Le previsioni aggiornate della United States Geological Survey sul rischio dei terremoti negli Stati Uniti mostrano un preoccupante legame indiretto tra terremoti e fracking che coinvolge milioni di cittadini.
Sette milioni di cittadini americani sarebbero esposti al rischio di terremoti causati o favoriti dall’attività umana. Il preoccupante dato è stato diffuso dalla United States Geological Survey, l’agenzia governativa americana che studia tre le altre cose anche i fenomeni sismici. Questa nuova tipologia di rischio definita ‘sismicità indotta’ è stata introdotta per la prima volta nelle nuove previsioni di rischio sismico dell’USGS ed in molte aree degli Stati Uniti evidenzia una corrispondenza geografica tra terremoti e fracking delle rocce con un legame che tuttavia risulterebbe meno diretto di quanto si sarebbe portati a supporre.
Iniezioni di fluidi, il legame tra terremoti e fracking
Secondo le valutazioni dell’USGS gli stati americani a maggiore rischio di sismicità indotta sono in ordine Oklahoma, Kansas, Texas, Colorado, New Mexico e Arkansas tutti concentrati nei territori centrali ed orientali degli Stati Uniti. Oklahoma e Texas in particolare sono i due stati con la maggiore popolazione esposta al rischio anche se come detto nel complesso sono circa 7 milioni i cittadini statunitensi interessati. Dal modello di valutazione sono al momento esclusi gli stati occidentali che sono caratterizzati da una sismicità naturale molto elevata rispetto alla quale una eventuale sismicità indotta non modificherebbe in maniera sostanziale il grado di rischio.
L’USGS spiega di aver raccolto negli ultimi 5 anni decine di migliaia di segnalazioni di allarme o di danni provenienti da zone storicamente poco esposte al rischio sismico. La percezione degli abitanti trova inoltre conferma nei dati dei sismografi che hanno evidenziato un notevole incremento degli eventi sismici rilavati annualmente. Particolarmente sorprendenti sono i numeri degli stati centrali degli USA. Qui i dati storici raccolti tra il 1973 ed 2008 mostravano una media di soli 24 terremoti all’anno di magnitudo uguale o superiore a 3. Tra il 2009 ed 2015 però questa media si è innalzata a 318 terremoti l’anno con un picco nel 2015 di ben 1.010 eventi. A metà marzo del 2016 inoltre si sono già registrati 226 terremoti (sempre considerando solo quelli di magnitudo maggiore o uguale a 3).
Incrociando varie tipologie di dati l’USGS ha quindi individuato 21 aree (v. immagine) con un rischio di sismicità indotta aumentato, tutte associate ad attività di iniezione nel sottosuolo di acque reflue (wastewater injection). Proprio questa attività costituisce il legame tra terremoti e fracking previsto anche da diversi modelli scientifici. Va comunque sottolineato che anche altre attività oltre al fracking prevedono l’immissione di fluidi in pressione nel sottosuolo anche se la loro diffusione negli Stati Uniti è meno elevata. Sempre in chiave statistica il terremoto più inteso associazione ad un pozzo di iniezione è stato registrato nel 2011 a Prague in Oklahoma ed ha raggiunto una magnitudo di 5,6.
Cos’è il fracking
Il fracking (o fratturazione idraulica) è una tecnica di estrazione usata dell’industria petrolifera per aumentare la produzione di idrocarburi (petrolio e gas in particolare) da un giacimento. Anche se i principi della fratturazione idraulica sono noti ed utilizzati già dall’Ottocento, è solo negli ultimi anni che queste ‘nuove trivelle‘ hanno trovato larga diffusione in particolare negli Stati Uniti. A partire dal 2011 infatti negli USA estensive campagne di esplorazione sono state portate avanti utilizzando il fracking come modalità standard di estrazione. La tecnica in particolare è stata utilizzata per estrarre idrocarburi da rocce bituminose a bassa permeabilità il cui sfruttamento con tecniche tradizionali era e resta quasi sempre non competitivo.
A livello economico questa politica energetica ha permesso agli Stati Uniti di aumenta la propria produzione interna di idrocarburi fino a rendere il Paese non solo autosufficiente ma anche esportare verso l’estero. Sull’altro piatto della bilancia però pesano i pesanti effetti ambientali che derivano da un diffuso ricorso alla fratturazione idraulica che per sua stessa natura modifica le proprietà degli strati rocciosi a cui è applicata. Tra questi effetti, come mostrano anche i dati dell’USGS, emerge anche una relazione tra terremoti e fracking e più in generale tra sismicità indotta ed iniezioni di fluidi nelle rocce.
La stessa agenzia osserva infatti che solo raramente il fracking può essere individuato come causa di un terremoto indotto, mentre sono numerose le correlazioni tra eventi sismici e pozzi di iniezione.
È opportuno in questo senso ricordare che il fracking in senso stretto è l’operazione con cui acqua ad alta pressione è iniettata in uno strato di roccia profondo per propagarne la fratturazione ed aumentarne in questo modo la permeabilità. All’acqua sono aggiunti additivi ed inerti solidi che una volta cessata la pressione evitano la richiusura delle fessurazioni e preservano la maggiore permeabilità raggiunta. L’intero processo genera la risalita in superficie di acque reflue il cui smaltimento avviene spesso -ma non necessariamente- con una nuova iniezione a pressione nel sottosuolo. Questa operazione avviene in strati rocciosi profondi con caratteristiche tali da evitare i rischi di contaminazione delle falde acquifere presenti.
Proprio questa fase, in circostanze specifiche, può diventare causa o concausa di un terremoto. Lo stesso sito dell’USGS spiega infatti che l’aumento di pressione nei liquidi di cui è premiata una roccia può agire riducendo la resistenza nelle zone di faglia e innescando in alcuni casi un movimento che poi si traduce in evento sismico.
Photo Credits | U.S. Geological Survey
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