Structure and mechanical properties of seismogenic fault zones in carbonates

In many seismically active areas (e.g. Italy, Greece) earthquakes, sometimes destructive, nucleate within (aftershocks surely do) and propagate through carbonates in the upper crust (e.g. L’Aquila earthquake, 2009, Mw 6.1). Seismology, geophysics and geodesy furnish key parameters related to the earthquake source (e.g. seismic moment, static stress drop, radiated energy) but lack sufficient resolution to constrain detailed three-dimensional fault zone geometry and coseismic on- and off-fault deformation processes at scales relevant to earthquake physics. In this thesis it is proposed to study the internal structure and mechanics of fault zones hosted in carbonate rocks using a multidisciplinary approach, complementary to the seismological-based one. This includes detailed structural survey to quantify the architecture of exhumed fault zones in carbonates both by field and remote sensed methods (e.g. use of a drone to get high-resolution aerial images), rock deformation experiments under conditions relevant to the seismic cycle (e.g. use of rotary shear apparatus, uniaxial press, Split Hopkinson Pressure Bar), microstructural-mineralogical characterization (optical and scanning electron microscopy, electron microprobe analyses, X-ray powder diffraction, cathodoluminescence, X-ray microtomography, white light interferometry, image analysis) of natural and experimental fault rocks to infer the physico-chemical processes occurring during earthquakes. Two fault zones cutting dolostones exhumed from < 3 km depth in the Italian Southern Alps were described: the Borcola Pass Fault Zone (BPFZ) and the Foiana Fault Zone (FFZ). In both cases the internal structure of the two fault zones was strongly influenced by the reactivation of preexisting anisotropies such as regional-scale joint sets and bedding surfaces. The BPFZ is a secondary strike-slip branch of the regional Schio-Vicenza Line that developed in a fluid-rich upper crustal environment. The microstructural characteristics of the principal and secondary slip zones of the BPFZ, including detailed analysis of the clast size distribution of injected cataclasites, suggested coseismic fluidization processes during faulting, most likely related to the propagation of ancient seismic ruptures in to the shallow crust. The FFZ is a major sinistral transpressive fault zone that developed in a fluid-poor upper crustal setting. Systematic along-strike and down-dip changes in the structure of the FFZ were recognized, allowing a comparison to be made between field observations and the predictions of three-dimensional earthquake rupture simulations. A noteworthy characteristic of the FFZ is the presence of thick belts (hundreds of meters) of in-situ shattered dolostones cut by discrete mirror-like fault surfaces. The origin of mirror-like fault surfaces and in-situ shattered dolostones in the FFZ was investigated using, respectively, low- to high-velocity (0.0001-1 m/s) rotary shear friction experiments on dolostone gouges and low- to high-strain rate (quasi-static 10-3 s-1, dynamic > 50 s-1) uniaxial compression tests on dolostone rock cylinders. At applied normal stresses and displacements consistent with those estimated for the FFZ, experimental mirror-like fault surfaces comparable to the natural examples (e.g. clast truncation along fault surfaces, similar surface roughness) were formed in rotary-shear experiments only at seismic slip rates (v ≥ 0.1 m/s). I suggest therefore that small-displacement mirror-like fault surfaces developed in dolostone gouge layers represent markers of seismic slip. In-situ shattered dolostones similar to those found within the FFZ (i.e. rock fragments up to a few millimeters in size elongated in the stress wave loading direction, incipient zones of microfracturing down to the micrometer scale) were formed during uniaxial compression tests only above strain rates of ~ 200 s-1. The association of in-situ shattered dolostones cut by discrete mirror-like fault surfaces is interpreted to record the propagation of multiple earthquake ruptures within the shallow crustal portions of the FFZ. Lastly, the structural complexity of the studied fault zones in terms of three-dimensional geometry of the fault-fracture network, fault rock spatial distribution, fault orientation and kinematics, compares favorably to the predicted damage distribution in three-dimensional earthquake rupture simulations, as well as the structure of active seismic sources hosted in carbonate rocks as illuminated by seismological techniques