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

From time to time, news comes to the fore about a devastating earthquake that affects a region of the planet, resulting in numerous deaths, injuries and countless buildings destroyed. Whenever this happens, the press quickly covers the issue. Newspapers, TV channels, radio and digital platforms flood us with a tsunami of information that goes from the smallest to the most scientifically proven. But in doing so, special attention is paid to misfortune and the most shocking pictures of the destruction caused.

Like any wave, including a tsunami, news attention recedes after three to four days. And then the “destructive earthquake” slumbers on the editorial shelves, waiting for its next awakening in another part of the world.

Seismic hazard does not change

Recently, we have had several earthquakes with a magnitude greater than 6.5 MW (momentary 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 to these major seismic events has grown exponentially. In fact, the 2011 Japan earthquake-tsunami (9.2 MW) was practically broadcast live.

However, the present and future seismic hazard 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 are dedicated precisely to setting limits on the likelihood that an earthquake of a certain magnitude will occur. To do this, we rely on the seismic history of the area, since the seismic forecast to this day continues to be a real chimera.

This time the border between Turkey and Syria became the punishable territory. Since the early hours of February 6, 2023, it has been hit by a real seismic storm, resulting in more than 500 earthquakes aligned in a northeast-southwest direction along a line of just over 180 kilometers that runs from the Mediterranean coast. inland from Turkey.

In this seismic swarm, two earthquakes with a magnitude of 7.9 MW and 7.5 MW are currently distinguished. These are the ones that caused the most destruction. The latest information so far is about 11,000 casualties, thousands of buildings destroyed and at least 20 people seriously injured with damage greater than or equal to Grade VIII. Yes, this is not a mistake: now we are talking about intensity (and not magnitude, Mw), a parameter of magnitude, which is quantitatively expressed in Roman numerals.

Intensity is not equal to magnitude

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

So, for example, an innocent increase in one of these values ​​(ML or Mw) from 5 to 7 means that the earth is moving with an amplitude of a hundred times greater, and the energy released is about a thousand times greater. This explains why large earthquakes are much more destructive than small ones: it is the difference in energy that does the damage.

Of course, seismograph instrumental data are not available for earthquakes before the 20th century, and in many countries even before the middle or end of this century. For this reason, for historical earthquakes, the only parameter that allows you to calculate their size is the intensity, a scale that measures the strength with which an earthquake is felt in a particular place.

Intensity is a semi-quantitative measure of the destructive power of an earthquake based on its effect on people, buildings, terrain, or nature. For this reason, it is driven by the standards and quality of the buildings: a major earthquake won’t hit Japan the way Haiti or Iran would.

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). The scale of the Italian geophysicist Mercalli was one of the first, and almost the majority of modern macroseismic scales are based on it.

As a curious fact, it should be noted that Mercalli was part of the International Scientific Mission that was present at the study of the earthquake in Arenas del Rey (Granada) on December 25, 1884. This is the last strong earthquake (6.9 MW) that had occurred in the interior areas of the Iberian Peninsula, almost completely destroying five cities (three of them had to be rebuilt elsewhere) and claiming the lives of 899 people, as well as 1,500 wounded. For this reason, it is assigned the maximum intensity X.

In short, for recent earthquakes, we measure the size of the earthquake in terms of magnitude (instrumental measurement) and intensity (based on damage). It is the latter that is associated with ground acceleration and is used to map seismic hazards in developed countries.

One magnitude, but several intensities

An earthquake should be of the same magnitude, but it can be felt at different intensities (strengths) in different locations depending on the type of ground, structure, etc. Typically, intensity decreases with distance from the seismic source, although complex geological or topographic amplifying effects can give a significant intensity even in areas remote hundreds of kilometers from the epicenter.

As for magnitude, this is a scale without mathematical limits, although the Earth has a physical limit related to its composition and size: 10.6 MW.

The strongest instrumentally recorded earthquake is the earthquake in Chile in 1960, 9.5 MW. However, various empirical relationships show that the meteorite impact that wiped out the dinosaurs 65 million years ago released energy equivalent to a magnitude 10+ mega-earthquake, causing a seismic, volcanic, atmospheric and climatic catastrophe and a giant mass-scale tsunami. , global. 12.1 MW would be tantamount to the planet being torn in half with no one to record it, so it wouldn’t even be news.

Once the difference between magnitude and intensity is clarified, we hope that when the next “destructive earthquake” occurs, we can all distinguish between its magnitude (energy) and intensity (damage).

Pablo Gabriel Silva Barroso, Professor at the University of Salamanca and Maria Puy Ayarza Arribas, Professor at the Faculty of Geology at the University of Salamanca

This article was originally published on The Conversation. Read the original.

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