Presentation on theme: "Κωνσταντίνος Ευταξίας"— Presentation transcript:
1Κωνσταντίνος Ευταξίας Αναπληρωτής Καθηγητής Τμήματος Φυσικής ΕΚΠΑSeeding light to fractures on geophysical scale (earthquakes)from nanoscale fracture findings
2Understanding how earthquakes occur is one of the most challenging questions in fault and earthquake mechanics (Shimamoto and Togo, 2012).Earthquakes in the labSCIENCE, 54, 2012In this direction, a main effort has been devotedin the study of earthquakes on laboratory scale via different methods.It has been found that opening cracks are accompanied byelectromagnetic emission (EME) and acoustic emissions (AE)ranging in a wide frequency spectrum, from kHz to MHz(laboratory seismicity)It is considered that the laboratory seismicitymimics the natural seismicity.Recent studies by means of MHz-kHz EME have permitteda real-time-like monitoring of fracture / failure process.
3A major difference between the laboratory and natural processes is the order of magnitude differences in scale in space and time.This allows the possibility of observation of a range of physical processes not observable on a laboratory scale. (Main, 2012).On the laboratory scalethe fault growth process is normally occurs violently in a fraction of a second(Lockner et al., 1999).
4If this concept is correct the expectation that fracture induced MHz-kHz EM fields would allow the clear monitoring in real-time and step-by-step of the gradual damage of stressed materials during earthquake preparation process, is not groundless.
6Based on the above mentioned idea we have installed a field experimental networkusing the same instrumentationas in laboratory experimentsfor the recording fractured/failure induced kHz and MHZ magnetic and electric correspondingly on geophysical scale.
7But why does nature paint such a picture? PHYSICS REPORT313, 1-108, 1999NatureVol. 397, 333, 1999
9Physical Review Letters, 92(6), 065702, 2004 Physical Review E., 74, /21, 2006Physical Review E, 77, 36101, 2008PUZZLINGFEATUREPUZZLING FEATURE
10Pre-seismic anomalies associated with the Kozani-Grevena EQ
11Because an earthquake is mainly a large-scale dynamic failure process, we attemptto formulate the observed EMEsthough a shift in thinkingtowardsbasic scienceoffracture and failure mechanics.Such a studyhad not previously attempted.
12OBJECTIVES In the frame of the aforementioned directions, The comprehensive understanding of EM precursors in terms of basic scienceis a path to achieve more sufficient knowledgeof the last stages of the EQ preparation processand strict definitions of EM precursors.OBJECTIVESIn the frame of the aforementioned directions,our effort is focusing, on asking three questions:How can we recognize a MHz or kHz EME as a pre-seismic one?(ii) How can we link an individual MHz or kHz EM precursor with a distinctive stage of the earthquake preparation?(iii) How can we identify precursory symptoms in EM observations which signify that the occurrence of the prepared EQ is unavoidable?
13We base on twowell establishedexperimental results
14the MHz radiation is observed prior to the kHz one. An important feature, observed both at laboratory and geophysical scale,is thatthe MHz radiation is observed prior to the kHz one.On the laboratory scale:ThekHz EM emission is launched in the tail of pre-fracture EM emissionfrom 97% up to 100% of the corresponding failure strength.On the geophysical scale:The MHz EM precursors are emerged duringthe last week before the EQ occurrence.The kHz EM precursors are launched froma half of hour up to a few decades of hoursbefore the EQ.
15PARADOX FEATURE EM silence in all frequency bands appears before the main seismic shock occurrence,as well as during the aftershock period.The appears of the above mentioned EM silenceis one of the most fundamental questions presentlyin EM precursors research.The view that«acceptance of “precursive” EM signals without co-seismic signalsshould not be expected»seems to be reasonable.PARADOX FEATURE
16OUR PROPOSAL Asignificant EQ is what happens when two surfaces of a major fault slip past one anotherunder the stresses rooted in the motion of tectonic plates.OUR PROPOSALHowever large stresses siege the major faultafter the gradual occurrence of a population of smaller EQsin the strongly heterogeneous region that surrounds the main fault.After a seismic event occurrence the stress are redistributed.The cracking events are correlated.A higher spatial correlation is emerged with the timebetween the cracking areas.Finally, the released stresses siege the main fault.
17RcriticalLOCATIONCOMLEXITYSymmetry breakingAdaptabilityComplexityThe challenge is to determine the “critical epoch” during which the “short-range” correlations evolve into “long-range” ones.CRITERION
18Nature seems to paint the following critical picture: MHz EM PRECURSOR FRACTURE OF HETEROGENEOUS MEDIANON-LINEAR NEGATIVE FEEDBACK MECHANISMIf the amplitude of fluctuations increases in a time intervalIt is likely to continue decreasing in the interval immediately followingTHIS MECHANISMS KICKS THE CRACKIN RATE AWAY FOR EXTREMESNature seems to paintthe followingcritical picture:
191. First, a population of single isolated cracking-events emerge in the system which, subsequently, grow and multiply.2. This leads to cooperative effects. The released stresses during the damage of material siege / produce other sub-regions / cracking events.3. Long-range correlations build up through local interactions until they extend throughout the entire system.4. Right at the “critical point” the subunits are well correlated even at arbitrarily large separation, namely, the probability that a subunit is well correlated with a subunit at distance away is unity and the correlation function follows long-range power-law decay.5. At the critical state appear self-similar structures both in time and space. This fact is mathematically expressed through power law expressions for the distributions of spatial or temporal quantities associated with the aforementioned self-similar structures.6. Below and above of the critical point a dramatic breakdown of critical characteristics, in particular long-range correlations, appears; the correlation function turns into a rapid exponential decay
20SYMMETRY BREAKING The challenge is to determine the “critical epoch” during which the “short-range” correlations evolve into “long-range” ones.SYMMETRY BREAKINGFrom the phase of non-directional almost symmetricalcracking distribution to a directional localized cracking zone that includesthe backbone of strong asperitiesThe siege of strong asperities begins.The prepared EQ will occurif and when the local stress exceeds fracture stresses of asperities.
21Phys. Rev. Lett, 89, 035701, 2002. Contoyiannis, Y.F., et al., Contoyiannis, Diakonos, Malakis:Intermittent Dynamics of Critical Fluctuations,Phys. Rev. Lett, 89, , 2002.The earth as a living planet:Human-type diseases in the earthquake preparation process.Y. F. Contoyiannis, S. M. Potirakis, and K. EftaxiasContoyiannis, Y.F., et al.,Phys. Rev. Lett, 93, , 2004.HEALTHYCRITICAL POINTSYMMETRY BREAKINGNEGATIVE FEEBACKMULTIFRACTALITYPATTIENTPHYSICAL REVIEW E, 82, 2010Ivanov, P. C., et al.,Multifractality inhuman heartbeat dynamics,Nature, 399, , 1999.Goldberger, A.L., et al.,Fractal dynamics in physiology:Alterations with disease and aging,PNAS, 99, , 2002.
22FRACTURE OF HETEROGENEOUS NEDIA MHz EM PRECURSORFRACTURE OF HETEROGENEOUS NEDIANON-LINEAR NEGATIVE FEEDBACK MECHANISMTHAT KICKS THE CRACKIN RATE AWAY FOR EXTREMESRight at the “critical point”the subunits are well correlated even at arbitrarily large separationThe aforementioned crucial features characterize a healthy state,since such a mechanism provides adaptability,the ability to respond to various stresses and stimuli of everyday challenges.FRE RISCK FRACTURES“Injury” states include characteristic features of the statewhich is away from the critical pointThe earth as a living planet: Human-type diseases in the earthquake preparation process.HEART INFRACTION.
24FIRST PHASE: Stick-slip-like sliding at low velocity A magnified view of fault surfaces reveals a rough looking surfacewith high asperities and low valleys.Two surfaces in sliding motion will contact first at these high asperities.The population of asperities distributed along the two fault surfaces hinders theirrelative motion. The initial phase of slip process refer to the cumulative damageof a critical number of asperities.If the external stress raises the local stress around of an asperity, the asperity drops, the slip instantaneously accelerates and in the following decelerates and stop. In this way, the frictional fault surfaces suddenly slip, lock and then slip again in a repetitive manner “stick-slip” state.
25Physical Review Letters, 92(6), 065702, 2004 Physical Review E., 74, /21, 2006Physical Review E, 77, 36101, 2008PUZZLINGFEATUREPUZZLING FEATURE
27SECOND PHASE:Sliding at high velocity characterized by a shear-thinning rheology.The repetition of such local damage-slip events intensifies fault wear and dynamic weakening. Material between the fault surfaces, which is called ‘’gouge’’, is produced and organized itself such away that it acts like a bearing.FineGraingougeSince in a bearing,one has rolling friction but no gliding friction,two fault surfaces slide against each otherwith a low friction
28PHYSICAL REVIEW LETTERS 92, , 2004Space-Filling Bearings in three DimensionSpace-filling bearingshave been introducedto explain the fact thattwo faces of fault slide against eachother with a friction much lessthan the expected one,without productionof any significant heat .SELF-SIMIRARITYLubrication
29During the local damage of a strong asperity an ‘’electromagnetic earthquake’’is emerged.The population of ‘’electromagnetic earthquakes’’included in the abruptly emerged intermittent avalanche-like strong EM emission may mirrors the fracture of a corresponding population of asperities
30The fracture of a strong contact is associated with a corresponding sharp stress drop. Laboratory experiments should reveal that the EM signals are emitted only during sharp drops in stress. Recent laboratory studies verify that this really happens, while the amplitude of the emitted EM fields is proportional to the stress rate.
31Numerical methodfor the determination of contac areasof a rock jointunder normal and shear loadsInternational Journal of Rock Mechanics,58, 8–22, 2013At the peak stage, the normal dilation was initiated, which led to a sharp drop in the contact area. Approximately 53% of the surface area remained in contact, supporting the normal and shear loads.After the peak stage, the contact area ratio decreased rapidly with increasing shear displacement, and few inactive elements came into contact until the residual stage. At the residual stage, only small fractions, 0.3%, were involved in contact.
32ENIGMATIC FEATURE! Two strong avalanche-like kHz EM anomalies have been detected before the Athens surface earthquake.The larger anomaly, the second one, contains approximately 80%of the total EM energy released;The second anomaly contains the remaining20%.ENIGMATICFEATURE!
33Satellite Synthetic Aperture Radar (SAR) interferometry Vertical displacements of rock surface are associated with each slip event.On the geophysical scalesuch vertical displacements cause deformations on the earth’s surface.Satellite Synthetic Aperture Radar (SAR) interferometryis an imaging technique for measuringthe topography of a earth’s surface,its changes over time,and other changes in the detailed characteristic of the surface.
34Satellite ERS2 SAR images leads to the fault model of the Athens earthquake This model predicts the activation of two faults. The main fault segment is responsible for the ~80% of the total seismic energy released,while the secondary fault segment for the remaining 20%.A UNIQUEEXPERIMENTALRESULT!Geophysical Research Letters28, , 2001
35self-affine nature of faulting and fracture The Earth's crust is extremely complex. However, despite its complexity, there are several universally holding scaling relations. Such universal structural patterns of fracture and faulting process should be included into an EM precursor which is rooted in the activation of a single fault.From the early work of Mandelbrotthe aspect ofself-affine natureof faulting and fractureis well documentedfrom field observations,laboratory experiments,theoretical and numerical studies
37persistent factional Brownian motion Fracture surfaces were found to beself-affinefollowing thepersistent factional Brownian motionmodelover a wide range of length scales
38A question arises whether the sequence of precursory kHz EM pulses(“EM-earthquakes”)is induced by the slipping of tworough and rigidBrownian profiles one over the other.THIS HAPPENS.
39when there is an intersection of the two profiles PHYSICAL REVIE LETTERS, 76, 2599, 1999PHYSICAL REVIEW E, %^, 1346, 1997Self-Affine Asperity ModelA model for fault dynamicsconsisting of two rough and rigidBrownian profilesthat slides one over the other is introduced.An “EM-EQ” occurswhen there is an intersection of the two profilesrepresenting the two fault faces.The model predicts that a seismic event releases energyin the interval [E, E+dE] with a probabilityP(E)dE, P(E) ~ E-BThe population of natural EQs follows the lawP(E) ~ E-B, where B ~ 1.6.The population of EM-EQsfollows the lawP(E) ~ E-B where B =1.6.
40Τhe Hurst-exponent indicates the “roughness” of the individual fault . Decreasing H increases the “sharpness” of the surface topography.The standard random walk profile corresponds to H = 1/2.The value H = 1 is an upper bound reached when the “roughness” of the fault is minimum, in other words a differentiable profile corresponds to H = 1.Finally the value H = 0 is a lower bound; as H tends towards 0 trends are more rapidly reversed giving a very irregular look.
41The roughness of the profile of H ~ 0.75 the observed was found to beH ~ 0.75weakly dependent on thenature of the material and onthe failure mode.This quantity was thenconjectured tobe universalThe roughnessof the profile ofthe observedKHz EM time seriesisH ~ 0.75The roughnessof thekHz pre-seismicEM time series isH ~ 0.75
42Geophysical Research Letters, The surface roughness of a recently studied strike-slip fault planehas been measured by laser scanners .The fault surface exhibits self-affine scaling invariance with a directional morphological anisotropy that can be described by two scalingroughness exponents,H = 0.7in the direction of slip andH = 0.8perpendicular to the direction of slip.Geophysical Research Letters,33, 04305, 2006
43The analysis for the whole Greece seismisity reveals that it is characterized byH ~ 0.77
44Analysis in terms of fractal dimension D The fractal dimension Dalso specifies the strength of the irregularityof the fBm surface topography.Measurementsas well as theoretical studies suggestthat a surface trace of a single fault is characterized byD ~ 1.2.FRACTALELECTRODYNAMICS
50PHYSICAL REVIEW LETTERS 92, , 2004Space-Filling Bearings in three DimensionSpace-filling bearingshave been introducedto explain the fact thattwo faces of faultslide against eachother with a friction much lessthan the expected one,without productionof any significant heat .The observed spontaneous formationof vorticity cells and clusters of rotating bearingsmay provide an explanation for the long standingheat flow paradox of earthquake dynamics.HEAT-FLOW PARADOX
51Rock Mechanics Rock Engineering 44, , 2011SILENCE
52the compressive strength, the greater the EMR energy generated, International Journal of Rock Mechanics57, 57–63 , 2013.Numerical simulationof electromagnetic radiationcaused by rock deformation and failure“The greaterthe compressive strength,the greaterthe EMR energy generated,especially during main failure”
53GRANULAR MATTER., 13, , 2011Precursors of failure and weakening in a biaxial test.Numerical simulations
54But why does nature paint such a picture? S = 103 – 106 m2 for each m2 Experimental results lead to the conclusion:The new surface areasgenerated during an EQ isS = 103 – 106 m2 for each m2of fault area.But why does naturepaint such a picture?
55PARADOX! Scale-free intermittent plastic flow from nanoscale up to geophysical scaleThat avalanche strains decease in inverse proportion to sample size explains why it is difficult to observe strain bursts in macroscopic samples. The energy release by contrast may be assumed to be proportional to the dissipated energy e, which is related to the strain by e = σsV, where σ is the stress and V is the volume. Hence, the cutoff of the energy released distribution is expected to increase with sample size ase ~ L2.PARADOX!