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The rocks that make up the earth’s crust are continuously subjected to enormous stress, which is the result of slow movements between the large plaques in which the most superficial layer of the Earth is divided.
When the stresses exceed the resistance limit of the rocks, these break suddenly releasing energy that propagates, in the form of seismic waves, from the epicentre in all directions, generating the earthquake.
Sometimes the fracture that generates the earthquake, called the fault line, is visible on the surface and forms the fault scarp, a permanent deformation that it is the effect of the process that took place in depth.
The breaking of the rocks frees an enormous amount of energy, which in turn generates powerful vibrations that propagate in the Earth: seismic waves. An earthquake produces different types. The main ones are P waves and S waves. The P waves (such as “Prime”) vibrate the ground in the same direction in which they propagate; compressing and expanding in sequence the rocks that pass through, like an accordion. Instead, the S waves (such as “Second”) vibrate the rocks perpendicularly to their direction of travel, such as a rope that is shaken.
P waves are faster than S waves (approximately 1.7 times), so they are the first to be recorded by the seismometers, followed by S waves. The surface waves are last and propagate only on the earth’s surface. The seismic waves cross through the layers of the Earth varying the speed and also their direction depending on the density of the layer that passes through: the higher the density, the greater the speed and different is the direction of propagation. Going towards the centre of the Earth, the transition from one layer to another causes the seismic rays to run curved trajectories and not straight.
Seismic waves generate effects on man and the environment and are also the best source of information for studying the Earth’s interior. Since the beginning of the twentieth century, the recording techniques of seismic waves and the methods to interpret them have made significant progress. Thanks to them we were able to understand the deep structure of the Earth.
The Earth is composed of layers with very different characteristics: the main layers are the crust, the mantle, and the core. The crust and the outermost part of the mantle form the lithosphere: earthquakes originate here. The rocks that make up the earth’s crust and upper mantle are continuously subjected to enormous stress, which is the result of slow movements between the large plaques in which the most superficial layer of the Earth is divided.
These movements are generated by convective motions of the mantle that push and pull the plaques creating stress that are maximum near the borders between the plaques and minimum in their interior.
The rocks that form the crust have a resistance limit, and the rocks break when the stress exceeds this limit. The fracture propagates quickly and violently, generating the earthquake, namely releasing energy in the form of elastic waves.
The magnitude of an earthquake is measured with two different values: magnitude and intensity. The magnitude (designed in 1935 by the famous American seismologist Charles F. Richter) is used to measure the strength of an earthquake, or rather to estimate how much elastic energy the earthquake unleashed. In fact, there is a very particular mathematical relationship between the size, or magnitude, and the energy of an earthquake. Whenever the magnitude rises one energy unit, the energy increases approximately 30 times. In other words, compared to an earthquake of magnitude 1, an earthquake of magnitude 2 is 30 times stronger, while one with a magnitude of 3 is 30 by 30 times or rather 900 times more powerful!
The highest magnitude ever recorded, equal to 9.5, and is that of the Chile earthquake in 1960. Smaller earthquakes perceived by man have very low magnitudes (around 2.0), while those that can cause damage mostly have a magnitude greater than 5.5.
Another way of measuring an earthquake is based on its intensity. Environmental effects on things and humans are observed. If the magnitude of a given earthquake is only one, on the other hand, the intensity may vary from place to place, according to what happened to things and people; in general, it decreases further away from the epicentre. The intensity of an earthquake is expressed using the Mercalli scale, named after the Italian seismologist that, in the early twentieth century, internationally disseminated the classification of earthquakes according to the effects and damage. This scale, modified by Cancani and Sieberg, consists of twelve levels: the higher the grade, the more disastrous the earthquake.
To estimate the intensity of an earthquake, one must observe and evaluate the effects that it has caused throughout the affected area. For this reason teams of specialist technicians perform surveys in the area affected by an earthquake and collect data to develop maps (macro seismic maps) where the different locations are grouped according to the intensity of the earthquake
The Richter ML magnitude and the Mercalli-Cancani-Sieberg scale are two extremely different measurements: the first is obtained using seismometers; the second is a classification of the effects that the earthquake had on people and things. The measurements are not always related; strong earthquakes in uninhabited areas or with anti-seismic buildings do not cause damage and, therefore, have low degrees of intensity. Conversely, small earthquakes in areas with poorly constructed buildings can cause damage and determine high levels of intensity.
I sismologi indicano la dimensione di un terremoto in unità di magnitudo. Sono molti e diversi tra loro i modi con cui la magnitudo è misurata a partire dai sismogrammi perché ogni metodo funziona solo su un intervallo limitato di magnitudo e di distanze epicentrali, oltre che con differenti tipi di sismometri. Alcuni metodi sono basati su onde di volume (che viaggiano in profondità all’interno della struttura della Terra), alcuni basati su onde superficiali (che viaggiano soprattutto lungo gli strati superficiali della Terra) e alcuni basati su metodologie completamente diverse. Tuttavia, tutti i metodi sono progettati per raccordarsi ben oltre l’intervallo di magnitudo dove sono affidabili. Valori preliminari di magnitudo, basati su dati incompleti ma disponibili già dopo poche decine di secondi dal terremoto vengono comunicati al Dipartimento della Protezione Civile e riportati su web. Tali valori preliminari di magnitudo, che possono differire dalla magnitudo definitiva anche notevolmente (circa 0.5), sono sufficienti per scopi di protezione civile e sono sostituiti da stime più accurate di magnitudo non appena altri dati sono disponibili. Nella maggior parte dei casi, la prima stima della magnitudo fornita dalla Sala Simica dell’INGV di Roma è la magnitudo Richter o magnitudo locale ML. Per eventi di magnitudo maggiore di circa 3.5, se ci sono dati disponibili, si calcola il meccanismo focale con la tecnica del Time Domain Moment Tensor (TDMT, http://cnt.rm.ingv.it/tdmt) e si ottiene anche la Magnitudo Momento MW.
The duration of the perception of an earthquake depends on the magnitude of the event, the distance of the epicentre and the geology of the ground on which it is found. Moreover, if the earthquake is sensed inside a building, the height and type of the building heavily influence the intensity and duration of the perception of the event. In general, the perceived duration ranges from a few seconds to more than a minute depending on the conditions described above.
Below is a detailed explanation of this issue.
The “duration of an earthquake” cannot be defined uniquely, since what can be calculated from instrumental data does not coincide with the duration of the shaking felt by people.
In fact, there are two ways to think of the duration of an earthquake: the first is the time needed for the fault line (the source of the earthquake) to break and the second is the shaking time perceived by a person in a given area.
The first is a given that, even if not immediately, is calculated by analysing the recorded seismic signals. The duration of the shaking at a certain point, however, can only be known by having a seismic station exactly at that point. Even so, we must consider that the duration of the shaking measured by an instrument is always greater than that perceived by a person in the same place, since instruments are far more sensitive than humans, and also record imperceptible shaking.
The earthquake is caused by the sudden sliding (or breakage) of two blocks of crust along a fracture, called fault line. The duration of the break (or sliding) of the fault line is related to how long a point on the fault line takes to slide and the time required for the rupture to propagate along the fault line. Therefore, we must think of an earthquake as an area rather than a point (as the epicentre is conventionally represented on maps). The earthquake starts at a point (the epicentre) and then the rupture propagates along the fault line at about 3 km/s. So, the greater the area of the fault line that breaks, the longer the duration of the earthquake. The greater the area of the fault line that breaks, the larger the magnitude of the earthquake. Therefore, there is a general relation between the duration and magnitude of an earthquake.
It is not possible to promptly indicate this type of duration on websites and INGV applications (as for all the earthquake research centres) because the calculation of how much time it took for a fault line to break is not immediate.
the earthquake takes to happen and how the waves move through the ground up to that point. Moreover, particular geological features (loose soils, alluvial deposits, etc.) can create amplification effects and let the shaking last longer than if it were to happen on hard soils, for example a solid rock (granite, limestone, etc.).
Another important aspect to take into consideration when studying the perception of the duration of an earthquake is, if the earthquake is sensed inside a building since the height and type of building heavily influence the intensity and duration of the perception of the event.
It is difficult to provide a unique and meaningful measurement since the duration of the shaking is quite variable from place to place depending on the distance and local conditions.
To understand the principle of a method for locating the epicentre, the figure below shows the method used when there were few seismic instruments and there were no computers. The so-called circle method is based on the difference between the arrival time of the P waves and that of the S waves, which can be determined on the seismograms, and after a few calculations, the distance between the epicentre and the station where the seismometer is located can be obtained. Tracing a circle with radius equal to the distance just calculated around the station and repeating the same process for at least two other stations can calculate the epicentre. The point in which the lines intersect through the intersection points between the various circumferences is where the epicentre is located.
The data for each earthquake with a magnitude greater or equal to 2.5, which takes place in Italy, is reported to the Civil Protection Data after a few minutes and published on the INGV website.
The time needed to localise an earthquake are shown below, parallel to the times with which INGV informs the Civil Protection Department for earthquakes of a magnitude of ML≥2.5.
Within 2 minutes from an event, it is possible to have an initial estimate of the epicentre position, the depth and the magnitude of the earthquake. This assessment takes place automatically and is based on data sent from the seismic stations closest to the event.
Within 5 minutes the seismograms of all stations of the National Seismic Network affected by the earthquake are available. In this case, even if it is still automatic, the estimate is more precise. Seismologists of the Earthquake Monitoring Control Room quickly assess these estimates, analyse the data, identify the timing at which the P and S waves arrive at different stations, and elaborate a highly accurate localisation and magnitude. The latter are communicated to the Civil Protection Department within 30 minutes from the event (in average after approximately 10-15 minutes).
This information is the first to be released to the media and citizens through INGV communication channels including the INGV websites, the Twitter @ingvterremoti account, the Facebook INGVterremoti page and the INGVterremoti iPhone application.
Timing may be slightly longer for some events, but never longer than 30 minutes. This is the case of some earthquakes that occur in the sea, in volcanic areas or geologically complicated areas. Since timely information, even if partial, is of great importance for civil protection, work is under way to reduce communication times.
Usually, earthquakes occur in areas already affected in the past, where the tectonic stress caused by the movement of the plaques that make up the external shell of the Earth is greater. Consequently, even the underground accumulation of energy and deformation is larger. In Italy, the strongest earthquakes have taken place in Sicily, in the eastern Alps and along the central and southern Apennines, from Abruzzo to Calabria. However, major earthquakes have also occurred in the central-northern area of the Apennines and Gargano. In the last 1000 years, there have been approximately 260 earthquakes with a Mw magnitude equal or greater than 5.5 – an average of one every four years (CPTI11).
Recent earthquakes are distributed mainly in those areas that have experienced maximum seismic intensity values in the past. It is not possible to conclude that earthquakes tend to repeat in the same places. Over the last 30 years, seismometers recorded more than 190,000 earthquakes in Italy and neighbouring countries, largely concentrated in mountainous and volcanic areas. The population did not perceive most of these, and 45 earthquakes have had a Richter ML magnitude equal or greater than 5.0.
Comparing the two maps, it may appear that more earthquakes have occurred in recent years. Actually, the implementation and technological development of the seismic monitoring network, which took place after 1980, made it possible to record ever more small earthquakes that were almost imperceptible. Indeed, these earthquakes occurred also in the past, but there were no instruments to record them and, therefore, there are no records of them.
30 very strong earthquakes (Mw≥5.8), some of which have been catastrophic, have occurred since 1900. They are listed below in chronological order. The strongest of these is the earthquake that in 1908 destroyed Messina and Reggio Calabria.
Catastrophic earthquakes in Italy and around the world have taught us that rapid and accurate information is essential to allow Civil Protection to organise first aid in the affected areas.
Therefore, in the case of earthquakes, we can get an accurate analysis of the phenomenon and within minutes send the Civil Protection Department the location of the epicentre, the Richter ML magnitude and the list of cities closest to the epicentre. This information is crucial to obtain a preliminary estimate of the possible effects to assess the resources needed to handle possible emergencies.
Seismic risk is the expected shaking of the ground at a site due to an earthquake. Since it is mainly a probabilistic analysis, one can define shaking associated only with the probability of occurrence in the near future. Therefore, it is not deterministic earthquake forecasting, a goal far from being achieved in the world, nor is it the highest possible earthquake in the area since the maximum earthquake has a very low probability of occurrence.
In 2004 this seismic risk map (http://zonesismiche.mi.ingv.it) was released, which offers an overview of the most dangerous areas in Italy. The seismic risk map of the country (GdL MPS, 2004; ref. The President of the Council of Ministers’ Order 3519 dated 28 April 2006,Annex 1 b) is expressed in terms of horizontal ground acceleration with a probability of exceeding 10% in 50 years, referring to hard soils (Vs30> 800 m/s; cat. A, Section 3.2.1 of Ministerial Decree dated 14/09/2005). The President of the Council of Ministers’ Order 3519/2006 made this map an official reference instrument for the country.
In 2008, Buildings Technical Standards were updated: for every place in the national territory seismic action is to be considered in designing and is based on this estimate of appropriately correct dangerousness to take account the actual characteristics of the ground at the local level.
The colours indicate the different ground acceleration values that have a 10% probability of being exceeded in 50 years. Indicatively, the colours associated with lower accelerations refers to less dangerous areas, where the frequency of the stronger earthquakes is lower than the most dangerous ones, but this does not mean that they cannot occur.
The strongest shaking with ground acceleration values greater than 0.225 g (g = 9.81 m/s2 gravitational acceleration), are expected in Calabria, southeastern Sicily, Friuli-Venezia Giulia and all along the south-central Apennines. Average values are related to the Salento Peninsula, along the Tyrrhenian coast of Tuscany and Lazio, Liguria, much of the Po Valley and along the entire Alpine region. Sardinia is the least dangerous region with moderate shaking values.
For the purposes of prevention, up to 2008, the probabilistic risk values have been simplified into classes, each of which corresponded to the parameters for the design of buildings.
Subsequently, Building Technical Standards imposed design criteria directly related to the risk map values for every place in the country. Seismic zoning remains in force as an administrative tool of the Regions, for prevention policies, risk reduction interventions, studies on assessing the vulnerability of buildings or soil response (micro-zoning). According to guidelines and criteria established at national level, regions can modify the classification of their area.
- Area 1 where strong earthquakes are very likely;
- Area 2 and Area 3 with strong moderately infrequent events or moderate but frequent earthquakes;
- Area 4 with rare moderate energy events. However, strong, but rare, earthquakes are possible.
Generally speaking, the buildings in zone 1 must be able to withstand, without collapse, a strong earthquake and even more so earthquakes of lower energy. In area 4, the safety of strategic buildings and high-occupancy buildings must be protected.
The earthquake risk is the estimated damage expected as a consequence of earthquakes that could take place in a given area and depends on:
- The riskiness of the area, or rather the seismic shaking that it is reasonably expected in a particular time interval;
- Exposure, or rather the presence of people and things that could be damaged (buildings, infrastructures, economic businesses, etc.);
- Vulnerability of the buildings and infrastructures of the area, or rather their greater or lesser tendency to be damaged by earthquakes.
A very high seismic hazard zone, but devoid of human activity has a very low seismic risk. Conversely, a low seismic hazard area that is highly populated, or whose buildings are poorly constructed or poorly preserved, has a very high seismic risk, since even a moderate earthquake could lead to serious consequences.
The vulnerability of buildings, which depends on the type of building and its maintenance level, remains the primary factor on which it is possible to intervene to reduce the earthquake risk in each area.
Promoting an active role of citizens in prevention is the main goal of “IO NON RISCHIO”: National awareness campaign on natural and anthropogenic risks affecting our country. IO NON RISCHIO earthquake is available in high seismic risk areas and in some big cities that could experience strong earthquakes. The initiative is promoted and organised by: Civil Protection Department, ANPAS, INGV and Reluis, according to the regions and the municipalities involved.
The seismic risk affects everyone, and everyone must do his or her part. The State improves awareness of the phenomenon and its effects by monitoring the territory and developing specific studies; promotes and implements policies to reduce the vulnerability of the private and public building heritage, make homes, schools, hospitals, cultural property and emergency structures safer; update the seismic classification and regulations, by indicating the criteria for building in risk areas and proper planning of the territory; performs training programs, exercises and activities to increase public awareness.
The individual citizen must, first of all, be informed: know the hazard level in its community, know the municipal emergency plan, and locate the nearest waiting area. He/she must make the home and workspace safer by fixing furnishings and distributing them rationally. He/she must also ensure that his/her house is built with the criteria established by seismic standards in the area where it is located and follow these standards in the event of restructuring.
Finally, emergency situations must be faced calmly and with accountability.
Visit the website www.iononrischio.it
What to do first?
The advice of a technician
- Sometimes you just have to reinforce the load-bearing walls or improve the connections between walls and floors. To make the right choice ask an expert technician for advice.
From the beginning
- Remove heavy furniture from the beds and sofas.
- Fix shelves, bookcases and other high furniture to walls, hang pictures and mirrors with closed hooks that prevent them from being removed from the wall.
- Put heavy objects on the lower levels of shelves; objects can be attached to higher shelves with double-sided tape.
- In the kitchen, use a latch for opening the cupboards where dishes and glasses are contained, so that they do not open during the earthquake.
- Find out where gas, water taps and the main switch of the lights are and how to exclude them.
- Keep a first aid box, electric torch, battery-run radio in the house and make sure everyone knows where these are located.
- Find out if the municipality has a Municipal emergency plan and what it entails. If your municipality does not have one, ask for one to be drafted so that you know how to act in the case of an emergency.
- Remove all situations that could be a danger to you and your family members during an earthquake.
- Learn about proper conduct during and after an earthquake and, especially, identify the safe areas in your home where to seek shelter during an earthquake.
During an earthquake
If you are in a close environment
Place yourself in a doorway inserted in a bearing wall (the thickest), close to a supporting wall or under a beam, or find shelter under a bed or a sturdy table. If you are found in the middle of a room, you could be hit by falling objects, pieces of plaster, suspended ceilings, furniture, etc.
Do not go outside until the earth has stopped moving.
If you are in an open environment
Move away from buildings, trees, streetlights and electrical lines: flowerpots, shingles and other materials that fall could hit you.
Pay attention to other possible consequences of the earthquake: the collapse of bridges, landslides, and gas leaks, etc.
Dopo un terremoto
Ensure the health condition of those around you and, if necessary, provide first aid.
Before leaving the building, turn off the gas, water, and lights and put shoes on. When exiting, avoid using the lift and be careful when using stairs as these may be damaged.
Be cautious once outside.
If you are in a tsunami risk area, stay away from the beach and reach a high spot.
Limit the use of the telephone as much as possible.
Limit the use of cars to avoid blocking the passage of emergency vehicles.
Go to the waiting area provided for by the Municipal emergency plan.