Did the Earth Move for You?

By James Piercy

Back in January I travelled to Malta to work with staff at Esplora a new science centre opening soon on the island. At about 06.00 on 13th January I was disturbed a loud noise and rattling. It only lasted a few seconds, I couldn’t see anything out of the window and thought no more of it. The next day I discovered that the noise was the result of an earthquake! The epicentre was in the Sicily Channel about 25 miles away.

Map showing Sicily, Malta and the epicentre of the earthquake

Map showing Sicily, Malta and the epicentre of the earthquake

Measuring an Earthquake

This was my first experience of an earthquake and fortunately it was only a very mild one. A team at the university of Malta use a network of seismometers to record tremors in the area and they are very common. You can see the list of latest records here.

The unit has 2 sensitive seismometers on the island and a third on nearby Gozo will soon come into operation,  you will notice that they record an event nearly every day with strengths ranging from 2-5. The detectors aren’t like the wobbly needles you have seen in films but digital detectors which stream real time responses to the most minor tremors in the earth. Pauline Galea from the Geoscience department at University of Malta explained how their system extends beyond just the island.

“At the same time, we have set up a “virtual” broadband network that gets data in real-time from a number of regional and global seismic stations, so that wherever in the world a significant earthquake occurs, we have very rapid epicentre location and alert.  The system is more finely tuned to concentrate on regional seismicity ie the Central Mediterranean, so we get data from quite a large number of stations in Italy, Tunisia, Spain.”

But what do those numbers mean?

The quake that I felt was described widely as measuring 4.4 on the Richter scale. Charles Richter developed the scale in 1935 as a way of recording the amount of energy released and therefore damage caused by tremors.

The difference between the smallest shakes and the largest catastrophic quakes is enormous but it was important to be able distinguish them. Therefore he devised his scale to be logarithmic. This means that an increase of 1 on the scale is actually a difference of ten times the amplitude of the tremor. My 4.4 scale shake was nearly 100 times more powerful than the 2.5 tremor detected about a week later.

This scale also describes the effects that you can expect from different strength earthquakes. However, these descriptions should be used with caution. Not only are these effects expected close to the epicentre and not further away where the strength should be less, things like the type of land, depth of the tremor and the type of buildings will have an effect of the damage caused.

The Richter Scale

Magnitude Description Effects
Less than 2.0 Micro Not felt or felt rarely
2.0-2.9 Minor Felt by some people, no damage to buildings
3.0-3.9 Minor Often felt by people, but rarely causes damage
4.0-4.9 Light Shaking of indoor objects and rattling. Felt by people in the area, very minimal damage, objects may fall off shelves or be knocked over
5.0-5.9 Moderate Can cause damage to poorly constructed buildings. Easily felt
6.0-6.9 Strong Damage to some buildings, even earthquake resistant buildings may show damage. Strong to violent shaking close to epicentre and felt hundreds of miles away from epicentre.
7.0-7.9 Major Damage to most buildings, some will collapse or suffer severe damage. Serious damage caused up to 250 Km from epicentre. Felt across great distances.
8.0-8.9 Great Major damage with buildings likely to be destroyed. Moderate to heavy damage even to earthquake-resistant buildings. Felt across very large region
9.0 and greater  Massive Total or near-total destruction. Heavy damage and shaking over great distances. Earthquake will result in changes to land formation.

Other Ways to Measure Earthquakes

Whilst we are all used to hearing earthquakes described by the Richter scale it is actually now not used to asses earthquakes.

In the 1960’s it was found that there were problems with using the Richter scale to measure and predict the effects of some kinds of earthquakes, especially those that happened at some distance from the detectors. Several modified systems were introduced, smaller quakes like the one I experienced are classified according to the ‘Local Magnitude Scale’.  Larger and more distant tremors are usually assessed by the Moment Magnitude Scale, These both use a different formula to Richter but are still logarithmic and give very similar results. The Moment Magnitude Scale gives figures very close to Richter for medium strength quakes, those between about 3.5 and 7. Since very small tremors are not noticed by most people and very large quakes are extremely rare most geologists don’t have a problem with the use of the term ‘Richter’ since this is largely understood by the public, but we should remember that this is probably not the scale that was really used to measure the quake.


 

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Posted in Geology, News