The fragility of the buildings and the lack of planning
Mario Tozzi on Printing Remember how myanmar that “It’s one History of earthquakesuntil the times of the empires. And it is a story in which a modern planning and, above all, an anti -seismic restoration of buildings did not be able to replace the ancient constructive wisdom “. History and the territory constantly urges us to intervene, but one always takes over one cyclical forgetfulness associated with poor interest to invest in safety because it does not offer immediate advantages.
The video circulated on the net of the 30 floors skyscraper under construction in Bangkok, with all the structures already performed, collapsed immediately On itself during the earthquake, it reminds us how fragile new buildings can also be fragile without adequate anti -seismic design. Faced with a high magnitude earthquake, a new construction can damage, even seriously, but it must not collapse If the anti -seismic details are proportionate to the strong seismic intensity that characterizes the construction area.
Site and liquefaction effects of the soil
Why we are telling of collapses to Bangkokmore than 1000 km away from the epicenter (point on the earth’s surface directly above the hypocenter)?
The tragedy of this earthquake immediately appeared of exponential value not only for the lack of attention to the design and anti -seismic recovery of the buildings, but also for the co -presence of already known soil amplification phenomena who put the structures to the test even hundreds of kilometers away. The so -called “Site effect” It is a phenomenon that has also been the protagonist in recent Italian earthquakes, for example in L’Aquila in 2009 but even earlier in Umbria in 1997. And overseas in the most recent seismological history, the effect of site has largely characterized the great earthquake of Mexico City of 1985. In that case the epicenter was in the ocean over 400 km from the Mexican capital. Normally the level of damage, with the same building quality and medium -sized characteristics of the land, decreases with the distance from the epicentral area.
With the exception of two punctual factors: greater expiry of the building quality of the buildingor for poor quality of the stratigraphy of the soil on which the construction is based. In the loose granular landsuch as limits and sands, or alluvial deposits, or subject to liquefaction phenomena (As happened in Emilia in the 2012 earthquake), in the presence of cavities, or in any case in particular local conditions concerning the stratigraphic structure of the rocks and land, dangerous amplifications of seismic waves are favored even if distant from the epicentral area.
“Geological “corridors” They lead the waves even at a great distance and then prove – possibly serious – because of the local conditions of the subsoil » (Mario Tozzi, La Stampa). Already in the earthquake of L’Aquila there were nearby houses built identical by the manufacturer himself, however damaging themselves differently for probable punctual amplifications of the land below. The so -called site effects can be known in advance by means of in -depth geological investigationswhich should always be carried out especially if you already know a story of the event of these phenomena during past earthquakes.
THE devastating damage of Myanmar’s earthquake seem to be traced back to phenomena of liquefaction of the sands. These land when Saturi tend to losing resistance under cutting stresses impressed by an earthquake, as a consequence of which, the ground reaches one condition of fluidity equal to that of a viscous liquid. The resistance to cutting the ground is thus canceled. It therefore happens that the concentrated weight of a building in a part of soil affected by the liquefaction causes a differential thrust (i.e. different from that present in the rest of the surface) downwards, which translates into a cutting effort that the liquid state of the soil it is unable to combatcausing the sinking and instability of the building itself, or even its overturning.
Against earthquakes, historical memory and prevention are essential
Reflections are always the same in the face of these tragedies, summarized in the words of Mario Tozzi, expert geologist: «But why do the earthquakes of New Zealand and California do not produce the victims and the damage of Asian and Anatolian ones? The usual: There is no aware territorial planning, There is no anti -seismic design, I do not choose suitable materials, no seismic zoning is proceeded, Forget the events of the past».
The catastrophes deriving from earthquakes are often assimilated to rare events, of which a culture does not criticize does not want to treasure errors but on the contrary trying in the time immediately following forgetting, although in seismic areas it is cyclical events that will return to repeat over time. We are always granted a period of peace between an earthquake and the next, which we do not always know (we want) exploit to progress towards a new and safer building culture.
