Permeability and seismic velocity evolution of thermally cracked Aji granite during cyclic loading

The time dependent Vp/Vs ratio (Scholz et al., 1973), preslip (Dieterich, 1978), and cascade models (Ellsworth and Beroza, 1995) are popular geophysical models used as earthquake precursors. Such geophysical models have generally focused on the physics of earthquake source regions (Yamashita et al., 2021). However, the physical properties of the off-fault region are also related to earthquake mechanics (Gudmundsson, 2004), which are strongly affected by the presence of fractures (Jayawickrama et al., 2024). While the presence of fractures affects the physical properties of rocks, inarguably, they alter the transport properties as well. In this investigation, it is speculated that such changes in the transport properties coupled with the seismic velocity in the off-fault regions could be used to develop an earthquake precursor model. To this end, cyclic deformation experiments were conducted on thermally cracked (at 800^oC) Aji granite, in analogy to damage, while simultaneously measuring the water permeability and the seismic velocity. However, unlike the conventional cyclic experiments, a stationary time was employed between each cycle (loaded to 0.6C, 0.7C, 0.8C, and 0.9C) around 0.1C (C: peak stress), in analogy to a stress relaxation period before the stress accumulation for the next event. With the initial stress build up, the seismic velocity increases while the permeability drops until the onset of dilation, where the trend is reversed from there onwards. Once the axial stress is released gradually, followed by an initial increase, the permeability becomes relatively stable, while the seismic velocity continues to drop. During the stationary period, a noticeable compaction was observed along with a seismic velocity increase and a permeability drop. Hence, indicated an overall healing in the damaged sample, under relaxed stress conditions. The dynamic crack density evolution also provides critical evidence for this. Moreover, with each cycle, the magnitude of velocity increment and the permeability drop becomes lower. Subsequent to the healing process as the stress builds up, the seismic velocity and the permeability follow a similar trend to the previous cycle. Hence the repeated compaction and dilation that the sample suffers with stress cycles is reflected by the seismic velocity and the permeability evolution. As such, the current investigation has proven the possibility of utilizing the evolution of permeability coupled with seismic velocity in the off-fault regions as a possible precursor to earthquakes.