Why here?

Studying the distribution of the bigger seisms, with the patient collection of thousands of data, in the Sixties, a map was created for the entire planet. This map shows that the earthquakes are not distributed at random, but the more frequent ones, with the greatest intensity, are distributed in very precise belts. Comparing this map with the map of the margins of the lithospherical plates or layers (the large rigid blocks, which the most superficial part of our planet is subdivided into, that move, and drift on the underlying plastic mantle), it can be seen that the distribution of the earthquakes, almost perfectly outlines the limits of the plates. However there is even more : if the earthquakes are divided according to the depth in which they occurred and the energy that was released, it can be seen that the more superficial and less powerful earthquakes are located near the ocean ridges (diverging plate margins) where the plates separate from each other and where the crust is thinner (3-5 km) and, breaking, allows magma to rise from the Earth’s mantle, and the formation of submarine volcanoes. The deeper earthquakes, the ones that release a greater quantity of energy, instead, are located in the areas where the plates collide (converging plate margins) : in this case the margin of one of the two plates is forced to slide under the other one, in a process called subduction, till, slowly heated, it is reabsorbed by the mantle in a kind of large “recycling” circuit of the lithosphere. Earthquakes in these areas are the result of the friction and deformations that the forced sliding of a plate under the other one produces, and the maximum depth of these earthquakes indicates the depth where the subducted plate is still rigid enough to break, thus giving origin to a seism : the maximum depth registered for these earthquakes is 640 km. Through a study of the earthquakes in these areas, it is possible to “follow” the sliding process of a strip of lithosphere towards the mantle: earthquakes are distributed along an inclined plane called the Wadati-Benioff plane, whose name comes from the researchers who first identified it, drawing the profile of the descending plate almost perfectly. It is therefore clear that the distribution of earthquakes is surely not a casual event and there are areas of our planet in which these events more than a risk are a certainty. For this reason it should not surprise us if seisms are repeated in the same areas. What happened in South East Asia is simply the consequence of the collision of the Pacific Ocean plate and the complex system of small plates to be found between Indonesia and the Philippines. Along the western coasts of Indonesia is one of the deepest trenches in the planet, the Java Trench, near which the subduction of the plates one under the other takes place, leading to the formation of the islands of the Indonesian archipelago. Observing the atlas, we can understand where the lithosphere plates collide and one of the two slides below the other. In fact if one observes the Pacific coasts it is possible to see numerous groups of islands with a typical arc shape, characterized by strong seismicity and intense volcanism; this indicates the presence of subduction plates. It is not a chance that volcanoes and earthquakes are often connected: the presence of an active volcano implies a constant high seismic risk. The study of the Earth’s surface therefore gives us precious elements and proofs of a possible seismic and volcanic risk.

Special reports

From the Multimedia section