during failure tests. We pair this novel technology with the study of acoustic emissions (AE). The latter allows us to study failure processes by measuring local microcracking and fast deformation that can be interpreted by the seismic signatures using traditional seismological tools. We show a rich spatio-temporal understanding of deformation and illustrate how damage accumulates as microcracks in the sample grow and coalesce into an eventual macrofracture that leads to catastrophic brittle failure. Understanding the interplay between slow and fast deformation and how damage localizes in geomaterial can improve our fundamental understanding of subsurface processes and provide quantitative means to improve numerical techniques.
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