Multiple Seismic Slip‐Rate Pulses and Mechanical and Textural Evolution of Calcite‐Bearing Fault Gouges

<jats:title>Abstract</jats:title><jats:p>Natural fault zones are complex, spatially heterogeneous systems. Rock deformation experimental studies simplify the complexity of natural fault zones either as a surface discontinuity between intact rocks (bare‐rock surfaces) or as a few mm‐thick gouge layer. However, depending on the simplified fault type and its slip history, the response to applied deformation can vary. In this work, we conduct laboratory experiments for investigating the evolution of mechanical parameters of simulated faults made of calcite gouge subjected to multiple (four) identical seismic slip‐rate pulses. We observed that, as the number of applied slip‐rate pulses increased, (a) initial friction and steady‐state friction remained approximatively constant, (b) peak friction and normalized strength excess increased and, (c) the slip distances to achieve peak and steady‐state friction, <jats:italic>D</jats:italic><jats:sub><jats:italic>a</jats:italic></jats:sub> and <jats:italic>D</jats:italic><jats:sub><jats:italic>c</jats:italic></jats:sub>, decreased. The greatest changes occurred between the first and the second slip‐rate pulse. From this pulse onward, the dissipated energy of the calcite gouge fault was similar to those obtained in bare‐rock surfaces experiments. Microstructural analysis showed that, strain is localized in up to two (recrystallized) principal slip zones (PSZ) with sub‐micrometric grain size, surrounded by low porosity sintered and non‐sintered comminuted gouge domains. We conclude that previous seismic slip episodes impact on both the structure and the strain localization processes within a fault, contributing to its shear fabric evolution. We highlight that the strain localization process identifies the PSZ, dissipating the least amount of energy within the entire experimental fault zone.</jats:p>

This study has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement 856559). MC participated in this work as Principal Investigator of the ERC project FEAR (Grant agreement 856559) under the European Community's Horizon 2020 Framework Programme. SA, CC, and ES participated in this work in the framework of the ERC project FEAR (Grant agreement 856559) under the European Community's Horizon 2020 Framework Programme. This work was also supported by Project FSE+ 2021–2027, Contributi premiali per i ricercatori e assegnisti di ricerca per rafforzarne la condizione professionale e potenziare il sistema della ricerca del Lazio (Atto n. G05411 del 05/05/2022) attributed to SA, ES and CC. GDT acknowledges the ERC CoG project 614705 NOFEAR and Progetto PRIN 2022 Di Toro - 2022WE2JY9. CC acknowledges Christopher Harbord for the technical development of the gouge sample holder, Stefano Castelli for the optical scans of the polished samples, and Giacomo Pozzi and Manuela Nazzari for assistance with the FEG-SEM at INGV, and Jacopo Nava and Leonardo Tauro for assistance with the FEG-SEM at University of Padua.

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