In this paper, we focus the attention on the formation of cementitious phases made of micro- and nano-scale fibrous structures, and the controls of the arrangement of these phases on mechanical properties. This is an area of rock physics still in its infancy. That requires the understanding of how structural arrangements at the micro and nano scale control the physical and mechanical properties at the macroscopic scale. Studying the mechanisms that control the rheology of rocks and geomaterials is crucial as much for predicting geological processes as for functionalizing geomaterials. There are in fact segments of active margins worldwide, from the Cascadia subduction zone, to the San Andreas strike-slip system, being traditionally associated with fibrous mineral phases (Louderback, 1942 Schleicher, et al., 2006 Kirby et al., 2014 Harris, 2017 Moore et al., 2018) so this study advances new knowledge of understanding the role of fibrous microstructures in controlling brittle-to-ductile rheological behaviors, and possibly, slow slip events (Voss et al., 2018 Nuyen and Schmidt, 2021). The applied value of this study lies in the possibility to further understand the physical properties of the brittle-ductile transitions-from large earthquakes occurring as sudden slip events expressing a catastrophic (brittle) failure to slow-slip or clusters of many small earthquakes characterizing regions of the crust that benignly creep for long periods of time (Obara K., 2002 Rogers and Dragert, 2003 Brenguier, et al., 2008 Chen and Bürgmann, 2017 Harris, 2017). Thus, microphysics-based models have been proposed as an alternative to classical RSF for interpreting laboratory and field observations (Van den Ende et al., 2018), which require knowledge of the micro-and nanogeometry of structural-rock samples to calculate the resistance of an interface to sliding. Train tremors identified by the present approach have been successfully used for seismic velocity monitoring of the San Jacinto Fault in previous studies. However, extra steps, such as beamforming or polarization analysis, are required to determine the dominant seismic sources and study the source characteristics, which are crucial to interpreting the velocity monitoring results. These findings have great potential for monitoring fault zones, including the San Jacinto Fault and the Ridgecrest area in Southern California, Napa in Northern California, and faults in central Japan. Multiple sites close to fault zones show high‐frequency CFs stable for an extended period of time. The algorithm is tested in California and Japan. We tackle the problem from a statistical point of view, considering that persistent, powerful seismic sources yield highly coherent correlation functions (CFs) between pairs of seismic sensors. In this work, we propose a systematic workflow to seek such powerful seismic sources in a rapid and straightforward manner. Anthropogenic seismic sources typically have fixed spatial distribution and provide new perspectives for velocity monitoring. It has been recently found that some anthropogenic, high‐frequency (>1 Hz) seismic sources are powerful enough to generate body waves that travel down to a few kilometers and can be used to monitor fault zones at seismogenic depth. The temporal variation of seismic velocity during the preparation phase of earthquakes has been well documented in laboratories but rarely observed in nature. Seismic velocities in rocks are highly sensitive to changes in permanent deformation and fluid content.
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