The course of history

Earthquakes are linked to geologic and tectonic situations and therefore tend to recur in time and in the same areas, often in a similar manner. It is very important, therefore to collect the historical data of seismic events that date as far back as possible in time. Some geological studies also enable the reconstruction of seismic events of the even more remote past, through studies, for example, of particular forms of the ground, ancient landslides, or the breakage of concretion inside caves. In this manner, maps of the seisms of the past are drafted for every region, enabling the creation of general seismic risk maps, which illustrate the so-called seismic zoning. For each area it is important to note the intensity of the different seisms that occurred in succession, in order to determine the “periodicity” of the seisms of greater intensity. In practice, the statistics of the more intense events are elaborated, and a forecast is made determining after how many years an earthquake of a certain magnitude has occurred. The earthquakes of a greater intensity generally have a longer periodicity, that can be measured in years or decades or centuries. It cannot be exactly forecasted when an earthquake will take place but it is known that it will occur within a certain amount of time, so the more time passes, the greater the probabilities that certain events might take place. If the Earth remains quiet, therefore, it must not mean we can stay off-guard – but rather, the contrary! A classic example is the San Andreas Fault in California, one of the most studied seismic zones. The San Andreas Fault is over 1000 km long and 32 km deep. It is where the North American plate clashes with the Pacific plate – here the calculation of the periodicity for a recurring seism of great intensity is estimated to be 100-150 years. The last event of a great intensity occurred in 1857, therefore it is increasingly probable that a strong earthquake might take place now or in a few years time. Therefore the wait has begun for what the Californians call “The Big One”. Other studies have highlighted an increase in the microseismicity and deformations around the zone of the fault, these are all precursory signs of a subsequent important movement: it is therefore estimated that there is a 60% probability that a violent earthquake may occur in the next 30 years. The Big One is awaited, hoping everything is ready to face it!

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