ERC Starting Grant

ERC Starting Grant 2010: consolidator panel

InteGrated Laboratories to investigate the mechanics of ASeismic
vs. Seismic faulting


Principal Investigator: Cristiano Collettini
Proposal duration from 01/10/2010 to 30/09/2015
Budget: 1.514.400 Euro





Synoptic view of GLASS

The extent to which earthquake phenomena can be accurately assessed depends on how well the underlying physical conditions and processes are understood. What methodologies are available to investigate processes occurring at a depth of several kilometres in the Earth’s crust? Seismologists characterize fast (seconds) and slow (days to months) deformation processes but they cannot shed light on the nature of the fault rocks responsible for seismic behaviour. Geologists document long‐term processes but in most cases they are unable to relate geological features to specific slip phenomena. Experimentalists test the physical properties of fault rocks, but have to face the complex problem of bridging the scaling gap between experimental and natural phenomena. 
Due to the impossibility of developing a comprehensive study of earthquake processes using only a single discipline, GLASS has tried to look at the mechanics of faulting from different angles since there is much learn by considering phenomena at the boundaries between different disciplines. Our interdisciplinary approach linked with the new state-of-the-art infrastructures allowed for a comprehensive understanding of the spectrum of fault slip behaviour from the crustal to the nano-scale and in the time-window spanning from the earthquake co-seismic phase (a few seconds) to the entire fault history (millions of years). 
Apart from earthquake phenomena, results from GLASS have potential impact on the energy field, environment and landslides. 
GLASS has been a fantastic opportunity for enhancing our research environment.

Technological achievements: the BRAVA rock deformation apparatus & the down-hole seismometers
With GLASS we have designed and developed the BRAVA rock deformation apparatus. In addition we have implemented the TABOO scientific infrastructure by instrumenting three shallow (~250 meter) boreholes with three‐component short‐period (2 hertz) seismometers sampled at 1 kilohertz. Collettini & Chiaraluce EOS, 2013; Collettini et al., IJRMMS 2014; Chiaraluce et al., 2014 Annals of Geophysics; 2014 Scientific Drilling.
The Brittle Rock deformAtion Versatile Apparatus is an is extremely versatile machine since it works as a biaxial apparatus (also with confining & fluid pressure), a triaxial apparatus capable of running on large cylindrical blocks (10 cm in diameter and 20 cm in length), and a biaxial for large (20x20 cm) sliding surfaces.
The borehole seismometers has increased the detection capability of the TABOO seismic network. Event 1 and 2 are two small earthquakes, ML = -1.0 and -0.6 respectively, showing an increase of signal to noise ratio with the depth of the sensor.  

Fault zone structure of carbonate/phyllosilicate bearing faults
We have Integrated detailed geological cross sections, field studies and high-resolution seismological data to improve the characterization of the fault structure of carbonate/phyllosilicate-bearing faults, from the km down to the nano-scale.  
Details in Tesei et al., JSG, 2013; 2017; Viti et al., CMP, 2013; Collettini et al., JSG, 2014; Valoroso et al., Geology 2014.   
In marly limestones the deformation is distributed along thick (up to 200 m) shear zones and it is characterized by pressure solution + frictional sliding along phyllosilicates.
In massive limestones the deformation occurs on parallel slipping zones with localization on thin slip surfaces (<300 mm). 
The slipping zones form several interacting segments that define a fault zone structure up to 1.5 km wide. A similar structure is imaged by the high-resolution aftershock locations of the L’Aquila 2009 sequence occurring on similar rocks.

Dynamic weakening of carbonate faults during earthquake propagation
We have documented that frictionally induced grain sensitive, grain boundary sliding deformation mechanisms can account for the onset of dynamic weakening of carbonate faults during earthquake propagation. The microstructures diagnostic of these mechanisms are nanograins aggregates exhibiting polygonal grain boundaries. With accumulating slip thermal decomposition of carbonates becomes more evident. Polygonal microstructures and decomposed aggregates have been firstly documented in the lab and then observed along natural faults. Details in De Paola et al., EPSL 2015; Collettini et al., JSG, 2014; Collettini et al., Geology 2013.

Strength, slip stability and healing of of carbonate & phyllosilicate bearing faults
We have developed rock deformation experiments with BRAVA to characterize frictional stability and healing behaviour of carbonate and phyllosilicate-rich faults. We have used filed studies, geological cross sections and the critical taper model to up-scale the lab results.
Details in Collettini et al., EPSL 2011; Tesei et al., EPSL 2014; Carpenter et al., JGR 2014; Tectonophysics 2015; GJI 2016, Tesei et al., Geology 2015; Giorgetti et al., JGR 2015; JSG 2016.
Frictional strength of intact fault rocks retrieved from depths above and below the smectite-illite transition, is very low (m = 0.17-0.26). Combining these data with the critical taper theory we suggest a long term weakness of décollements without invoking diffuse near lithostatic fluid pressure. 
Slip stability of carbonate-bearing faults is affected by normal stress, sliding velocity, fault zone structure, cementation, but see also works by Verberne et al., for the role of temperature.
Healing in calcite is significantly faster than in other granular materials such as quartz. The fast re-strengthening results from the numerous physico-chemical processes active on calcite at laboratory conditions.

The continuum mode of fault slip behaviour and laboratory earthquake precursors
We have integrated mechanical data, obtained by rock deformation experiments spanning the field of slow to regular earthquakes, with ultrasonic wave characterization, obtained by using standard seismological techniques (cross-correlation of P-wave coda), to characterize the continuum mode of fault slip behaviour. We have observed precursory changes of wave speed in laboratory faults for the complete spectrum of failure modes observed for tectonic faults.  Scuderi et al., Nature Geoscience 2016; Tinti et al., JGR, 2016; Scuderi et al., Geology, 2017. 

Unconventional aspects of the research
Unconventional aspects of GLASS regard the energy field, the environment and landslides.
Details in Trippetta et al., IJGGC; Giorgetti et al., JSG 2016; Collettini and Scuderi Nature Scientific Reports, 2016.
A multidisciplinary characterization of a natural example of a CO2 geological reservoir in central Italy suggests that: (1) tectonics and structural position, and (2) lithology and associated mode of deformation, play a key role for channelling and trapping deep-seated CO2-rich crustal fluids.
Using the BRAVA apparatus in a double direct configuration within a pressure vessel (left panel), we tested the mechanical parameters that control frictional stability at different levels of fluid pressure and showed that they evolve from rate-strengthening to neutral, suggesting that fluid overpressure plays an important role in controlling the mode of fault slip. These results have important implications for induced vs. triggered seismicity.