Per la classificazione di un terremoto si ricorre spesso a indicatori sintetici espressi sotto forma di scala. La Scala Mercalli e la Scala Richter sono probabilmente le più note al pubblico ma seguono percorsi sostanzialmente diversi e tra loro non direttamente comparabili.
La Scala Mercalli e gli effetti di un terremoto
La Scala Mercalli deve il suo nome al sismologo milanese Giuseppe Mercalli che la propose alla comunità scientifica tra la fine dell’ottocento ed i primi anni del novecento. La scala riprendeva alcuni elementi della preesistente Scala Rossi-Forel e proponeva la classificazione di un terremoto in funzione degli effetti che questo genera sulle persone (ai gradi più bassi), sugli oggetti o sulle strutture (ai gradi più alti). In questo senso la Mercalli misura gli effetti di un terremoto e non direttamente la sua intensità intesa come energia liberata. L’attenzione della scala viene cioè posta sulle conseguenze percettibili di un terremoto sulle attività umane e non sulla misura diretta o indiretta di grandezze fisiche legate all’evento sismico.
Una prima conseguenza di questa formulazione è che la classificazione di un terremoto sulla Scala Mercalli ha solo validità locale e non consente in maniera diretta di confrontare eventi sismici che si registrano in luoghi diversi. È infatti evidente che a parità di altre caratteristiche (energia liberata, geologia dei luoghi, profondità dell’epicentro, capacità di trasmissione del suolo, etc.) un terremoto avrà effetti più marcati in zone in cui le strutture sono state costruite senza criteri antisismici rispetto a zone in cui al contrario le opere dell’uomo sono state progettate tenendo in conto delle problematiche sismiche. In altre parole quindi una scossa di terremoto che la Scala Mercalli classifica come ‘rovinosa’ in una certa regione potrebbe invece essere classificata come ‘moderata’ in un diverso contesto.
I 12 gradi della Scala Mercalli
Le prime formulazioni della Scala Mercalli prevedevano una classificazione in 10 gradi. In tempi successivi il fisico italiano Adolfo Cancani estese la Scala Mercalli introducendo due ulteriori gradi di classificazione. Un ulteriore contributo alla definizione della scala arrivò dal geofisico tedesco August Heinrich Sieberg che ne riformulò la struttura. Per effetto di questo percorso evolutivo è talvolta in uso la denominazione di Scala Mercalli-Cancani-Sieberg (MCS) anche se nelle comunicazioni meno formali si continua a parlare semplicemente di Scala Mercalli. Ogni grado della scala è caratterizzato da una nomenclatura sintetica e da una descrizione degli effetti avvertiti. Si tratta come vedremo di indicazioni non troppo dettagliate ma che hanno il pregio di individuare in maniera pressoché immediata la natura del danno provocato dalla scossa sismica. In dettaglio una formulazioni molto comune della Mercalli sarà impostata in questa forma:
- Grado I: impercettibile (Registrata solo dagli strumenti).
- Grado II: molto leggera (Avvertita dalle persone solo in condizioni particolari).
- Grado III: leggera (Avvertita da poche persone. Oscillazione degli oggetti appesi. Vibrazioni).
- Grado IV: moderata (Avvertita da molte persone. Oscillazioni e vibrazioni).
- Grado V: piuttosto forte (Avvertita anche nel sonno. Caduta di alcuni oggetti).
- Grado VI: forte (Prime lesioni agli edifici).
- Grado VII: molto forte (Lesioni agli edifici).
- Grado VIII: rovinosa (Gravi danni a qualche edificio. Possibili vittime isolate).
- Grado IX: distruttiva (Crolli in alcuni edifici. Numerose vittime).
- Grado X: completamente distruttiva (Crolli in molti edifici. Molte vittime. Crepe al suolo).
- Grado XI: catastrofica (Distruzione dei centri abitati. Molte vittime; Crepe al suolo, frane e possibili maremoti).
- Grado XII: apocalittica (Distruzione di ogni struttura. Pochi superstiti. Modifiche al profilo del territorio).
Corrispondenze tra Scala Mercalli e Scala Richter
Nella classificazione moderna dei terremoti la Scala Mercalli convive da tempo con la Scala Richter. Quest’ultima offre una misurazione del terremoto in tempo reale basata sulla magnitudo del terremoto a sua volta legata all’energia effettivamente liberata nell’evento sismico. Da questo punto di vista quindi non è possibile individuare una corrispondenza esatta tra i gradi della Mercalli e la magnitudo della Richter anche in conseguenza delle considerazioni fatte in precedenza sui criteri antisismici adottati nella regione esaminata.
Si può tuttavia osservare che terremoti con un valore di magnitudo fino a 3 rientrano nella maggior parte dei casi all’interno dei primi 4 gradi della Scala MCS. All’estremo opposto i gradi più elevati della Scala Mercalli (dal IX al XII) corrispondono tipicamente a terremoti di magnitudo superiore a 6, eventi quindi numericamente limitati ma non rari nelle zone ad alto grado di sismicità.
Quando si usa la Scala Mercalli
Come abbiamo visto la Scala Mercalli offre una serie di criteri qualitativi per la classificazione di una scossa di terremoto. Il suo uso più diretto quindi si ha quando c’è la necessità di fornire una indicazione sintetica dei danni generati da un terremoto. La classificazione sulla Mercalli può ad esempio essere utilizzata per indirizzare l’opera dei soccorritori: l’indicazione sulla presenza di feriti o vittime e sulla natura dei danni alle strutture sono infatti parametri importanti per definire la modalità d’intervento. La Scala Mercalli può quindi essere d’aiuto a comprendere rapidamente l’entità dei danni e di conseguenza orientare la scelta delle attrezzature e delle professionalità che sarà necessario schierare sul campo.
A livello storico invece la Scala Mercalli permette di esaminare indirettamente l’evoluzione delle tecniche costruttive e delle normative antisismiche. Terremoti di pari magnitudo con il passare degli anni vengono solitamente classificati con valori decrescenti sulla Mercalli per effetto dei miglioramenti introdotti nelle modalità di realizzazione delle strutture e -in tempi più recenti- delle normative per le costruzioni in zona sismica. In questo contesto la Scala Mercalli può essere assunta come un indice temporale indiretto dei progressi compiuti nello studio e nel contenimento dei rischi connessi ai terremoti.
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