The earthquake of 6 April 2009 Aquila has sparked heated debate in Italy about the possibility of predicting large earthquakes. The prediction of earthquakes has been a dream of many people involved in professional or amateur of earth sciences, and then an event like that of April 6, 2009 may appear as if it were a golden opportunity to demonstrate that the problem is solvable. Unfortunately, the prediction of earthquakes is a field full of half- verità, vicoli cechi, approssimazioni e delusioni. Esso viene offuscato ancora di più da periodiche dichiarazioni provenienti da ciarlatani che, di fronte al pubblico e ai mass media, riescono ad simulare una sembianza di credibilità (Dearing e Kazmierczak 1993). E' comprensibile che il pubblico a rischio, le pubbliche autorità delle zone sismiche e gli stessi ricercatori tengano alla speranza che ci sia un metodo attendibile di preavvisare le popolazioni di un evento sismico imminente, ma la realtà è assai più complessa.
Nell'ottica delle dichiarazioni del dott. Giampaolo Giuliani sulle anomalie nei flussi di radon che hanno preceduto il sisma abruzzese ( La Repubblica 2009), e del successivo dibattito the reliability of radon as an indicator of seismic activity, the objective of this short article is to clarify certain aspects of the problem of earthquake prediction. I will refer to observations obtained from the literature on social and physical warning of natural disasters and associated operational issues.
The prediction of earthquakes
The prediction of earthquakes can be divided between long periods of food-related initiatives, and short-term. As for the long term, there are good models of the seismicity of the Central that facilitate the detailed understanding of seismic zones and therefore the risk areas (Galadini and Gallo 2000). With reference to the average term recurrence intervals of earthquakes of a given destructive power are known (Costa et al 1995).
should be noted, however, that the registration of seismic events and the continued publication of new geophysical research help to refine the model geotectonic that necessitate periodic revisions to the anti-seismic regulations for the towns. In this way, the area most affected by the earthquake of April 2009 was included in the first category seismic (maximum likelihood) for a short time. Given the lack of active seismic retrofit programs, very few buildings had been adapted to the new rules. In fact, the seismic standards have a common fault of not protecting the existing buildings that in many cases constitute the majority of the housing stock and above all most at risk.
Radon and forecasting short-term
The forecast short-term refers to the ability to recognize the warning signs of a seismic event with days or hours before the event the main shock, that they can issue a notice in at-risk populations with enough time to request a specific response to social self-protection.
Radon (Rn-222) is a heavy, radioactive element that belongs to the category of noble gases or inert. It is present in many rock formations and can these transfers to and from the ground, in the form of heavy gas, or dissolved in groundwater or in the form of oxides with carbon or fluoride.
In the classical model of the generation of an earthquake, is a lively process of micro-cracks in the earth's crust where the earthquake was born. The constant opening and closing of small cracks may emit radon from the rocks and do accumulate in groundwater and, in gaseous form, into the soil. The imposing deformation processes in the area of \u200b\u200bthe fault under stress can cause a substantial increase in the flow of radon, followed by the hour before the strong shaking a sharp attenuation in the period in which the movement of the fault begins to dynamically spread and small cracks close (Scholz et al 1973).
methods of monitoring the flow of radon have been developed in the 70s (eg. Noguchi and Wakita 1977) and since then have been practiced in many seismic areas of the world, with continual improvements in precision instruments (Planinic et al 2004). Thus, in India through monitoring of soil gas radon and the radon content of some groundwater and Singh Virk (1994) were able to identify the "symbol" of an earthquake of magnitude 6.5 to 7.0 in one week advance of the shock. Igarashi et al (1995) have found abnormalities in the flow of radon for months before the earthquake in Kobe, Japan, 1995, with a peak of this activity nine days before the main shock. The researcher islandese Hauksson (1981) ha avanzato l'ipotesi che più grande sarà l'eventuale terremoto, più territorio sarà interessato da anomalie del flusso di radon.
Malgrado diverse storie di successo nel monitoraggio del radon come precursore dei terremoti, in uno studio di Hauksson e Goddard sul radon nelle acque freatiche davanti a 64 piccoli terremoti avvenuti in Islanda il metodo ha avuto un'entità di successo di solo 14 per cento. Nella maggior parte dei casi, la prevista anomalia non si è manifestata e in sette casi l'anomalia è arrivata ma senza precedere attività sismica.
La ricerca di Claesson et alii (2004) ha rivelato precursori, non soltanto in radon, ma in 15 altri elementi e minerali, e questi sono stati attribuiti a processi di apertura e chiusura delle faglie, soprattutto in rocce fratturate e sature di acqua.
Valutazione della capacità premonitrice
Per quanto riguarda la generale abilità di prevedere i terremoti, e il ruolo specifico del radon, si può fare diverse osservazioni, come segue.
La generazione dei terremoti mette in moto processi geologici e meccanici assai complicati che danno luogo a sofisticati cambiamenti fisico-ambientali. Molti dei fenomeni che potrebbero dare luogo a precursori sono sepolti nella crosta terrestre e avvengono con temperature e pressioni altissime che impediscono sia il monitoraggio che la previsione. Perciò, rimane difficile prevedere eventi of which the elements are not easily replicable.
According to scientific consensus, all possible precursors should be considered, not only some choice. Thus, phenomena seismological, meteorological, electrical, magnetic, geochemical, geodetic and hydrological are involved. However, in the case of Parkfield, California, a period of precursors in the '80s and '90s gave rise to a seismic alarm duration of 18 months and a forecast of an earthquake measuring at least 6.0. During this period, scientists have been monitoring precursory phenomena intensively with 300 instruments, but the earthquake did not happen (Mileti et al 1994).
Many of the best prediction of an earthquake, using radon or other precursors, were performed after the event by "backcasting", which is the post hoc analysis of data collections. A case very well known than the Kobe earthquake that occurred at January 17, 1995. Many post-event analysis revealed precursory phenomena (see eg. Nagai et al 2002), but before the earthquake did not look for these signs because the estimates were focused initiatives in other parts of Japan.
In sum, the radon in some cases proved to be a good precursor, but there are few studies of its reliability. The aforementioned Icelandic (Hauksson and Goddard 1981) but not very hopeful.
forecast and warning
The long experience dei processi di previsione dei terremoti del Servizio Geologico statunitense (USGS) rivela che in molti casi le probabilità di avvenimento del sisma sono basse, e le 'finestre di tempo' della validità della previsione sono lunghe. L'imprecisione dei dati non aiuta a destare una risposta ottimale da parte dei beneficiari della previsione, ovvero le popolazioni a rischio (Geller 1997).
In ogni caso il problema della previsione a corto termine dell'evento sismico catastrofico è secondario a quello dell'uso che verrebbe fatto della previsione. Il termine 'preavviso' può essere definito come "una raccomandazione o un ordine di compiere un'azione in base ad una previsione a corto termine" (Alexander 1993). I preavvisi consistono components in physical, administrative and social (see figure). Thus, an alarm is to be interpreted by the relevant scientific, civil protection authorities and, if necessary, information is transformed into a practical form and transmitted to the public, which would be bound to give all'autoprotezione.
The lack of any of the three components makes the notice failed and potentially dangerous. Therefore, it is sufficient to inform the population of scientific anomaly if there are solid plans for evacuation, reception and made safe. Furthermore, although the precursory phenomena may be clear, their times of operation and validity are not easily interpretable. If you tratta di un fenomeno che vada avanti per mesi, non si può pensare che ci sia, da parte della popolazione a rischio, una lunghissima reazione temporanea che comprende la sospensione delle normali attività e l'interdizione di luoghi, prevalentemente urbani, che sono considerati pericolosi.
Nel campo della previsione dei terremoti il senno di poi è una triste afflizione. Ciò che è visibile o interpretabile dopo l'evento non lo è in ogni caso prima. La strategia più saggia sarebbe di concentrare gli sforzi sul potenziamento delle misure a medio e lungo termine, cioè la riduzione del rischio sismico, la pianificazione di misure per soccorrere e tutelare le popolazioni a rischio, e forse in terzo luogo il miglioramento links between the sides technical, administrative and social system notice. Before you can use the information on earthquake precursors in place, we must create a culture of sensitivity to seismic risk and self-protection. Otherwise, the forecast still would not be usable.
Citations
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