EARTHQUAKE

An earthquake (or seismic event) is a sudden and unpredictable movement of the earth's crust's deep layers that has varying effects on the surface depending on the released energy.

It's a natural phenomenon that stems from the Earth's structure, which is made up of inhomogeneous rocks that react differently to pressure and heat, and contributes to the modelling of the planet's surface. Earthquakes begin in the lithosphere, the outermost layer of the earth's crust, and can reach a depth of 700 kilometres. Although a seismic event can occur anywhere on the planet, it is more common near specific areas, such as the Pacific Ocean's edges and major mountain ranges (Alps, Himalaya Range, Rocky Mountains and Cordillera de los Andes). Because they occur in the deepest layers of the earth's crust, the most common and violent earthquakes are referred to as "tectonic." According to Plate Tectonics Theory, the earth's crust is divided into rigid portions that "move" and "collapse" into one another, causing compressions, cracks (called "faults"), and deformations. Tectonic earthquakes are distinguished by "volcanic" earthquakes, which are lower-intensity seismic events that occur near volcanic craters and are caused by magmatic fluid movement.

In general, the "hypocentre" is the depth at which energy is released, whereas the "epicentre" is the point of maximum intensity recorded on the earth's surface. The energy propagates in waves from the epicentre, striking people, buildings, and the entire territory.

Three phases can be identified during an earthquake:

• Foreshocks: a series of premonitory (but not always detectable) shocks that occur prior to the actual seismic movement;

• Mainshocks: the main shock that is released directly from the hypocentre;

• Aftershocks: or replicas, are very common and have a decreasing intensity.

The sequence of seismic movements of more or less similar intensity is referred to as "seismic swarm", while the set of three phases is referred to as "seismic wave period."

Two different measurement units are used to define the strength of an earthquake: magnitude and macroseismic intensity. Magnitude is a numerical value based on the Richter’s scale that expresses the amount of released energy. A comparison of the maximum amplitude of the wave (recorded by a seismogram) and a reference-earthquake is used to determine the magnitude. The amplitude of the wave detected by the seismograph is then normalised using a logarithm that expresses the values on a decimal basis: each Richter's scale degree corresponds to a 10 times greater displacement of the ground and a 30 times greater release of energy. This scale has no upper limit, and the highest recorded value is currently 9.5. (Valdivia earthquake, Chile 1960).

The Mercalli's scale is used to assess macroseismic intensity. The depth of the hypocentre in relation to the epicentre, the intrinsic characteristics of the soil, and the human presence in the affected area are all elements that influence the intensity. As a result, the reference scale for this parameter (known as Mercalli-Càncani-Sieberg's scale) takes the destructive effects of the seismic event into account. The scale is divided into 12 degrees, with the lowest values representing events that have not visible effects. The energy created in the hypocentre causes the rock particles to vibrate, which spreads in all directions. Friction between the particles contributes to energy dispersion, which is why the shock is less intense further away from the epicentre.

There are four distinct types of waves, two deep (P and S) and two superficial (L and R):

• P waves: high-speed waves that cause longitudinal particle displacement as well as compression and expansion of the ground;
• S waves: slow waves that only affect solids and cause transversal particle displacement with perpendicular oscillations;
• L waves: surface waves produced by the collision of P and S waves, causing transverse oscillations;
• R waves: surface waves produced by the collision of P and S waves, causing elliptical movements.

To mitigate the negative effects of seismic events, Italian legislation has enacted a number of measures and regulations based on the concept of "seismic classification", which is determined by the magnitude and historical frequency of seismic events in the area, as well as enforcing specific building and infrastructure regulations.

The Italian legislation, in particular, establishes technical standards for both countering and resisting less powerful earthquakes without suffering damage, as well as responding to stronger earthquakes without collapsing. The current seismic classification criteria were established in 2003 and are the result of more recent elaborations relating to the seismic hazard for the national territory, which refers to the likelihood that an earthquake will strike a specific territory within a certain time frame (called "return period", usually 50 years).

The State has delegated to the Regions the task of adopting the seismic classification by Order of the President of the Council of Ministers no. 3274 of March 20, 2003 (Official Gazette no. 105 of May 8, 2003), resulting in the compilation of a list of Municipalities with the corresponding attribution to four areas of decreasing danger:

• Zone 1: High probability of a strong earthquake;
• Zone 2: Moderate probability of a strong earthquake;
• Zone 3: Low probability of a strong earthquake;
• Zone 4: Very low probability of a strong earthquake.

However, even between areas that are very close to each other, it is common to see how the negative effects of an earthquake could differ. These situations are influenced by the land's specific characteristics, as well as the state of maintenance and seismic adaptation of the existing buildings.

Seismic Microzonation (MS) studies have been introduced to deal with this situation, and they identify the stability (or instability) of municipal areas (stable areas, stable areas susceptible to local amplification, unstable areas and subject to landslides, surface ruptures due to faults and dynamic soil liquefaction). This data is critical for effective territory management, as well as the design and planning of post-emergency intervention and reconstruction.

Seismic microzoning studies can be used to guide urban planning decisions, plan surveys, establish guidelines, priorities, and intervention methods in territorial management, as well as in emergency planning to identify strategic elements in a given area and, in the event of criticality, to intervene in terms of safety. Finally, during the post-event reconstruction phase, seismic microzonation aids in determining the best locations for temporary structures and new constructions, as well as providing useful elements for assessing possible reconstructions (Seismic Microzonation, see below).

Did you know?

Seismic activity can be caused by humans as well. This is the case with "collapse" and "explosion" earthquakes: both are low-intensity rare events linked to the deliberate or unintentional destruction of structures (buildings or infrastructures) or the use of explosives.