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seismic off-fault Deformation: A multi-scale iMAGing to constrain Earthquake energy budget

Periodic Reporting for period 1 - DAMAGE (seismic off-fault Deformation: A multi-scale iMAGing to constrain Earthquake energy budget)

Reporting period: 2020-01-13 to 2022-01-12

Earthquakes are ruptures that nucleate, propagate and terminate mostly on pre-existing faults with a sudden and possibly destructive release of elastic strain energy, making them one of the world's deadliest natural hazards. Most of the earthquake energy is dissipated into heat by friction due to localized slip in the core of faults at depth. The remaining energy amount is partly driven to the Earth’s surface by seismic waves, but is also spent in the rupture propagation and the generation of extensive rock deformation in the vicinity of seismic faults. Vast research has been directed in the years to investigate localized shear deformation within the core of faults (i.e. on-fault deformation), whereas much less is known about the physical processes and energy sinks associated to distributed off-fault deformation. The objective of DAMAGE was to quantify off-fault coseismic rock deformation at different scales by combining targeted geological and geophysical surveys with rock deformation experiments. The action specifically focused on carbonates, the rocks that host most of the destructive seismicity affecting Europe. In particular, geological and geophysical surveys were conducted within active extensional fault zones of the central Italian Apennines, one of the regions with the highest seismic hazard worldwide.
The interdisciplinary approach of the project enabled me to collect novel information about the distribution and intensity of rock deformation in seismogenic carbonate-hosted fault zones, to determine with unprecedented detail the scale-dependent elastic properties of damaged rocks, and to investigate the conditions of rock damage initiation and accumulation under dynamic stress wave loading, by exploiting state-of-the art synchrotron-based imaging techniques.
Within the DAMAGE project I carried out field geological studies, near-surface geophysical surveys, rock deformation experiments, and microstructural-petrophysical analyses.
The field geological studies included the production of high-resolution structural maps quantifying over few kilometres along strike the fault/fracture networks and the spatial distribution of distinct structural units within two exceptionally exposed extensional seismogenic fault zones. These unique surveys highlighted the occurrence of cohesive cataclastic fault cores and fault damage zones dominated by heavily fractured to fragmented dolomitized carbonates with small to negligible shear strain accommodation (i.e. shattered in-situ). The intensity and the extent of the off-fault rock deformation systematically increased at fault geometrical and kinematic complexities (fault bends and oversteps) or at mechanical asperities represented by inherited misoriented structures (e.g. older thrust zones). Part of these new observations were presented in Fondriest et al. (2020). Additional structural surveys were conducted along single profiles across other seismogenic extensional fault zones of the region, with the aim to progressively build a catalogue containing key information (e.g. thickness of the in-situ shattered fault rocks, fault displacement, host rock) to investigate the controlling factors of fault-related damage in carbonates.
Near-surface active seismic surveys were carried out within the two fault zones mapped in detail to quantify their internal seismic velocities down to ~25 m depth. Multiple P- and S-wave high-resolution seismic profiles were acquired at different structural sites, moving from the principal fault surface into the outer damage zone. The derived first-arrival tomography models clearly highlight a heterogeneous velocity structure with fault-bounded rock bodies, each with different velocity, that correlate with the structural mapping. At the length scale of the seismic surveys, the cataclastic fault cores and the outer low-strain damage zones appear as relatively “fast” (stiff) bodies in contrast with very “slow” (compliant) heavily fractured damage zones. Ground Penetrating Radar (GPR) surveys at different frequencies were successfully coupled with the seismic ones to image high- and low-angle reflectors (i.e. extensional faults and older thrusts) and to infer fracture intensity variations in the subsurface. These combined investigations provided a novel and detailed image of the velocity contrasts within carbonate-hosted fault zones, due to the juxtaposition of rock units with different physical properties. To this end, ultrasonic wave velocities were measured in the laboratory on oriented samples of the fault zone rocks, together with detailed microstructural analyses. The discrepancy between the systematically higher ultrasonic velocities compared to the tomography-derived ones was investigated through an effective medium approach accounting for the effect of meso-scale fractures.
The mechanical behaviour of different carbonate host rocks was investigated through dynamic stress-wave loading experiments reproducing the high-strain rate deformation associated to earthquakes. The experimental apparatuses were two split Hopkinson pressure bars installed at the host institution and at the European Synchrotron Radiation Facility (ESRF) in Grenoble (collaboration with Prof. Francois Renard). The state-of-the-art imaging equipment at ESRF allowed us to capture ultra-high-speed X-ray radiographs through the samples and track almost “real-time” damage nucleation and accumulation under dynamic loading. In addition, ultrasonic wave velocities and X-ray computed tomography were employed to examine microscale rock damage after successive milder loadings. The experiments highlighted the occurrence of a significant scale effect on critical strain rates and peak stresses to rock shattering and pulverization, and the role of both initial damage (veins and microcracks) and heterogeneities (diagenetic porosity) in promoting microscale damage.
This project has provided the opportunity to collect a wealth of novel geological and geophysical data to quantify off-fault rock deformation along seismogenic fault zones in carbonates. In particular, it was documented how the distribution and intensity of rock deformation is mainly controlled by fault geometry and structural inheritance, by producing maps of large fault zone exposures with unprecedented detail. Near-surface geophysical surveys provided entirely new information on the internal seismic velocity structure of carbonate-hosted fault zones and revealed extreme internal variability not commonly found in the “low-velocity” zones detected by larger-scale seismic surveys. These findings have significant implications on how the near-surface mechanical behaviour of fault zones and fault-affected slopes can be more realistically modelled and how heavily fractured and compliant damage zones can influence the distribution of shallow deformation. On the other hand, additional work is needed to model the evolution at larger depths of the fault zone velocities derived from the seismic surveys, and to investigate the variations of seismic velocities at different length scales and source frequencies. Finally, the rock deformation experiments coupled with a state-of-the-art X-ray imaging system allowed us to obtain unique data to push further our comprehension of carbonate rocks deformation under extreme coseismic loading.
synoptic view of DAMAGE project