SISMICITA' NATURALE, INDOTTA e ATTIVATA - dal rapporto ICHESE un saggio sulle differenze

04/05/2014 2446

SISMICITA' NATURALE, INDOTTA e ATTIVATA - dal rapporto ICHESE un saggio sulle differenze

In queste settimane si è parlato molto del rapporto ICHESE sul SISMA in EMILIA e, in particolare, sulle conclusioni, l'unica parte del documento riportato anche in italiano. Ne abbiamo parlato anche su INGENIO riprendendo sia le anticipazioni delle SCIENZE che riportando il rapporto integrale.

Nei giorni successivi - nei consueti approfondimenti fatti dalla redazione di INGENIO - ci ha però colpito un'intervista fatta a un geologo italiano, Aldo Piombino, sugli esiti del rapporto ma, soprattutto, sulla frenesia con cui il mondo di internet e non solo si è affrettato a richiamare, in modo spesso approssimato, la relazione. Proprio Aldo Piombino nel suo BLOG "scienzeedintorni" inizia un post citando Douglas Adams "la gente vuole sapere le cose, poi che siano giuste o sbagliate non è importante."

Nell'intervista il dott. Piombino evidenzia come "E, soprattutto oggi, quasi tutti parlano per sentito dire. Mi domando quanti di quelli che si sono espressi in materia abbiano letto l'articolo di Science (pochi, perché fuori dalle università è a pagamento...) e quanti hanno letto il rapporto ICHESE. Ma anche quanti di quelli che l'hanno davvero letto hanno capito (o “voluto capire”) quello che c'era scritto."

Condividiamo quando espresso dal geologo, anche perchè il lungo rapporto è stato radatto al termine di un'aprofondita analisi da una commissione tecnico-scientifica è costituita da 6 personaggi di grande esperienza nel campo delle Scienze della Terra, italiani e stranieri. E si tratta di un ottimo saggio sui meccanismi che generano i terremoti. Per questo avevamo voluto pubblicare il rapporto nella sua interezza.

Ma come afferma Piombino, probabilmente per la lunghezza, o per la lingua, pochi lo hanno letto. Per questo con INGENIO torniamo su una delle parti del rapporto quella in cui si distingue tra sismicità naturale e antropica, suddividendo poi quest'ultima in sismicità indotta e sismicità attivata.

Tutti sappiamo che il terremoto si genera per degli stress, degli sforzi che agiscono sulle faglie. Le faglie sono dei segmenti già rotti: quando agisce uno stress molto forte lungo la rottura avviene un cedimento e i due blocchi di roccia scorrono l’uno sull’altro, provocando il terremoto.
 
Sostanzialmente ci sono sostanzialmente terremoti di tre tipi: terremoti naturali,  che avvengono a prescindere dalla presenza dell’uomo; terremoti indotti, in cui l'attività dell’uomo genera nuove frattureterremoti attivati, dove l’attività dell’uomo attiva delle faglie esistenti che sono nelle vicinanze.
 
Il terremoto indotto, quello che genera nuove fratture, non è ovviamente facile da generare. Occorrono azioni di grande portata generale e su questo tema vi sono grandi discussioni, soprattutto negli Stati Uniti dove vi si opera nel settore petrolifero attraverso il cosiddetto meccanismo del fracking. In realtà, come già riportato in alcuni articoli riprodotti anche su INGENIO, si sta dimostrando che più l'azione di fracking incida l'azione di iniezione di acqua calda e contaminata in grandi quantità. Nel ciclo produttivo del fracking si ha una risalita di grandi quantitativi di acqua, che deve essere buttata da qualche parte e ovviamente non può essere buttata in superficie; la si reinietta a maggiori profondità a 6/7/8 km di profondità, e in quelle profondità ci possono essere faglie e ci possono essere stress tali da generare terremoti. 
 
Comunque, per saperne di più è molto interessante leggere il relativo capitolo del rapporto ICHESE in cui si spiegano questi meccaninismi.
 
Ecco il testo.

Review of state of knowledge of antropogenically influenced seismicity
 
A. Introduction 

Earthquakes almost always occur when the forces acting to generate movement (shear stress) along a pre-existing fracture exceed the frictional forces (normal stress) acting to resist that movement. When that fracture/fault moves it radiates energy into the surrounding rock in a complex way as a combination of wave types depending on where the fracture is located with respect to a free surface and other geological discontinuities. The radiated energy is transported away by a sequence of wave trains of which the first but not the largest is a compressional wave (P-Wave) where the direction of cyclic deformation is parallel to the direction of transport, followed by waves which produce shear deformations perpendicular to the direction of propagation, called not surprisingly shear waves (S-Wave). If a free surface is relatively close to the failure then strong deformations can occur and propagate at and below that surface as Rayleigh (vertically polarised) and Love (horizontally polarised) wave trains. The S, Rayleigh and Love waves are slower than the P waves and the two latter have frequency dependent velocities (dispersion). These seismic waves transport energy and can be detected on sensitive instruments. If the earthquake magnitude is in excess of 1.5-2.0 local magnitude (ML), the waves may be felt; and if magnitudes are higher (probably in excess of 4.0 ML) the waves can cause significant damage and possible loss of life. 

B. Anthropogenically Influenced Seismicity 

In areas, which are geologically active, such as zones of active rifting or active thrusting in the forelands of mountain belts, it is very likely that the crustal and cover rocks are in a critically stressed state. In such areas minor perturbations to an already precariously balanced stress system can initiate fault movements with associated, sometimes large, earthquakes. The important distinction made by [1] and [2] is between induced and triggered events. For induced seismicity human activity accounts for either most of the stress change or most of the energy associated with the earthquakes. In triggered seismicity human activity accounts for only a small fraction of the stress change and of the energy associated with the earthquakes, whereas tectonic loading plays the primary role. It is conceptually possible to divide earthquakes into a number of different categories but it should be appreciated that the boundaries between these are diffuse: 

  • Tectonic Earthquakes, due to naturally existing stress systems, where the tectonic stress has already exceeded the resisting frictional stress and the region was seismogenically ‘ripe’. 
  • Anthropogenic Earthquakes, where human activity has played some part in bringing the stress system to failure: 

a. Induced Earthquakes, where external anthropogenic activities produce stress changes, which are sufficiently large as to produce a seismic event. The rock-mass may not necessarily have been in a stress-state, which would have led to an earthquake in the reasonably foreseeable future (in a geological sense). Earthquakes produced by procedures such as thermal or hydraulic stimulation of a rock, such as Hydraulic Fracturing and Enhanced Geothermal Systems, fall into this category. 

b. Triggered Earthquakes where a small perturbation generated by human activity has been sufficient to move the system from a quasi-critical state to an unstable state. The event would have eventually occurred anyway although probably at some unknown, later time. That is, these activities have advanced the earthquake clock. In this case the additional perturbing stress is often very small in comparison with the pre-existing stress system. The necessary condition for the occurrence of seismicity is a tectonically pre-stressed fault near the human operations altering the stress field, where ‘near’ can be even tens of km away depending on the duration and type of the stimulus. Under certain circumstances, such stress changes can eventually cause the loaded fault to fail. Importantly, since technological operations act only to activate the tectonic stress release process, the magnitudes of such earthquakes can be high, and within the same range as those of natural earthquakes, depending on the amount of elastic strain accumulated on the fault due to tectonic loading. 

1. How do we tell the difference between natural and triggered/induced seismicity? 

It is clear that there are many, many possible mechanisms which can bring about the minor stress changes which are necessary to generate seismic events during anthropogenic activities, The magnitude of these man-made events can be large and is controlled by the ambient stress field, the magnitude and the duration of the perturbation and the dimensions of the faults which are available to be stimulated. Some of the physical mechanisms are illustrated in Figure II.1.

Dahm et al [3] sums up the situation very well: 

“Human operations, such as mining, hydrocarbon production, fluid withdrawal or injection, drilling, hydro-fracturing and reservoir impoundments, can positively and negatively impact tectonic stresses, pore pressure, fluid migration and strain in the sub-surface. Earthquakes occurring in spatial and temporal proximity to such operations are immediately under suspicion to be triggered or induced. The discrimination between natural, triggered, and induced earthquakes is a difficult task, and clear rules and scientific methods are not well established or commonly accepted”. 

Although at present it is not possible to discriminate unequivocally between man-made and natural tectonic earthquakes, some characteristics of seismic processes have already been identified, which can speak for or against possible connections between seismicity and human technological activity. 

There are seven discriminatory criteria which are often applied in regions where injection or extraction of fluids takes place (modified after [5]). These are: 

  • i. Are these events the first known earthquakes of this character in the region? 
  • ii. Is there a clear correlation between injection/abstraction and seismicity? 
  • iii. Are epicentres near wells (within 5 km)? 
  • iv. Do some earthquakes occur at or near injection/abstraction depths? 
  • v. If not, can known geologic structures channel flow to sites of earthquakes? 
  • vi. Are changes in fluid pressures at well bottoms sufficient to generate seismicity? 
  • vii. Are changes in fluid pressures at hypocentral distances sufficient to generate seismicity? 

These can be useful in many cases to improve the confidence that any particular event or set of events is induced/triggered. This was the case for the 2011 Hydraulic Stimulation events (Fracking) detected in Blackpool Lancashire ([6]). More recent studies show, however, that these criteria are not appropriate in all cases. When there are many activities occurring in a region which is itself seismically active then these criteria cannot be simply applied and it is necessary to look very carefully at spatial and temporal relationships between seismicity and operational parameters associated with pre-existing faults either mapped on the surface or from seismic investigations and also statistical parameters of the seismic events themselves. 

The threshold epicentral distance of 5 km used by [5] now seems to be too short compared to observed cases (e.g.[4]). Sometimes the depth of induced/triggeredevents correlates well with the injection depth, however at other times the hypocentral depth can significantly exceed the injection interval (e.g. [7]). Violation of the criteria of [5] seems to occur particularly often for triggered earthquakes. 

Several cases of delayed seismicity are reported in literature. Keranen et al. [7] report an 18 yr. long lag between the start of fluid injection and the occurrence of Oklahoma, US earthquake sequence from 2011. The lag inferred for the Romashkino Oil Field, the biggest oil field in Russia, was 28 yr. (from 1954 to 1982, [8]). Induced/triggered seismicity may continue even long after termination of injection operations. 

The induced, and specifically the triggered, seismic response to injections is complex and variable among cases and its correlation with technological parameters is far from being fully known (e.g. [9], [10]). 


TAG: SISMICITA' NATURALE, SISMICITA' ANTROPICA, SISMICITA' INDOTTA, SISMICITA' ATTIVATA, rapporto ICHESE, SISMA EMILIA, terremoti naturali