Mirror fault formation and coseismic slip at surface conditions: an example of faulting in unconsolidated deposits in the Central Apennines (Italy)

<p>Faulting in seismically active regions commonly involves the deformation of unconsolidated to poorly lithified sediments at shallow to near-surface depths. When compared to classic crustal strength profiles that predict a velocity-strengthening behaviour for the first few km of depth, the propagation of seismic rupture to the surface appears counterintuitive. Rock deformation experiments have shown an inverse relationship between normal stress and displacement needed to the onset of dynamic weakening during seismic slip, meaning that for a seismic rupture to be able to propagate towards the surface, displacements should be large enough to counter the progressive decrease of normal and confining stresses.</p><p>In this contribution, we document the occurrence of mirror-like faults that formed within 20-30 m-thick, unconsolidated colluvium fan deposits at the hanging wall of the active Vado di Corno Fault Zone (VCFZ) in the Central Apennines, Italy. The deposits lie in direct contact with the master normal-fault surface, are Late Pleistocene to Holocene in age, and consist of angular carbonate clasts with grain size ranging ~0.1-10 mm derived from the dismantling of the adjacent VCFZ footwall. Field observations of cross cutting relationships and marker layer displacements suggest a maximum formation depth of the faults of c. 20-30 m and slip accommodated along single faults on the order of few cm. Faults are organised in three sets: subvertical, N-S and NE-SW trending faults, and WNW-ESE striking faults, synthetic and antithetic to the VCFZ master fault surface (N195/55°). Faults are commonly lineated with a dip-slip to slightly oblique kinematic.</p><p>Detailed microstructural analysis of the mirror faults shows extreme strain localization on a 2-5 µm thick principal slip zone composed of calcite nanograins ranging 10s-100s nm in size with amorphous material and phyllosilicates occurring along grain boundaries and within intragranular porosity. Locally, aggregates of nanograins coalesce and transition to µm-sized polygonal, larger grains. Calcite nanograins are mostly equant, with straight grain boundaries, 120° dihedral angles, and negligible porosity. These microstructures strongly resemble high temperature recrystallization structures documented along seismic faults exhumed from >5 km of depth, where stresses are significantly larger. In our case, field constraints show that deformation occurred in very confining stress conditions and with limited displacement.</p><p>Collectively, our observations provide new documentation on the conditions for the formation of mirror faults and new insights into the mechanics of faulting and strain accommodation in the shallowest part of the crust (< 1 km).</p>

Impact of coseismic off-fault damage on the overall energy budget.

<p>In the brittle part of the crust, deformation is usually perceived to be the result of displacement along fault planes, whose behaviors are controlled by their frictional properties. However, fault zones not only consist of a narrow fault core where slip occurs, they are also surrounded by a complex structure which is of key importance in the mechanics of faulting, hence in determining the overall energy budget. Indeed, as pointed out by the numerous field, geophysical, mechanical and laboratory observations, if the behavior of fault zones is intrinsically linked to the properties of the main sliding plane, it also depends on those of the surrounding medium.<strong>  </strong>In parallel, fault displacements may induce a substantial change in the physical properties of the surrounding medium. As a consequence, to improve our understanding of active fault zones, fault slip and the evolving physical properties must be studied as a unique system of stress accommodation and no longer as two distinct entities. To tackle this problem, we have developed a micromechanics-based constitutive model, thermodynamically argued, that can determine the inelastic behavior at macroscopic scale that arises from structural rearrangements at microscale. It is therefore the compulsory tool to emulate the strong coupling between the bulk and the fault that prevails during earthquakes. With this code, we can reproduce the strain rate sensitive, non-linear stress-strain relationship that leads to off-fault damage as a seismic event is propagating. We explore different scenarii and we show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture, that is also observed in the field.  We define, in return, the impact of damage on the propagation of the earthquake in itself and the generated waves. We conclude by assessing the kinetic energy, the dissipated energy and the radiated energy to define how energy is consumed within crustal systems during seismic events.</p>

Signature of coseismic slip in unconsolidated Quaternary gravels, Campo Imperatore, Italy

<p>Faulting in seismically active regions commonly involves the deformation of unconsolidated to poorly lithified sediments. The seldom occurrence of seismic slip within these deposits appears to be counterintuitive if compared to classic crustal strength profiles that predict a velocity-strengthening behaviour for the first few km of depth. Therefore, the investigation of geological evidence for coseismic faulting within unconsolidated deposits is a key step towards a broader understanding of mechanisms of strain accommodation at shallow to near-surface depth.</p><p>Here we document the occurrence of minor faults within an unconsolidated colluvial fan at the hanging wall of the Vado di Corno Fault Zone (VCFZ) in the Central Apennines, Italy. The VCFZ is part of the active Campo Imperatore Fault System and accommodated 1-2 km of displacement since Early-Pleistocene. The deposits lie in direct contact with the master fault surface, are Late-Pleistocene to Holocene in age, and consist of angular carbonatic clasts, up to tens of centimetres in size, derived from the dismantling of the VCFZ footwall.</p><p>Studied faults are organised in two main sets: (i) subvertical, N-S trending dip-slip faults, parallel to the fan long axis, and (ii) WNW-ESE striking faults, synthetic and antithetic to the VCFZ master fault surface (N195/55°). Both fault sets are striated and commonly have positive relief with respect to the host deposits. Some of these faults show a fault core up to 5-6 cm thick, bounded by discrete and well-developed polished surfaces. Locally, particularly in fine-grained gravel levels, the occurrence of extreme strain localisation (i.e. millimetric ultracataclastic layers with truncated clasts) along mirror-like fault surfaces is observed. Grain size analysis of undeformed and faulted gravels shows an increase of the power-law exponent (fractal dimension) from values of D = 1.65-2.2 in the undeformed host rocks up to D = 2.9 in the cataclastic slip zones. Microstructural analysis suggests cataclasis is the main deformation mechanism leading to grain size reduction along faults, whereas intergranular pressure solution becomes widespread moving away from the slip zone where fluid circulation was present.</p><p>Collectively, our observations provide new insights into the mechanics of faulting and strain accommodation in the shallowest part of the crust (< 1 km) and new evidence to understand the propagation of seismic ruptures within shallow unconsolidated deposits.</p>

Detecting long-lasting transients of earthquake activity on a fault system by monitoring apparent stress, ground motion and clustering

Damaging earthquakes result from the evolution of stress in the brittle upper-crust, but the understanding of the mechanics of faulting cannot be achieved by only studying the large ones, which are rare. Considering a fault as a complex system, microearthquakes allow to set a benchmark in the system evolution. Here, we investigate the possibility to detect when a fault system starts deviating from a predefined benchmark behavior by monitoring the temporal and spatial variability of different micro-and-small magnitude earthquakes properties. We follow the temporal evolution of the apparent stress and of the event-specific residuals of ground shaking. Temporal and spatial clustering properties of microearthquakes are monitored as well. We focus on a fault system located in Southern Italy, where the Mw 6.9 Irpinia earthquake occurred in 1980. Following the temporal evolution of earthquakes parameters and their time-space distribution, we can identify two long-lasting phases in the seismicity patterns that are likely related to high pressure fluids in the shallow crust, which were otherwise impossible to decipher. Monitoring temporal and spatial variability of micro-to-small earthquakes source parameters at near fault observatories can have high potential as tool for providing us with new understanding of how the machine generating large earthquakes works.

Coupled deformation-diffusion effects in the mechanics of faulting and failure of geomaterials

This review article discusses the applications of poroelasticity to the mechanics of faulting and failure in geomaterials. Values of material parameters inferred from laboratory and field studies are summarized. Attention is focused on solutions for shear dislocations and shear cracks. A common feature is that undrained response, invoked by rapid slip or deformation, is stiffer than drained response, which occurs for slower slip or deformation. The time and spatial variation of the stress and pore pressure is different for slip on permeable and impermeable planes. These solutions are applied to interpretation of water well level changes due to slip, earthquake precursory processes, and stabilization of spreading slip zones. Inclusion models for reservoirs, aquifers, and other inhomogeneities are formulated and the results are applied to stress and strain changes caused by fluid mass injection or withdrawal. This article has 120 references.