Dynamic fluid pressure surge in a fracture as a mechanism for earthquake triggering

Geological environments having abundant fluids are prone to earthquake dynamic triggering due to passing seismic waves. Because the incident wave stress is usually very low in amplitude (~kPa), the prevailing triggering mechanism has been the critical state argument in which the earth crust is critically stressed and is close to failure and a small stress perturbation is capable to cause earthquakes. Here, we present another mechanism we found using both numerical modeling and lab physical modeling. We found the fluid pressure in a fracture could be amplified greatly. We call this phenomenon the transient pressure surge (PS). In our lab experiment, we made a low-frequency source (XFrac-S) which can vibrate to produce single-frequency sinusoid pressure waves in water. Currently, it can output frequency in the range of about 10 to 70 Hz. We also designed and made a thin pressure transducer of a disc shape (Xfrac-H) of 0.2mm in thickness. The Xfrac-H sensor can record up to ~600Hz and can be easily placed in thin fractures to directly measure fluid pressure waveforms during wave propagation. To quantify the pressure amplification, we define a pressure surge factor (PSF) as the amplitude ratio between the measure fluid pressure in the fracture and that of the incident wave pressure. The fracture was made by placing two glass blocks (dimension ~ 1m) on top of each other. The experiment was conducted in water in a large tank. By varying the fracture aperture from 0.2mm to 9mm, we achieved a maximum PSF more than 20 at about 29 Hz and for an aperture of 0.95mm. PSF depends on fracture geometry, size, aperture, and the stimulating wave frequency. Extrapolating PS to the field scale (100m fracture), we anticipate observing large PSF (> 100) at frequencies around 0.1 Hz or less. If PS is the underlying mechanism in causing earthquakes, it offers a possibility for us to estimate the maximum magnitude of a potential induced/triggered earthquake. It also allows us to probe the earthquake failure stress threshold by studying incident waves of different frequencies, corresponding to different pressure amplification factors.