Final Report Summary - NOFEAR (New Outlook on seismic faults: From EARthquake nucleation to arrest) Earthquakes or abrupt ground motions that we feel at the Earth's surface are the result of ruptures propagating in the crust (usually for tens of kilometers in the case of large and destructive earthquakes) along geological structures called faults (the “earthquake engine”). The NOFEAR (or "New Outlook on seismic Faults: from EARthquake nucleation to arrest") project funded by the European Research Council aimed at understanding the intimate nature of earthquakes by investigating the basic ingredients of the earthquake engine: the elastic strain energy stored in the wall rocks (the fuel), the fault and its mechanical properties (the engine and its performance). We integrated (1) field studies of seismic faults (we looked inside the earthquake engine by lifting up its bonnet) and, (2) experiments that reproduced the extreme deformation conditions typical of earthquakes (we measured the performances of the earthquake engine) in (3) numerical models to include the geological complexity of natural faults and their mechanical behavior as measured in the laboratory. These models allowed us to produce simulated earthquakes to be compared with natural ones (i.e. make the earthquake engine work). The NOFEAR project involved a group of twenty scientists with different background (geologists, physicists, engineers, seismologists) and age (from B.Sci. students to more experienced scientists). They produced 42 scientific publications and delivered 200 contributions to international meetings plus tens of presentations in schools, “research nights” and mass media for popular dissemination. Some of the most relevant scientific results, published in very selective scientific journals, include the evidence that the formation of rock nano-particles (< 20 nm in size) due to intense fragmentation along faults, triggers grain-size- and temperature-dependent fault processes which sustain rupture nucleation and propagation during earthquakes. These findings shake our knowledge of earthquake mechanics: rather than the expression of the "brittle" behaviour of the Earth's crust as presented in text-books, at the microscopic scale, earthquakes are the result of “ductile” crystal-plastic processes. A second relevant finding of the experimental studies performed on materials recovered from drilling projects of active tsunamigenic faults (New Zealand, Costa Rica, Japan and Sumatra) is that these materials, rather than impeding the propagation of seismic ruptures as previously thought, lubricate faults. As a consequence, seismic rupture can easily propagate up to the ocean seafloor during earthquakes, resulting in ocean seafloor uplift, which in turn uplifts the sea surface triggering devastating tsunamis. A third relevant result is that geologically- and experimentally-based numerical simulations of earthquakes matched in detail the general features of natural earthquakes (e.g. rupture duration) estimated from the analysis of seismic waves. However, as expected, when we modelled the Mw 9.0 Tohoku earthquake that generated the large tsunami wave that hit Japan in March 2011, slight variations in the initial and boundary conditions of the model (poorly constrained at the state-of-the-art) resulted in a gamut of earthquake types (megathrust earthquakes, tsunami earthquakes and normal-type thrust earthquakes). These findings suggest that, though our understanding of earthquake physics has made large advancements in the last years, unfortunately a robust physically-based short term (days to weeks) probabilistic earthquake forecasting approach remains a huge challenge for Earth's scientists. Lastly, we investigated the causes of seismicity induced by human activities, especially those related to the injection of CO2 in deep basaltic reservoirs. The aim of these engineering projects is to dissolve the basalt through interaction with carbonated water (CO2 + H2O) and fix the CO2 gas in a mineral (calcite or dolomite). Mineral carbonation could be a very efficient technique for long-term storage of CO2 to decrease its concentration in the atmosphere and possibly buffer global warming. Unfortunately, experimental evidence suggest that the storing of significant volumes of CO2 will require the injection of extremely large volumes of water which will likely trigger earthquakes, given the low solubility of CO2 in water at the operating conditions expected at injection sites. It is clear that the safe exploitation of these reservoirs cannot be performed without a sound knowledge of earthquake physics.
