Probing Fault Zone Evolution with Ultrasonic Measurements: Seismic Imaging in Laboratory Experiments

Faults can slip in diverse modes, ranging from slow, aseismic creep to dynamic, earthquake-generating rupture. Geological observations reveal that many fault zones consist of localized slip zones surrounded by a broader damage zone, where microcracks and fractures interact in complex ways. Friction laws propose that whether a fault will host a slow slip event or a fast dynamic rupture depends on the relative stiffness of these slip zones and the surrounding material. Conversely, the local stress field imposes such structures' evolution. Although the evidence for these complex interactions indicates the opportunity to incorporate this knowledge in the theoretical framework, collecting data on the spatiotemporal evolution of elastic properties at seismogenic depth is inherently challenging, leaving this possibility mostly unexplored.

Laboratory experiments provide a controlled environment for studying the evolution of fault mechanics and elastic properties. Elastic waves are governed by the same equations that relate wave speeds to dynamic moduli. Therefore, they offer a pathway to link laboratory observations to natural fault processes.

This study investigates how microstructural reorganization during fault deformation and fault zone structure formation affects fault zone stiffness and slip behavior using synthetic quartz gouge layers sheared in double-direct shear (DDS) configuration.
Our DDS setup is instrumented with piezoelectric transducers designed to generate and record predominantly compressional (P) or shear (S) waves. By carefully characterizing the source time function, we ensure that early arrivals in each recorded signal represent a single wave mode with minimal mode conversion or side reflections.

We then apply Full Waveform Inversion (FWI) to these early arrivals to reconstruct velocity models for both P- and S-waves as deformation progresses. The inverted models reveal spatiotemporal variations in the bulk and shear moduli, which we interpret as signatures of contact-area changes, grain size reduction, and other micromechanical processes relevant to frictional stability. In particular, the evolving elastic properties allow us to gauge how the local stiffness of the gouge zone evolves relative to applied stress, linking the observed velocity changes to the constitutive laws underpinning rate-and-state friction (RSF). While RSF implicitly links frictional strength to contacts dynamic through a state variable, our results illustrate how ultrasonic waveform acquisition and modeling can provide hints toward the explicit rewriting of such laws in terms of the evolution of elastic properties, an intermediate level of description easier related to micromechanical processes.

This approach highlights the potential for ultrasonic measurements in earthquake laboratory experiments to probe fault zone mechanics and outline a framework for integrating seismic imaging with frictional mechanics to better understand fault behavior across scales.