Understanding the physics of earthquakes is the sine qua non of coping with them, one of the first steps to prevent their dramatic effects that constantly pose a severe threat to society, both in terms of human lives and economic damage.
This research is mainly centered on the seismic potential of basalts. Basalts have been recently proposed as a viable road for in-situ CO2 storage. However, injection of pressurized fluids into basaltic formations decreases the effective stress that holds ancient faults in place, eventually reactivating them and potentially inducing seismicity. Also, injection of fluids modifies the stress state even far beyond the fluid pressure front. One reasonable explanation invokes stress transfer resulting from aseismic slip/slow slip events, which may promote earthquake slip on nearby critically stressed faults.
Specifically, this research project aims to:
1) determine the frictional strength and fault stability of faults cross-cutting basalts;
2) measure the permeability to deionized water at an effective pressure range relevant for CO2 storage.
Moreover, the proposed experimental work will also be the first one that strives to find a correlation between fault permeability, rate of mineral carbonation and frictional properties of basalt faults subjected to injection of CO2-rich water.
Unravelling the coupled frictional - hydromechanical properties of basalt-built faults will contribute to gain insights on earthquake physics in basalts, natural or induced by fluid injection, and thus help to improve the available seismic hazards models and seismic risk maps.
Although basalts are the main constituents of the oceanic crust, their mechanical behaviour has not been fully characterized. So far, only a couple of frictional studies have been performed on basalt simulated faults, but a) at coseismic slip conditions (i.e., slip velocity > 1 m/s: Violay et al., 2014); b) at subseismic slip velocity conditions in shear stress control which, however, does not allow to unravel the mode of fault slip (Giacomel et al. , 2018), and c) at subseismic slip velocity conditions, by performing load-point velocity steps experiments to get the constitutive friction parameters, but at pressure and temperature conditions relevant for subduction zones (Zhang et al., 2017).
It has been documented that differences in fabric may affect the rheological behavior of rocks (e.g., De Paola et al., 2009). As a consequence, despite several friction experiments have been carried out in gabbro (e.g., He et al., 2007; Marone and Cox, 1994), which is the equivalent intrusive of basalt, the difference in fabric between these two lithologies may also reflect differences in their frictional behavior.
Therefore, none of the above literature fully address the relation between frictional strength and fault stability of basalt-built faults at the stress conditions with direct implications for CO2 storage sites in basalts.
The planned rock deformation experiments with BRAVA will lay the ground for a better understanding of the mode of basalt fault slip in these two contexts, whereas SHIVA experiments will contribute 1) to gain insights to the role of fluid pressure on fault reactivation during repeated seismic cycles (i.e. with increasing cumulated slip) following the protocol pointed out in Giacomel et al., 2018 , and 2) to get the frictional sliding criterion at different experimental conditions (i.e., room humidity, water saturated, high fluid pressure).
Specifically, these experiments will also allow to compare the friction coefficient retrieved from two different deformation apparatuses, SHIVA and BRAVA, which differ both in stiffness and the way how fault slips (rotary shear apparatus vs. biaxial deformation apparatus).
Finally, permeability measurements that will be measured by flowing deionized water through pre-cut samples will permit to check the diffusion time required to fully pressurize the experimental faults during the tests at high fluid pressure on SHIVA. Conversely, the permeability that is programmed to be measured to CO2-rich water, will be proposed as a potential novel method for monitoring the evolution of mineral carbonation in a CO2 storage site, in order to couple these parameters with the potential change in friction coefficient of carbonatized basalts.
References
De Paola, N., et al., 2009. Journal of Geophysical Research: Solid Earth, 114(B6).
Giacomel, P., et al., 2018, Geophysical research letters, 45(12), 6032-6041.
He, C et al., 2007. Tectonophysics, 445(3-4), 353-362.
Marone, C., & Cox, S. J. D., 1994. pure and applied geophysics, 143(1-3), 359-385.
Violay, M., et al., 2014, Geophysical research letters, 41(2), 348-355.
Zhang, L., et al., 2017,. Marine Geology, 394, 16-29.