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Magnitude vs. Intensity
Earthquake magnitude and intensity are both important measurements obtained after an earthquake. They refer to two different aspects of the seismic event, the size and the damage, but they are often confused with one another. The magnitude is a number indicating the size of the seismic event. An earthquake has only one value of magnitude. Such a value is obtained by analyzing seismic signals. The more seismic signals are analyzed, and the more time is spent on the analysis, the more accurate the resulting magnitude value will be. The intensity is a number (written as a Roman numeral) indicating the damage caused by an earthquake. An earthquake has several intensity values, because the shaking and the damages can vary from place to place depending on the distance from the epicenter. The intensity is calculated using human observations and reports of felt shaking and damage. |
Why does the magnitude change in the aftermath of an earthquake?
It is not rare that, in the aftermath of an earthquake, seismological agencies report a first estimate of the earthquake magnitude, but subsequently modify it -downgrading or upgrading it. Calculating an accurate preliminary magnitude is difficult because the earthquake itself is a complex process. Besides, several different techniques exist to calculate the earthquake magnitude. Some techniques are easier to apply and can return an approximate magnitude value within a few seconds from the earthquake onset. Other methods, on the contrary, are more robust, but require large data sets, extensive analysis, hence longer computing time. When an earthquake strikes, it is important both for the seismological agencies and the citizens to have rapid preliminary information about the earthquake size. With more data coming in, and more time dedicated to the analysis, seismologists can, in a longer time range, return a refined and more accurate value of the earthquake magnitude. |
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Why does the magnitude of an earthquake differ from place to place?
Earthquakes do not happen randomly in the Earth’s crust. If we look at the distribution of earthquakes around the globe for the past decades, we will notice that earthquakes mostly occur at the margins of the tectonic plates. You might also have noticed that certain regions of the world (e.g., Chile, Japan, Indonesia) are often hit by large (M>7) earthquakes. These regions are located over the so-called subduction zones, where one tectonic plate moves under the other and sinks into the mantle. The earthquake magnitude is related to the amount of seismic energy released during the seismic event. Hence the Earth’s most energetic seismic events typically occur in the subduction zones. |
I felt an earthquake, where can I find more information?
If you are looking for rapid and reliable earthquake information after an earthquake, you can download and use the LastQuake app.
Share your testimony by following the “I felt an earthquake” button. Personal information is private. We do not share, sell, or trade your personal information with anyone. |
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Are there going to be aftershocks?
Aftershocks are smaller seismic events that follow a major earthquake. If you live near the epicenter of the earthquake, you are likely going to experience aftershocks in the hours and days that follow the main seismic event. Aftershocks are unpredictable. However, their frequency in time and magnitude follow well-established empirical, statistical laws (respectively, the Omori’s Law and the Bath’s Law). To feel aftershocks in an area that has been hit by an earthquake is absolutely normal. Aftershocks are indeed the way the Earth’s crust rebalances itself after the main shock. |
Can we predict earthquakes?
Seismologists cannot predict earthquakes at present. They can, nonetheless, estimate the probability of experiencing a seismic event in a specific area within a time window. This is what seismologists call a probabilistic earthquake forecast. |
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Is there a tsunami risk?
If you live along the coast in an earthquake-prone region, you may be at risk from tsunamis. Tsunamis consist of a series of waves that rushes ashore with powerful currents. Tsunamis can travel farther inland for several hundreds meters, causing flooding and damages. The first wave may not be the largest or the most damaging one. Strong waves can indeed last for several hours after the arrival of the first wave. In the Mediterranean Sea, every coast is exposed to tsunami risk due to the high seismicity of the area. Since no one can predict with certainty when an earthquake will happen, and a tsunami triggered, it is important that everyone gets prepared in advance. For any further information on the tsunami risk and alert in the Mediterranean Sea, please visit the CENALT (Centre d'alerte aux tsunamis). |
Some earthquakes have no magnitude, why?
When an earthquake occurs, reliable earthquake information can take several minutes before being published on the official websites and the LastQuake app. Crowdsourced earthquake detection is very fast. It can happen sometimes that an earthquake is felt by the population, but seismological data takes more time than usual to be analyzed. In these cases, we display a possible felt event in a certain region, even though precise information on the earthquake parameters, such as the magnitude, are not available yet. Detailed information on the seismic event is updated and published as soon as the earthquake parameters have been retrieved with confidence. |
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Traveling to earthquake prone regions
If you are travelling to a seismically active region, it is best to be aware of your surroundings and take some simple precautions, so you do not put yourself at unnecessary risk. The information provided below is general guidance that follows best international practice.
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What to do in case of an earthquake?
Experiencing an earthquake can be scary: you may fear for your life and the lives of the people around you. Since no one can predict with certainty when an earthquake will happen, it is important that everyone gets prepared in advance.
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