The viscous fluids present in the fault zones of the earth's crust where earthquakes originate condition their evolution and magnitude. This is what emerges from a study recently published in the journal Nature Communications (https://www.nature.com/articles/s41467-019-09293-9) conducted by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), the École Polytechnique Fédérale de Lausanne (EPFL), the University of Padua and the University of Durham, England.
“In fault zones”, explains EPFL researcher Chiara Cornelio, “the interaction between rocks and fluids has mechanical and chemical effects on the nucleation, propagation and arrest of earthquakes: these effects condition both natural and anthropic in which the stimulation with fluids with different viscosities and chemical characteristics is necessary for mining and for geothermal energy".
Therefore, the aim of the work is to study the effect of fluid viscosity on fault stability. Viscous fluids can have a dual behavior and can act as lubricants or abrasives depending on other parameters, such as fluid pressure, sliding speed and initial roughness of the contact surface.
“Faults can be thought of as surfaces of rock in frictional contact. It is the friction that determines how the faults will behave, how big the slip will be and, therefore, the magnitude of the event", adds INGV researcher Elena Spagnuolo, "In the presence of a lubricant, the earthquake can propagate easily, while otherwise, it can originate slower and therefore less energetic events, or even stop their movement”.
Despite the relevance of the interaction between fluids and rock for the origin of earthquakes, the physics of this process has long remained conjectured and assigned to theoretical models since the technology of experimentation in conditions of deformation close to those of an earthquake and in the presence of fluids was often prohibitive. Thanks to the contribution of European projects such as USEMS (Uncovering the Secrets of an Earthquake: Multidisciplinary Study of Physico-Chemical Processes During the Seismic Cycle) and NOFEAR (New Outlook on seismic faults: from earthquake nucleation to arrest), the Laboratorio Alte Pressurei – Alte Temperature (HPHT) of INGV has equipped itself with tools able to make these simulations possible. Together with EPFL's Laboratory of Experimental Rock Mechanics (LEMR), and thanks to the new European BEFINE (Mechanical BEhavior of Fluid-INduced Earthquakes) project, the authors designed novel experiments to make it possible to study these phenomena.
“Understanding how earthquakes behave in the presence of fluid also has consequences on our ability to model and understand their behavior”, the authors conclude, “studies like this one show that a better understanding of the properties and effects of fluids on fault behavior is vital in the prevention and reduction of induced seismicity”.
Image - Fluids of variable composition (eg gas, water, frost, hydrocarbon seepage and liquid-gas mixtures) are often present in fault zones and the viscosity of these fluids can vary by several orders of magnitude. Viscous fluids can also form as a result of the partial transformation of earthquake energy into heat. The temperature increase causes the contact rocks to melt, as in the figure. This effect has been verified in the HPHT laboratory of the INGV in Rome with the experimental machine called SHIVA.