Abstract
The advent of novel technologies revealed that other geophysical signals than those directly related to fault motion could be used to probe the state of deformation of the Earth's crust. ...Electromagnetic signals belonging to this category have been increasingly investigated in the last decade in association to natural earthquakes and laboratory rock fractures. These studies are hampered by the lack of continuous recordings and a systematic mathematical processing of large data sets. Indeed, electromagnetic signals exhibit characteristic patterns on a specific frequency band (the very low frequency, VLF) that correlate uniquely with the paroxistic rupture of rocks specimens under uniaxial laboratory tests and were also detected in the atmosphere, in association to moderate magnitude earthquakes. The similarity of laboratory and atmospheric VLF offers an unique opportunity to study the relation between VLF and rock deformation on at least two different scales and to enlarge the dataset by combining laboratory and atmospheric data. In this paper we show that the enlarged VLF dataset can be successfully used, with a neural network approach based on LSTM neural networks to investigate the potential of the VLF spectrum in classifying rock rupture precursors both in nature and in the laboratory. The proposed approach lays foundation to the automatic detection of interesting VLF patterns for monitoring deformations in the seismically active Earth’s crust.
Large seismogenic faults consist of approximately meter-thick fault cores surrounded by hundreds-of-meters-thick damage zones. Earthquakes are generated by rupture propagation and slip within fault ...cores and dissipate the stored elastic strain energy in fracture and frictional processes in the fault zone and in radiated seismic waves. Understanding this energy partitioning is fundamental in earthquake mechanics to explain fault dynamic weakening and causative rupture processes operating over different spatial and temporal scales. The energy dissipated in the earthquake rupture propagation along a fault is called fracture energy or breakdown work. Here we review fracture energy estimates from seismological, modeling, geological, and experimental studies and show that fracture energy scales with fault slip. We conclude that although material-dependent constant fracture energies are important at the microscale for fracturing grains of the fault zone, they are negligible with respect to the macroscale processes governing rupture propagation on natural faults.
Earthquake ruptures propagate on geological faults and dissipate energy in fracture and frictional processes from micro- (less than a millimeter) to macroscale (centimeters to kilometers).
The energy dissipated in earthquake rupture propagation is called fracture energy (
G
) or breakdown work (
W
b
) and scales with coseismic slip.
For earthquake ruptures in natural faults, the estimates of
G
and
W
b
are consistent with a macroscale description of causative processes.
The energy budget of an earthquake remains controversial, and contributions from different disciplines are required to unravel this issue.
Numerous field and laboratory studies have been conducted to investigate the relationship between radon variation and seismic events, as well as the complex link between radon emission and rock ...deformation mechanisms. However, a clear understanding of this correspondence and systematic observations of these phenomena are still lacking, and recent experimental studies have yet to yield conclusive results. In this study, we investigate the possible relationships between radon migration dynamics and rock deformation at the micro-scale through laboratory experiments using the SHIVA apparatus under shear stress-controlled conditions and simultaneous high-resolution radon measurements. We studied the behaviour of three different lithologies to show that radon emission varies in response to rock deformation and this variation is highly dependent on the mineralogy and microstructure. This study represents the first attempt to define radon gas as an indicator of transient and rapid rock deformation at the micro-scale.
Rupture fronts can cause fault displacement, reaching speeds up to several ms(-1) within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing ...rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.
Evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred, as well as some attributes of the earthquake such as energy output. Biomarker thermal ...maturity can elucidate structures within a fault zone that were hotter than surrounding rock, and therefore likely to have hosted earthquakes. We performed a series of friction experiments on thin gouge layers of Woodford shale at slip rates of 1 m/s. Our results show that biomarker thermal maturity can clearly delineate zones of slip that are as thin as 150 μm. We establish that the kinetics of methylphenanthrene reaction previously determined at longer durations and lower temperatures are applicable at seismic slip rates and timescales. Temperature estimates show that once the slipping zone reached ∼500 °C, biomarker reaction was significant, as predicted by the reaction kinetics. Our results demonstrate that temperature rise in fault zones is a function of both the work density and power density during the earthquake. Biomarker reactions, along with other Arrhenius-style reactions such as those that lead to dynamic shear weakening, are both a function of slip speed and frictional work.
•Experiments at seismic slip rates demonstrate that biomarkers react on the timescale of earthquakes.•Biomarker thermal maturation is kinetically-controlled by both temperature and seismic slip duration.•Biomarkers reaction is greatest in principal slip zones where slip is highly localized.
Many earthquakes propagate up to the Earth's surface producing surface ruptures. Seismic slip propagation is facilitated by along-fault low dynamic frictional resistance, which is controlled by a ...number of physico-chemical lubrication mechanisms. In particular, rotary shear experiments conducted at seismic slip rates (1 ms
) show that phyllosilicates can facilitate co-seismic slip along faults during earthquakes. This evidence is crucial for hazard assessment along oceanic subduction zones, where pelagic clays participate in seismic slip propagation. Conversely, the reason why, in continental domains, co-seismic slip along faults can propagate up to the Earth's surface is still poorly understood. We document the occurrence of micrometer-thick phyllosilicate-bearing layers along a carbonate-hosted seismogenic extensional fault in the central Apennines, Italy. Using friction experiments, we demonstrate that, at seismic slip rates (1 ms
), similar calcite gouges with pre-existing phyllosilicate-bearing (clay content ≤3 wt.%) micro-layers weaken faster than calcite gouges or mixed calcite-phyllosilicate gouges. We thus propose that, within calcite gouge, ultra-low clay content (≤3 wt.%) localized along micrometer-thick layers can facilitate seismic slip propagation during earthquakes in continental domains, possibly enhancing surface displacement.
On August 24, 2016, a Ml=6.0 earthquake occurred in Central Apennines, Italy, between the towns of Norcia and Amatrice, causing severe destruction and casualties in a wide area around the epicenter. ...We present a preliminary analysis of continuous radon concentration data collected from the second half of 2012 to the day after the earthquake by a long term radon monitoring station, installed at Cittareale (Rieti, Italy), about 11 km south-west of the epicenter. We combine the field data analysis with the outcome of dedicated laboratory experiments, aimed to study real time radon emission dynamics from rock samples subject to normal and shear stress loads in absence of fluid transport and migration phenomena. Our results suggest the possibility of a minor role played by phenomena related to fluid migration for the Amatrice seismic event with respect to other recent Apennine earthquakes.
This paper examines how migration redefines family narratives and dynamics. Through a parallel between the mother and the mother tongue, I unravel the emotional, linguistic, social, and ideological ...connotations of the mother–daughter relationship, which I define as a ‘condensed narrative about origin and identity’. This definition refers to the fact that the daughter’s biological, affective, linguistic, and socio-cultural identity grounds in the mother. The mother–daughter tie also has a gendered dimension, which opens up interesting gateways into the female condition. Taking this assumption as a starting point, I examine how migration, impacting on the mother–daughter relationship, can redefine gender roles and challenge models of femininity, which are culturally, socially, geographically, and linguistically embedded. I investigate this aspect from a linguistic perspective, through a reading of a corpus of narratives written by four Italian-Canadian writers. The movement from Italy to Canada enacts ‘the emergence of alternative family romances’ and draws new routes to femininity. This paper seeks to illustrate how, in the narratives I examine, these new routes are explored through linguistic means. The authors in my corpus use code-switching to highlight contrasting views of femininity and reposition themselves with respect to politics of gender.
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