Earthquakes: why it is important to differentiate between intensity and magnitude

From time to time the news of a devastating earthquake that affects a region of the planet, causing many victims, injuries, and countless destroyed buildings, comes to the fore. Whenever it happens, the press quickly addresses the issue. Newspapers, television channels, radio and digital platforms flood us with a tsunami of information that goes from the least to the most scientifically proven. But that places special emphasis on the misfortune and the most shocking images of the destruction caused.

Like any wave, including a tsunami, news attention recedes in three or four days. And then, the “devastating earthquake” remains dormant on the shelves of newsrooms, waiting for its next awakening in another part of the world.

Seismic hazard does not change

We have recently had several earthquakes with magnitude above 6.5 Mw (moment magnitude), for example in Indonesia (2004), L’Aquila (2009), Haiti (2010), Chile (2010), Japan (2011), Ecuador (2016), Amatrice (2017) and Nepal (2018), some even above 8.5 Mw. Media attention on these major seismic events has multiplied exponentially. In fact, the 2011 Japan earthquake-tsunami (9.2 Mw) was virtually televised live.

However, the present and future seismic danger of the affected areas is the same all the time, when it is in the media and when it is not. Those of us who study earthquakes dedicate ourselves, precisely, to establishing the margins of probability that an earthquake of a certain size will occur. To do this, we base ourselves on the seismic history of an area, since to this day seismic prediction continues to be a true chimera.

This time, the area punished has been the border between Turkey and Syria. Since the early morning of February 6, 2023, it has been subjected to a true seismic storm, with a swarm of more than 500 earthquakes aligned in a northeast-southwest direction along a line of just over 180 kilometers, which runs from the Mediterranean coast inland from Turkey.

Within this seismic swarm, for the moment, two earthquakes of magnitude 7.9 Mw and 7.5 Mw stand out. These are the ones that have caused the most destruction. The latest information speaks, for the moment, of around 11,000 victims, thousands of buildings destroyed and at least 20 populations seriously affected with damage of intensity greater than or equal to grade VIII. Yes, it is not a mistake: now we are talking about intensity (and not about magnitude, Mw), a size parameter that is quantified with Roman numerals.

Intensity is not the same as magnitude

The magnitude is measured on scales such as the Richter (ML) or the seismic moment (Mw), logarithmic scales based on the maximum amplitude of the seismic waves recorded on the seismograms. They are quantitative scales that refer to the energy released by an earthquake. Each unit jump on these scales is equivalent to a displacement of the ground 10 times greater, and a release of energy about 33 times greater.

Thus, for example, an innocent increase in one of these magnitudes (ML or Mw) from 5 to 7 implies that the ground moves with a hundred times greater amplitude and the energy that is released is approximately one thousand times greater. This explains why big earthquakes are so much more devastating than small ones: it is that difference in energy that does the damage.

Of course, the instrumental data provided by seismographs are not available for earthquakes prior to the 20th century, and in many countries even up to the middle or end of that century. For this reason, for historical earthquakes, the only parameter that allows calculating their size is intensity, a scale that measures the force with which an earthquake is felt in a certain location.

Intensity is a semi-quantitative estimate of the destructive power of an earthquake based on its effects on people, buildings, and the terrain or nature. For this reason, it is conditioned by the standards and quality of the constructions: a great earthquake does not hit Japan in the same way as Haiti or Iran.

In order to avoid ambiguities, since the middle of the 19th century, intensity scales have been developed which today are divided into twelve degrees (I-XII). That of the Italian geophysicist Mercalli was one of the first, and practically the majority of current macroseismic scales are based on it.

As a curious fact, it should be noted that Mercalli was part of the International Scientific Mission that attended the study of the earthquake in Arenas del Rey (Granada) on December 25, 1884. This is the last strong earthquake (6.9 Mw) that has been occurred in the interior of the Iberian Peninsula, almost completely destroying five towns (three of them had to be rebuilt elsewhere) and causing 899 fatalities, in addition to 1,500 injuries. For this reason, it is assigned a maximum intensity of X.

Arenas del Rey after the earthquake that affected various towns in Granada (Spain) on December 25, 1884.
Wikimedia Commons

In short, for recent earthquakes we measure the size of an earthquake with magnitude (instrumental measurement) and intensity (based on damage). It is the latter that is linked to ground acceleration and is the one used to design seismic hazard maps in developed countries.

One magnitude but several intensities

An earthquake should have a single magnitude value, but it can be felt with different intensity (strength) in different locations depending on the type of soil, constructions, etc. Normally, the intensity decreases as we move away from the seismic focus, although complex geological or topographic amplification effects can give significant intensities even in areas hundreds of kilometers away from the epicenter.

Regarding the magnitude, it is a scale with no mathematical limit, although the Earth has a physical limit related to its composition and size: 10.6 Mw.

The largest instrumentally recorded earthquake is the 1960 Chile earthquake, 9.5 Mw. However, different empirical relationships indicate that the meteorite impact that wiped out the dinosaurs 65 Ma ago released energy equivalent to a mega-earthquake of magnitude greater than 10, causing a seismic, volcanic, atmospheric and climatic disaster and a gigantic tsunami on a massive scale. global. A 12.1 Mw would be the equivalent of the planet ripping in half and there would be no one left to record it, so it wouldn’t even be news.

Once the differences between magnitude and intensity have been clarified, we hope that when the next “devastating earthquake” occurs we will all be able to differentiate between its magnitude (energy) and its intensity (damage).

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